George Burgess IV
2015-Jan-14 18:30 UTC
[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Inline - George> On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org> wrote: > > > >> On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at google.com> wrote: >>> On 13 January 2015 at 22:11, Daniel Berlin <dberlin at dberlin.org> wrote: >>> This is caused by CFLAA returning PartialAlias for a query that BasicAA can prove is NoAlias. >> >> One of them is wrong. Which one? > CFL-AA. > > Right now it checks whether two things come out to be the same stratified info and have the same index, and if so, returns PartialAlias. This is wrong, because it never knows why two things got unified, only that they did. > > Therefore, the current check it does where it returns PartialAlias seems wrong. > > George, why does it return PartialAlias?It shouldn't -- unless I misinterpreted an earlier discussion, I was under the impression that we were testing with that line updated to return MayAlias. We do need to update that in LLVM though; thanks for the fix.> IMHO, you can't ever prove any kind of must-alias or partialalias info with CFL-AA, unless i'm missing something.Agreed.> >> I'm not sure from your description that this is a chaining issue. PartialAlias doesn't chain and isn't supposed to, it's a final answer just like NoAlias and MustAlias are. You return partial alias when you have proven that the accesses are overlapping, meaning that it is proven to neither be no alias nor must alias. > Well, it's still chaining wrong, because it did not change on MayAlias, but you are right that it is not the issue i identified :) > > Anyway, updated patch attached. > >> >> Nick >> >>> I've attached a patch that fixes it, but truthfully, chaining seems somewhat badly designed IMHO. >>> >>> It should let you pass along the result you've gotten so far, so that you don't have CFLAA get PartialAlias, chain, and have BasicAA get MayAlias. >>> >>> (IE it should let it work strictly down the hierarchy). >>> >>> Note that this patch breaks the CFLAliasAnalysis tests because it no longer returns PartialAlias in some cases it is expecting it. >>> >>> I don't have time ATM to fix chaining completely, so if someone wants to shepherd this patch through, that would be appreciated. >>> >>> >>> >>> >>> >>>> On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at dberlin.org> wrote: >>>> Can you send me actual LLVM IR or a preprocessed source from using -E? >>>> I don't have a machine handy that has headers that target that arch. >>>> >>>> >>>>> On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at dberlin.org> wrote: >>>>> Anything other than noalias or mustalias should be getting passed down the stack, so either that is not happening or CFL aa is giving better answers and something does worse with those better answers. I'll take a look this evening >>>>> >>>>>> On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos at codeaurora.org> wrote: >>>>>> Hi folks, >>>>>> >>>>>> Moving the discussion to llvm.dev. >>>>>> >>>>>> None of the changes we talked earlier help. >>>>>> >>>>>> Find attached the C source code that you can use to reproduce the issue. >>>>>> >>>>>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast -fno-math-errno test.c -S -o test.s -mllvm -debug-only=licm >>>>>> LICM hoisting to while.body.lr.ph: %21 = load double** %arrayidx8, align 8, !tbaa !5 >>>>>> LICM hoisting to while.body.lr.ph: %arrayidx72 = getelementptr inbounds double* %11, i64 1 >>>>>> LICM hoisting to while.body.lr.ph: %arrayidx81 = getelementptr inbounds double* %11, i64 2 >>>>>> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64 >>>>>> >>>>>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast -fno-math-errno test.c -S -o test-cflaa.s -mllvm -use-cfl-aa -mllvm -debug-only=licm >>>>>> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64 >>>>>> >>>>>> Why CFL AA cannot allow hoisting this out: %21 = load double** %arrayidx8, align 8, !tbaa !5 >>>>>> >>>>>> Which leads to this extra load in assembly code: ldr x14, [x4, x9, lsl #3] >>>>>> >>>>>> Thanks! >>>>>> Ana. >>>>>> >>>>>> -----Original Message----- >>>>>> From: Hal Finkel [mailto:hfinkel at anl.gov] >>>>>> Sent: Tuesday, January 13, 2015 12:51 AM >>>>>> To: Chandler Carruth >>>>>> Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel Berlin; George Burgess IV >>>>>> Subject: Re: question about enabling cfl-aa and collecting a57 numbers >>>>>> >>>>>> ----- Original Message ----- >>>>>> > From: "Chandler Carruth" <chandlerc at gmail.com> >>>>>> > To: "Ana Pazos" <apazos at codeaurora.org>, "Daniel Berlin" <dberlin at dberlin.org>, "George Burgess IV" >>>>>> > <george.burgess.iv at gmail.com>, "Hal Finkel" <hfinkel at anl.gov> >>>>>> > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com>, "Ana Pazos" >>>>>> > <apazos at quicinc.com> >>>>>> > Sent: Monday, January 12, 2015 6:43:44 PM >>>>>> > Subject: Re: question about enabling cfl-aa and collecting a57 numbers >>>>>> > >>>>>> > Not quite: >>>>>> > >>>>>> > >>>>>> > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos < apazos at codeaurora.org > >>>>>> > wrote: >>>>>> > >>>>>> > Thanks George and Daniel, >>>>>> > >>>>>> > >>>>>> > >>>>>> > If the recommended order is “–basicaa –cfla-aa” it means we should fix >>>>>> > the trunk code that processes the flags -mllvm use-cfl-aa-in-codegen >>>>>> > and –mllvm use-cfl-aa, right? >>>>>> > >>>>>> > >>>>>> > >>>>>> > In Transforms/IPO/PassManagerBuilder.cpp and CodeGen/Passes.cpp we see >>>>>> > this sequence: >>>>>> > >>>>>> > >>>>>> > >>>>>> > if (UseCFLAA) >>>>>> > >>>>>> > PM.add(createCFLAliasAnalysisPass()); >>>>>> > >>>>>> > PM.add(createTypeBasedAliasAnalysisPass()); >>>>>> > >>>>>> > PM.add(createScopedNoAliasAAPass()); >>>>>> > >>>>>> > PM.add(createBasicAliasAnalysisPass()); >>>>>> > >>>>>> > >>>>>> > >>>>>> > So are you recommending changing to the sequence below instead? >>>>>> > >>>>>> > >>>>>> > Not quite: >>>>>> > >>>>>> > >>>>>> > addPass(createTypeBasedAliasAnalysisPass()); >>>>>> > >>>>>> > addPass(createScopedNoAliasAAPass()); >>>>>> > You want it here. >>>>>> > >>>>>> > addPass(createBasicAliasAnalysisPass()); >>>>>> > >>>>>> > if (UseCFLAA) >>>>>> > >>>>>> > addPass(createCFLAliasAnalysisPass()); >>>>>> > >>>>>> > >>>>>> > The way to think about this is as follows: >>>>>> > >>>>>> > >>>>>> > if TBAA or ScopedNoAlias *can* return results, you want them to. They >>>>>> > are reflecting a-priori knowledge of aliasing. >>>>>> > >>>>>> > >>>>>> > if both of those say "maybe", you want to ask CFL. If CFL says "maybe" >>>>>> > you want to ask "BasicAA". >>>>>> >>>>>> I could very well be misunderstanding something, but I recall that we add BasicAA last so that it will be queried first. We do this because of our BasicAA delegation hack: We want a MustAlias from BasicAA to override a NoAlias from TBAA, so that we can catch cases of basic local type punning. Thus, assuming CFL delegates properly, I fail to see how switching its order w.r.t. the metadata-based analysis changes anything. >>>>>> >>>>>> -Hal >>>>>> >>>>>> > >>>>>> > >>>>>> > It's possible that you could as a compile-time hack flip the last two >>>>>> > to your suggested ordering, but a) it should really only be a compile >>>>>> > time hack, and b) we haven't really tested that BasicAA actually >>>>>> > delegates rather than directly returning "maybe". >>>>>> > >>>>>> > >>>>>> > Thanks, >>>>>> > >>>>>> > Ana. >>>>>> > >>>>>> > >>>>>> > >>>>>> > From: Daniel Berlin [mailto: dberlin at dberlin.org ] >>>>>> > Sent: Friday, January 09, 2015 6:41 PM >>>>>> > To: George Burgess IV; Hal Finkel >>>>>> > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu; chandlerc >>>>>> > >>>>>> > >>>>>> > Subject: Re: question about enabling cfl-aa and collecting a57 numbers >>>>>> > >>>>>> > >>>>>> > Right. I would not try with -cfl-aa -basicaa as the order, it should >>>>>> > be the other way around. >>>>>> > >>>>>> > >>>>>> > If you discover performance regressions with *that*, that would be >>>>>> > highly interesting. >>>>>> > >>>>>> > >>>>>> > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV < >>>>>> > george.burgess.iv at gmail.com > wrote: >>>>>> > >>>>>> > >>>>>> > >>>>>> > Thanks for the notes and observations Ana! :) >>>>>> > >>>>>> > >> Hal, what do you think? Does it meant that cfl-aa is more >>>>>> > >> conservative (note I set the right order -cfa-aa followed by >>>>>> > >> -basicaa)? >>>>>> > Currently, yes. The implementation as it is is focused heavily on >>>>>> > being fast, and on trying to cover cases that BasicAA can’t easily >>>>>> > cover itself. If our end goal is to use this as a replacement for >>>>>> > BasicAA, there’s probably quite a few low-hanging fruit for accuracy >>>>>> > improvements, and we can start playing with sacrificing speed in >>>>>> > pursuit of greater accuracy. On the other hand, if the end goal is to >>>>>> > append this pass to the list of AA passes already being performed, >>>>>> > then I’d be interested in seeing the difference in perf between >>>>>> > `-basicaa` and `-basicaa -cfl-aa`, if any. >>>>>> > >>>>>> > WRT this issue in particular, I believe CFLAA answers conservatively >>>>>> > because: >>>>>> > A. it doesn’t take into account constant pointer offsets (i.e. it >>>>>> > considers a[0]..a[n] all as the same “address”) B. CFLAA is entirely >>>>>> > context insensitive >>>>>> > >>>>>> > IIRC, BasicAA is somewhat context sensitive, and it does take into >>>>>> > account constant pointer offsets, so it can be more accurate in this >>>>>> > case. That being said, I don’t have access to the SPEC code, so I can >>>>>> > only speculate. :) >>>>>> > >>>>>> > George >>>>>> > >>>>>> > > On Jan 9, 2015, at 4:25 PM, Hal Finkel < hfinkel at anl.gov > wrote: >>>>>> > > >>>>>> > > Ana, >>>>>> > > >>>>>> > > Thanks so much for looking into this. I'm adding some additional >>>>>> > > relevant people to the CC line... >>>>>> > > >>>>>> > > -Hal >>>>>> > > >>>>>> > > ----- Original Message ----- >>>>>> > >> From: "Ana Pazos" < apazos at codeaurora.org > >>>>>> > >> To: "Ana Pazos" < apazos at quicinc.com >, "Hal Finkel" < >>>>>> > >> hfinkel at anl.gov >, "Jiangning Liu" < Jiangning.Liu at arm.com > >>>>>> > >> Sent: Friday, January 9, 2015 3:12:04 PM >>>>>> > >> Subject: RE: question about enabling cfl-aa and collecting a57 >>>>>> > >> numbers >>>>>> > >> >>>>>> > >> Hi Hal and Jiangning, >>>>>> > >> >>>>>> > >> I started to look at the effect of cfl-aa on SPEC. >>>>>> > >> >>>>>> > >> The first observation is that I see more redundant loads not being >>>>>> > >> hoisted out of loops by LICM. >>>>>> > >> >>>>>> > >> Due to license restrictions, I cannot paste SPEC code here, but I >>>>>> > >> think you have access to it. >>>>>> > >> >>>>>> > >> I tried to simplify a top function in equake (quake.c smvp) to show >>>>>> > >> the issue. See the attached reduced test case. >>>>>> > >> >>>>>> > >> If you look at the the source code you will see something like: >>>>>> > >> for { >>>>>> > >> ...load v[i][0]... >>>>>> > >> while { >>>>>> > >> ...load v[i][0]... >>>>>> > >> store w[col][0].... >>>>>> > >> } >>>>>> > >> ...load w[i][0]... >>>>>> > >> store w[i][0]... >>>>>> > >> } >>>>>> > >> >>>>>> > >> It should be possible to avoid load &v[i][0], &v[i][1], &vi[i][2] >>>>>> > >> inside the while loop. >>>>>> > >> >>>>>> > >> In the simplified example that would be this instruction: >>>>>> > >> %21 = load double** %arrayidx8, align 8, !tbaa !5 >>>>>> > >> >>>>>> > >> And the pointer for the load instruction: >>>>>> > >> %arrayidx8 = getelementptr inbounds double** %v, i64 %idxprom >>>>>> > >> >>>>>> > >> The decision to hoist it fails at LICM.cpp:189: AliasSet &AS >>>>>> > >> CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod. >>>>>> > >> >>>>>> > >> With basicaa, only loads are in the alias set, but with cfl-aa you >>>>>> > >> find loads and stores, so isMod returns true. >>>>>> > >> >>>>>> > >> AliasSet with basicaa: >>>>>> > >> AliasSet[0x2b35aa0, 7] may alias, Ref Pointers: (double*** >>>>>> > >> %arrayidx32, 8), (double** %12, 8), (double** %arrayidx36, 8), >>>>>> > >> (double** %arrayidx8, 8), (double** %arrayidx68, 8), (double** >>>>>> > >> %arrayidx77, 8), (double** %arrayidx85, 8) >>>>>> > >> $5 = void >>>>>> > >> >>>>>> > >> AliasSet with cfa-aa: >>>>>> > >> AliasSet[0x2b37df0, 20] may alias, Mod/Ref Pointers: (i32* >>>>>> > >> %arrayidx30, 4), (double*** %arrayidx32, 8), (double** %12, 8), >>>>>> > >> (double* %13, 8), (double** %arrayidx36, 8), (double* %15, 8), >>>>>> > >> (double* %arrayidx42, 8), (double* %arrayidx45, 8), (double* >>>>>> > >> %arrayidx51, 8), (double* %arrayidx54, 8), (double** %arrayidx8, >>>>>> > >> 8), (double* %21, 8), (double** %arrayidx68, 8), (double* %23, 8), >>>>>> > >> (double* %arrayidx72, 8), (double** %arrayidx77, 8), (double* %26, >>>>>> > >> 8), (double* %arrayidx81, 8), (double** %arrayidx85, 8), (double* >>>>>> > >> %29, 8) >>>>>> > >> $8 = void >>>>>> > >> >>>>>> > >> You can reproduce it with the commands: >>>>>> > >> opt -O3 -S -cfl-aa -basicaa -licm -debug-only=licm reduce.ll -o out >>>>>> > >> >>>>>> > >> opt -O3 -S - -basicaa -licm -debug-only=licm reduce.ll -o out LICM >>>>>> > >> hoisting to while.body.lr.ph : %21 = load double** %arrayidx8, >>>>>> > >> align 8, !tbaa !5 LICM hoisting to while.body.lr.ph : %arrayidx72 >>>>>> > >> getelementptr inbounds double* %11, i64 1 LICM hoisting to >>>>>> > >> while.body.lr.ph : %arrayidx81 = getelementptr inbounds double* >>>>>> > >> %11, i64 2 >>>>>> > >> >>>>>> > >> Hal, what do you think? Does it meant that cfl-aa is more >>>>>> > >> conservative (note I set the right order -cfa-aa followed by >>>>>> > >> -basicaa)? Could it be an issue with building the AliasSet tracker? >>>>>> > >> Any pointers on how to fix this? >>>>>> > >> >>>>>> > >> Thanks, >>>>>> > >> Ana. >>>>>> > >> >>>>>> > >> >>>>>> > >> >>>>>> > >> -----Original Message----- >>>>>> > >> From: Jiangning Liu [mailto: Jiangning.Liu at arm.com ] >>>>>> > >> Sent: Wednesday, December 24, 2014 6:27 PM >>>>>> > >> To: Hal Finkel; Ana Pazos >>>>>> > >> Subject: RE: question about enabling cfl-aa and collecting a57 >>>>>> > >> numbers >>>>>> > >> >>>>>> > >> Hi, >>>>>> > >> >>>>>> > >> I saw big regressions on cortex-A57 for the following SPEC >>>>>> > >> benchmarks after using "-mllvm -use-cfl-aa ". >>>>>> > >> >>>>>> > >> spec.cpu2006.ref.462_libquantum 9.22% spec.cpu2000.ref.179_art >>>>>> > >> 5.32% >>>>>> > >> spec.cpu2000.ref.256_bzip2 4.91% >>>>>> > >> >>>>>> > >> Thanks, >>>>>> > >> -Jiangning >>>>>> > >> >>>>>> > >>> -----Original Message----- >>>>>> > >>> From: Hal Finkel [mailto: hfinkel at anl.gov ] >>>>>> > >>> Sent: Wednesday, December 24, 2014 3:56 AM >>>>>> > >>> To: Ana Pazos >>>>>> > >>> Cc: Jiangning Liu >>>>>> > >>> Subject: Re: question about enabling cfl-aa and collecting a57 >>>>>> > >>> numbers >>>>>> > >>> >>>>>> > >>> ----- Original Message ----- >>>>>> > >>>> From: "Ana Pazos" < apazos at codeaurora.org > >>>>>> > >>>> To: "Hal Finkel" < hfinkel at anl.gov >, "Jiangning Liu" >>>>>> > >>>> < Jiangning.Liu at arm.com > >>>>>> > >>>> Sent: Monday, December 22, 2014 4:25:07 PM >>>>>> > >>>> Subject: question about enabling cfl-aa and collecting a57 >>>>>> > >>>> numbers >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> Hi Hal, >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> Can you clarify to enable cfl-aa from clang we need both these >>>>>> > >>>> flags? >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>> >>>>>> > >>> Hi Ana, >>>>>> > >>> >>>>>> > >>> Thanks for following-up on this! You only need (-mllvm >>>>>> > >>> -use-cfl-aa) >>>>>> > >>> to >>>>>> > >>> use CFL AA during the main optimization pipeline, and just this is >>>>>> > >>> enough to reproduce the performance regressions as far as I know. >>>>>> > >>> You >>>>>> > >>> can also use CFL AA during code generation (-mllvm >>>>>> > >>> -use-cfl-aa-in-codegen), which matters only if your using AA at >>>>>> > >>> all for code generation (which AArch64 does only for the >>>>>> > >>> Cortex/A53). >>>>>> > >>> >>>>>> > >>> -Hal >>>>>> > >>> >>>>>> > >>>> >>>>>> > >>>> -mllvm -use-cfl-aa >>>>>> > >>>> >>>>>> > >>>> -mllvm -cfl-aa-in-codegen >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> Hi Jiangning, >>>>>> > >>>> >>>>>> > >>>> Did you collect a57 perf results for SPEC 2000 and 2006 enabling >>>>>> > >>>> these two flags? >>>>>> > >>>> >>>>>> > >>>> Did you notice any correctness and performance regression? >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> Thanks, >>>>>> > >>>> >>>>>> > >>>> Ana. >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> Ana Pazos >>>>>> > >>>> Qualcomm Innovation Center, Inc. >>>>>> > >>>> The Qualcomm Innovation Center, Inc. is a member of the Code >>>>>> > >>>> Aurora Forum, a Linux Foundation Collaborative Project. >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>>> >>>>>> > >>> >>>>>> > >>> -- >>>>>> > >>> Hal Finkel >>>>>> > >>> Assistant Computational Scientist >>>>>> > >>> Leadership Computing Facility >>>>>> > >>> Argonne National Laboratory >>>>>> > >> >>>>>> > >> >>>>>> > >> -- IMPORTANT NOTICE: The contents of this email and any attachments >>>>>> > >> are confidential and may also be privileged. If you are not the >>>>>> > >> intended recipient, please notify the sender immediately and do not >>>>>> > >> disclose the contents to any other person, use it for any purpose, >>>>>> > >> or store or copy the information in any medium. Thank you. >>>>>> > >> >>>>>> > >> ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1 >>>>>> > >> 9NJ, Registered in England & Wales, Company No: 2557590 ARM >>>>>> > >> Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1 >>>>>> > >> 9NJ, Registered in England & Wales, Company No: 2548782 >>>>>> > >> >>>>>> > > >>>>>> > > -- >>>>>> > > Hal Finkel >>>>>> > > Assistant Computational Scientist >>>>>> > > Leadership Computing Facility >>>>>> > > Argonne National Laboratory >>>>>> >>>>>> -- >>>>>> Hal Finkel >>>>>> Assistant Computational Scientist >>>>>> Leadership Computing Facility >>>>>> Argonne National Laboratory >>> >>> >>> _______________________________________________ >>> LLVM Developers mailing list >>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > <cflaafix.diff>-------------- next part -------------- An HTML attachment was scrubbed... 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Daniel Berlin
2015-Jan-14 20:23 UTC
[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Yeah, it was this (which you mentioned) *and* the fact that if query()
returned MayAlias in alias(), we did not delegate there either.
I had assumed the PartialAlias was intentional because you have a test for
it explicitly test/Analysis/CFLAliasAnalysis/must-and-partial.ll
Note that i actually agree that those cases in must-and-partial are
partialalias/etc.
You may even be able to prove them in CFL-AA if you had "never touched
arguments in this set", "never touched globals in this set", and
all
variables in the set were SSA variables.
But otherwise, ...
For example, you are also adding edges between arguments.
So in this case:
; Function Attrs: nounwind
define void @test(double*** nocapture readonly %A, i32* nocapture readonly
%Index) #0 {
entry:
br i1 undef, label %for.body.lr.ph, label %for.end
for.body.lr.ph: ; preds = %entry
br label %for.body
for.body: ; preds = %for.body, %
for.body.lr.ph
%arrayidx22 = getelementptr inbounds double* %Index, i64 2
%0 = load double* %arrayidx22, align 8, !tbaa !1
%arrayidx25 = getelementptr inbounds double* %A, i64 2
%1 = load double* %arrayidx25, align 8, !tbaa !1
%mul26 = fmul double %0, %1
br i1 undef, label %for.end, label %for.body
for.end: ; preds = %for.body,
%entry
ret void
}
It believes %Index PartialAlias %A, which is clearly wrong :)
If you have any time to do the work necessary to make partialalias work for
the other cases, let me know.
Otherwise, i'd say we disable it and XFAIL the testcases for now.
On Wed, Jan 14, 2015 at 10:30 AM, George Burgess IV <
george.burgess.iv at gmail.com> wrote:
> Inline
>
> - George
>
> On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org>
wrote:
>
>
>
> On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at
google.com>
> wrote:
>
>> On 13 January 2015 at 22:11, Daniel Berlin <dberlin at
dberlin.org> wrote:
>>
>>> This is caused by CFLAA returning PartialAlias for a query that
BasicAA
>>> can prove is NoAlias.
>>>
>>
>> One of them is wrong. Which one?
>>
>
>>
> CFL-AA.
>
> Right now it checks whether two things come out to be the same stratified
> info and have the same index, and if so, returns PartialAlias. This is
> wrong, because it never knows why two things got unified, only that they
> did.
>
> Therefore, the current check it does where it returns PartialAlias seems
> wrong.
>
> George, why does it return PartialAlias?
>
> It shouldn't -- unless I misinterpreted an earlier discussion, I was
under
> the impression that we were testing with that line updated to return
> MayAlias. We do need to update that in LLVM though; thanks for the fix.
>
> IMHO, you can't ever prove any kind of must-alias or partialalias info
> with CFL-AA, unless i'm missing something.
>
> Agreed.
>
>
>
>> I'm not sure from your description that this is a chaining issue.
>> PartialAlias doesn't chain and isn't supposed to, it's a
final answer just
>> like NoAlias and MustAlias are. You return partial alias when you have
>> proven that the accesses are overlapping, meaning that it is proven to
>> neither be no alias nor must alias.
>>
> Well, it's still chaining wrong, because it did not change on MayAlias,
> but you are right that it is not the issue i identified :)
>
> Anyway, updated patch attached.
>
>
>> Nick
>>
>> I've attached a patch that fixes it, but truthfully, chaining seems
>>> somewhat badly designed IMHO.
>>>
>>> It should let you pass along the result you've gotten so far,
so that
>>> you don't have CFLAA get PartialAlias, chain, and have BasicAA
get MayAlias.
>>>
>>> (IE it should let it work strictly down the hierarchy).
>>>
>>> Note that this patch breaks the CFLAliasAnalysis tests because it
no
>>> longer returns PartialAlias in some cases it is expecting it.
>>>
>>> I don't have time ATM to fix chaining completely, so if someone
wants to
>>> shepherd this patch through, that would be appreciated.
>>>
>>>
>>>
>>>
>>>
>>> On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at
dberlin.org>
>>> wrote:
>>>
>>>> Can you send me actual LLVM IR or a preprocessed source from
using -E?
>>>> I don't have a machine handy that has headers that target
that arch.
>>>>
>>>>
>>>> On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at
dberlin.org>
>>>> wrote:
>>>>
>>>>> Anything other than noalias or mustalias should be getting
passed down
>>>>> the stack, so either that is not happening or CFL aa is
giving better
>>>>> answers and something does worse with those better answers.
I'll take a
>>>>> look this evening
>>>>> On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos
at codeaurora.org> wrote:
>>>>>
>>>>>> Hi folks,
>>>>>>
>>>>>> Moving the discussion to llvm.dev.
>>>>>>
>>>>>> None of the changes we talked earlier help.
>>>>>>
>>>>>> Find attached the C source code that you can use to
reproduce the
>>>>>> issue.
>>>>>>
>>>>>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57
-Ofast
>>>>>> -fno-math-errno test.c -S -o test.s -mllvm
-debug-only=licm
>>>>>> LICM hoisting to while.body.lr.ph: %21 = load
double** %arrayidx8,
>>>>>> align 8, !tbaa !5
>>>>>> LICM hoisting to while.body.lr.ph: %arrayidx72 =
getelementptr
>>>>>> inbounds double* %11, i64 1
>>>>>> LICM hoisting to while.body.lr.ph: %arrayidx81 =
getelementptr
>>>>>> inbounds double* %11, i64 2
>>>>>> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32*
%Index to i64
>>>>>>
>>>>>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57
-Ofast
>>>>>> -fno-math-errno test.c -S -o test-cflaa.s -mllvm
-use-cfl-aa -mllvm
>>>>>> -debug-only=licm
>>>>>> LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32*
%Index to i64
>>>>>>
>>>>>> Why CFL AA cannot allow hoisting this out: %21 = load
double**
>>>>>> %arrayidx8, align 8, !tbaa !5
>>>>>>
>>>>>> Which leads to this extra load in assembly code: ldr
x14, [x4, x9,
>>>>>> lsl #3]
>>>>>>
>>>>>> Thanks!
>>>>>> Ana.
>>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Hal Finkel [mailto:hfinkel at anl.gov]
>>>>>> Sent: Tuesday, January 13, 2015 12:51 AM
>>>>>> To: Chandler Carruth
>>>>>> Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel
Berlin; George
>>>>>> Burgess IV
>>>>>> Subject: Re: question about enabling cfl-aa and
collecting a57 numbers
>>>>>>
>>>>>> ----- Original Message -----
>>>>>> > From: "Chandler Carruth" <chandlerc
at gmail.com>
>>>>>> > To: "Ana Pazos" <apazos at
codeaurora.org>, "Daniel Berlin" <
>>>>>> dberlin at dberlin.org>, "George Burgess
IV"
>>>>>> > <george.burgess.iv at gmail.com>, "Hal
Finkel" <hfinkel at anl.gov>
>>>>>> > Cc: "Jiangning Liu" <Jiangning.Liu at
arm.com>, "Ana Pazos"
>>>>>> > <apazos at quicinc.com>
>>>>>> > Sent: Monday, January 12, 2015 6:43:44 PM
>>>>>> > Subject: Re: question about enabling cfl-aa and
collecting a57
>>>>>> numbers
>>>>>> >
>>>>>> > Not quite:
>>>>>> >
>>>>>> >
>>>>>> > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos <
apazos at codeaurora.org
>>>>>> >
>>>>>> > wrote:
>>>>>> >
>>>>>> > Thanks George and Daniel,
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > If the recommended order is “–basicaa –cfla-aa” it
means we should
>>>>>> fix
>>>>>> > the trunk code that processes the flags -mllvm
use-cfl-aa-in-codegen
>>>>>> > and –mllvm use-cfl-aa, right?
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > In Transforms/IPO/PassManagerBuilder.cpp and
CodeGen/Passes.cpp we
>>>>>> see
>>>>>> > this sequence:
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > if (UseCFLAA)
>>>>>> >
>>>>>> > PM.add(createCFLAliasAnalysisPass());
>>>>>> >
>>>>>> > PM.add(createTypeBasedAliasAnalysisPass());
>>>>>> >
>>>>>> > PM.add(createScopedNoAliasAAPass());
>>>>>> >
>>>>>> > PM.add(createBasicAliasAnalysisPass());
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > So are you recommending changing to the sequence
below instead?
>>>>>> >
>>>>>> >
>>>>>> > Not quite:
>>>>>> >
>>>>>> >
>>>>>> > addPass(createTypeBasedAliasAnalysisPass());
>>>>>> >
>>>>>> > addPass(createScopedNoAliasAAPass());
>>>>>> > You want it here.
>>>>>> >
>>>>>> > addPass(createBasicAliasAnalysisPass());
>>>>>> >
>>>>>> > if (UseCFLAA)
>>>>>> >
>>>>>> > addPass(createCFLAliasAnalysisPass());
>>>>>> >
>>>>>> >
>>>>>> > The way to think about this is as follows:
>>>>>> >
>>>>>> >
>>>>>> > if TBAA or ScopedNoAlias *can* return results, you
want them to.
>>>>>> They
>>>>>> > are reflecting a-priori knowledge of aliasing.
>>>>>> >
>>>>>> >
>>>>>> > if both of those say "maybe", you want
to ask CFL. If CFL says
>>>>>> "maybe"
>>>>>> > you want to ask "BasicAA".
>>>>>>
>>>>>> I could very well be misunderstanding something, but I
recall that we
>>>>>> add BasicAA last so that it will be queried first. We
do this because of
>>>>>> our BasicAA delegation hack: We want a MustAlias from
BasicAA to override a
>>>>>> NoAlias from TBAA, so that we can catch cases of basic
local type punning.
>>>>>> Thus, assuming CFL delegates properly, I fail to see
how switching its
>>>>>> order w.r.t. the metadata-based analysis changes
anything.
>>>>>>
>>>>>> -Hal
>>>>>>
>>>>>> >
>>>>>> >
>>>>>> > It's possible that you could as a compile-time
hack flip the last
>>>>>> two
>>>>>> > to your suggested ordering, but a) it should
really only be a
>>>>>> compile
>>>>>> > time hack, and b) we haven't really tested
that BasicAA actually
>>>>>> > delegates rather than directly returning
"maybe".
>>>>>> >
>>>>>> >
>>>>>> > Thanks,
>>>>>> >
>>>>>> > Ana.
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > From: Daniel Berlin [mailto: dberlin at
dberlin.org ]
>>>>>> > Sent: Friday, January 09, 2015 6:41 PM
>>>>>> > To: George Burgess IV; Hal Finkel
>>>>>> > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu;
chandlerc
>>>>>> >
>>>>>> >
>>>>>> > Subject: Re: question about enabling cfl-aa and
collecting a57
>>>>>> numbers
>>>>>> >
>>>>>> >
>>>>>> > Right. I would not try with -cfl-aa -basicaa as
the order, it should
>>>>>> > be the other way around.
>>>>>> >
>>>>>> >
>>>>>> > If you discover performance regressions with
*that*, that would be
>>>>>> > highly interesting.
>>>>>> >
>>>>>> >
>>>>>> > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV
<
>>>>>> > george.burgess.iv at gmail.com > wrote:
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > Thanks for the notes and observations Ana! :)
>>>>>> >
>>>>>> > >> Hal, what do you think? Does it meant
that cfl-aa is more
>>>>>> > >> conservative (note I set the right order
-cfa-aa followed by
>>>>>> > >> -basicaa)?
>>>>>> > Currently, yes. The implementation as it is is
focused heavily on
>>>>>> > being fast, and on trying to cover cases that
BasicAA can’t easily
>>>>>> > cover itself. If our end goal is to use this as a
replacement for
>>>>>> > BasicAA, there’s probably quite a few low-hanging
fruit for accuracy
>>>>>> > improvements, and we can start playing with
sacrificing speed in
>>>>>> > pursuit of greater accuracy. On the other hand, if
the end goal is
>>>>>> to
>>>>>> > append this pass to the list of AA passes already
being performed,
>>>>>> > then I’d be interested in seeing the difference in
perf between
>>>>>> > `-basicaa` and `-basicaa -cfl-aa`, if any.
>>>>>> >
>>>>>> > WRT this issue in particular, I believe CFLAA
answers conservatively
>>>>>> > because:
>>>>>> > A. it doesn’t take into account constant pointer
offsets (i.e. it
>>>>>> > considers a[0]..a[n] all as the same “address”) B.
CFLAA is entirely
>>>>>> > context insensitive
>>>>>> >
>>>>>> > IIRC, BasicAA is somewhat context sensitive, and
it does take into
>>>>>> > account constant pointer offsets, so it can be
more accurate in this
>>>>>> > case. That being said, I don’t have access to the
SPEC code, so I
>>>>>> can
>>>>>> > only speculate. :)
>>>>>> >
>>>>>> > George
>>>>>> >
>>>>>> > > On Jan 9, 2015, at 4:25 PM, Hal Finkel <
hfinkel at anl.gov > wrote:
>>>>>> > >
>>>>>> > > Ana,
>>>>>> > >
>>>>>> > > Thanks so much for looking into this. I'm
adding some additional
>>>>>> > > relevant people to the CC line...
>>>>>> > >
>>>>>> > > -Hal
>>>>>> > >
>>>>>> > > ----- Original Message -----
>>>>>> > >> From: "Ana Pazos" < apazos
at codeaurora.org >
>>>>>> > >> To: "Ana Pazos" < apazos at
quicinc.com >, "Hal Finkel" <
>>>>>> > >> hfinkel at anl.gov >, "Jiangning
Liu" < Jiangning.Liu at arm.com >
>>>>>> > >> Sent: Friday, January 9, 2015 3:12:04 PM
>>>>>> > >> Subject: RE: question about enabling
cfl-aa and collecting a57
>>>>>> > >> numbers
>>>>>> > >>
>>>>>> > >> Hi Hal and Jiangning,
>>>>>> > >>
>>>>>> > >> I started to look at the effect of cfl-aa
on SPEC.
>>>>>> > >>
>>>>>> > >> The first observation is that I see more
redundant loads not
>>>>>> being
>>>>>> > >> hoisted out of loops by LICM.
>>>>>> > >>
>>>>>> > >> Due to license restrictions, I cannot
paste SPEC code here, but I
>>>>>> > >> think you have access to it.
>>>>>> > >>
>>>>>> > >> I tried to simplify a top function in
equake (quake.c smvp) to
>>>>>> show
>>>>>> > >> the issue. See the attached reduced test
case.
>>>>>> > >>
>>>>>> > >> If you look at the the source code you
will see something like:
>>>>>> > >> for {
>>>>>> > >> ...load v[i][0]...
>>>>>> > >> while {
>>>>>> > >> ...load v[i][0]...
>>>>>> > >> store w[col][0]....
>>>>>> > >> }
>>>>>> > >> ...load w[i][0]...
>>>>>> > >> store w[i][0]...
>>>>>> > >> }
>>>>>> > >>
>>>>>> > >> It should be possible to avoid load
&v[i][0], &v[i][1], &vi[i][2]
>>>>>> > >> inside the while loop.
>>>>>> > >>
>>>>>> > >> In the simplified example that would be
this instruction:
>>>>>> > >> %21 = load double** %arrayidx8, align 8,
!tbaa !5
>>>>>> > >>
>>>>>> > >> And the pointer for the load instruction:
>>>>>> > >> %arrayidx8 = getelementptr inbounds
double** %v, i64 %idxprom
>>>>>> > >>
>>>>>> > >> The decision to hoist it fails at
LICM.cpp:189: AliasSet &AS >>>>>> > >>
CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod.
>>>>>> > >>
>>>>>> > >> With basicaa, only loads are in the alias
set, but with cfl-aa
>>>>>> you
>>>>>> > >> find loads and stores, so isMod returns
true.
>>>>>> > >>
>>>>>> > >> AliasSet with basicaa:
>>>>>> > >> AliasSet[0x2b35aa0, 7] may alias, Ref
Pointers: (double***
>>>>>> > >> %arrayidx32, 8), (double** %12, 8),
(double** %arrayidx36, 8),
>>>>>> > >> (double** %arrayidx8, 8), (double**
%arrayidx68, 8), (double**
>>>>>> > >> %arrayidx77, 8), (double** %arrayidx85,
8)
>>>>>> > >> $5 = void
>>>>>> > >>
>>>>>> > >> AliasSet with cfa-aa:
>>>>>> > >> AliasSet[0x2b37df0, 20] may alias,
Mod/Ref Pointers: (i32*
>>>>>> > >> %arrayidx30, 4), (double*** %arrayidx32,
8), (double** %12, 8),
>>>>>> > >> (double* %13, 8), (double** %arrayidx36,
8), (double* %15, 8),
>>>>>> > >> (double* %arrayidx42, 8), (double*
%arrayidx45, 8), (double*
>>>>>> > >> %arrayidx51, 8), (double* %arrayidx54,
8), (double** %arrayidx8,
>>>>>> > >> 8), (double* %21, 8), (double**
%arrayidx68, 8), (double* %23,
>>>>>> 8),
>>>>>> > >> (double* %arrayidx72, 8), (double**
%arrayidx77, 8), (double*
>>>>>> %26,
>>>>>> > >> 8), (double* %arrayidx81, 8), (double**
%arrayidx85, 8), (double*
>>>>>> > >> %29, 8)
>>>>>> > >> $8 = void
>>>>>> > >>
>>>>>> > >> You can reproduce it with the commands:
>>>>>> > >> opt -O3 -S -cfl-aa -basicaa -licm
-debug-only=licm reduce.ll -o
>>>>>> out
>>>>>> > >>
>>>>>> > >> opt -O3 -S - -basicaa -licm
-debug-only=licm reduce.ll -o out
>>>>>> LICM
>>>>>> > >> hoisting to while.body.lr.ph : %21 = load
double** %arrayidx8,
>>>>>> > >> align 8, !tbaa !5 LICM hoisting to
while.body.lr.ph :
>>>>>> %arrayidx72 >>>>>> > >>
getelementptr inbounds double* %11, i64 1 LICM hoisting to
>>>>>> > >> while.body.lr.ph : %arrayidx81 =
getelementptr inbounds double*
>>>>>> > >> %11, i64 2
>>>>>> > >>
>>>>>> > >> Hal, what do you think? Does it meant
that cfl-aa is more
>>>>>> > >> conservative (note I set the right order
-cfa-aa followed by
>>>>>> > >> -basicaa)? Could it be an issue with
building the AliasSet
>>>>>> tracker?
>>>>>> > >> Any pointers on how to fix this?
>>>>>> > >>
>>>>>> > >> Thanks,
>>>>>> > >> Ana.
>>>>>> > >>
>>>>>> > >>
>>>>>> > >>
>>>>>> > >> -----Original Message-----
>>>>>> > >> From: Jiangning Liu [mailto:
Jiangning.Liu at arm.com ]
>>>>>> > >> Sent: Wednesday, December 24, 2014 6:27
PM
>>>>>> > >> To: Hal Finkel; Ana Pazos
>>>>>> > >> Subject: RE: question about enabling
cfl-aa and collecting a57
>>>>>> > >> numbers
>>>>>> > >>
>>>>>> > >> Hi,
>>>>>> > >>
>>>>>> > >> I saw big regressions on cortex-A57 for
the following SPEC
>>>>>> > >> benchmarks after using "-mllvm
-use-cfl-aa ".
>>>>>> > >>
>>>>>> > >> spec.cpu2006.ref.462_libquantum 9.22%
spec.cpu2000.ref.179_art
>>>>>> > >> 5.32%
>>>>>> > >> spec.cpu2000.ref.256_bzip2 4.91%
>>>>>> > >>
>>>>>> > >> Thanks,
>>>>>> > >> -Jiangning
>>>>>> > >>
>>>>>> > >>> -----Original Message-----
>>>>>> > >>> From: Hal Finkel [mailto: hfinkel at
anl.gov ]
>>>>>> > >>> Sent: Wednesday, December 24, 2014
3:56 AM
>>>>>> > >>> To: Ana Pazos
>>>>>> > >>> Cc: Jiangning Liu
>>>>>> > >>> Subject: Re: question about enabling
cfl-aa and collecting a57
>>>>>> > >>> numbers
>>>>>> > >>>
>>>>>> > >>> ----- Original Message -----
>>>>>> > >>>> From: "Ana Pazos" <
apazos at codeaurora.org >
>>>>>> > >>>> To: "Hal Finkel" <
hfinkel at anl.gov >, "Jiangning Liu"
>>>>>> > >>>> < Jiangning.Liu at arm.com
>
>>>>>> > >>>> Sent: Monday, December 22, 2014
4:25:07 PM
>>>>>> > >>>> Subject: question about enabling
cfl-aa and collecting a57
>>>>>> > >>>> numbers
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>> Hi Hal,
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>> Can you clarify to enable cfl-aa
from clang we need both these
>>>>>> > >>>> flags?
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>
>>>>>> > >>> Hi Ana,
>>>>>> > >>>
>>>>>> > >>> Thanks for following-up on this! You
only need (-mllvm
>>>>>> > >>> -use-cfl-aa)
>>>>>> > >>> to
>>>>>> > >>> use CFL AA during the main
optimization pipeline, and just this
>>>>>> is
>>>>>> > >>> enough to reproduce the performance
regressions as far as I
>>>>>> know.
>>>>>> > >>> You
>>>>>> > >>> can also use CFL AA during code
generation (-mllvm
>>>>>> > >>> -use-cfl-aa-in-codegen), which
matters only if your using AA at
>>>>>> > >>> all for code generation (which
AArch64 does only for the
>>>>>> > >>> Cortex/A53).
>>>>>> > >>>
>>>>>> > >>> -Hal
>>>>>> > >>>
>>>>>> > >>>>
>>>>>> > >>>> -mllvm -use-cfl-aa
>>>>>> > >>>>
>>>>>> > >>>> -mllvm -cfl-aa-in-codegen
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>> Hi Jiangning,
>>>>>> > >>>>
>>>>>> > >>>> Did you collect a57 perf results
for SPEC 2000 and 2006
>>>>>> enabling
>>>>>> > >>>> these two flags?
>>>>>> > >>>>
>>>>>> > >>>> Did you notice any correctness
and performance regression?
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>> Thanks,
>>>>>> > >>>>
>>>>>> > >>>> Ana.
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>> Ana Pazos
>>>>>> > >>>> Qualcomm Innovation Center, Inc.
>>>>>> > >>>> The Qualcomm Innovation Center,
Inc. is a member of the Code
>>>>>> > >>>> Aurora Forum, a Linux Foundation
Collaborative Project.
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>>
>>>>>> > >>>
>>>>>> > >>> --
>>>>>> > >>> Hal Finkel
>>>>>> > >>> Assistant Computational Scientist
>>>>>> > >>> Leadership Computing Facility
>>>>>> > >>> Argonne National Laboratory
>>>>>> > >>
>>>>>> > >>
>>>>>> > >> -- IMPORTANT NOTICE: The contents of this
email and any
>>>>>> attachments
>>>>>> > >> are confidential and may also be
privileged. If you are not the
>>>>>> > >> intended recipient, please notify the
sender immediately and do
>>>>>> not
>>>>>> > >> disclose the contents to any other
person, use it for any
>>>>>> purpose,
>>>>>> > >> or store or copy the information in any
medium. Thank you.
>>>>>> > >>
>>>>>> > >> ARM Limited, Registered office 110
Fulbourn Road, Cambridge CB1
>>>>>> > >> 9NJ, Registered in England & Wales,
Company No: 2557590 ARM
>>>>>> > >> Holdings plc, Registered office 110
Fulbourn Road, Cambridge CB1
>>>>>> > >> 9NJ, Registered in England & Wales,
Company No: 2548782
>>>>>> > >>
>>>>>> > >
>>>>>> > > --
>>>>>> > > Hal Finkel
>>>>>> > > Assistant Computational Scientist
>>>>>> > > Leadership Computing Facility
>>>>>> > > Argonne National Laboratory
>>>>>>
>>>>>> --
>>>>>> Hal Finkel
>>>>>> Assistant Computational Scientist
>>>>>> Leadership Computing Facility
>>>>>> Argonne National Laboratory
>>>>>>
>>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
> <cflaafix.diff>
>
>
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Ana Pazos
2015-Jan-14 20:32 UTC
[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Daniel, your patch does not apply cleanly. Are you on the tip? The code I see there is no line if (QueryResult == MayAlias|| QueryResult == PartialAlias) to be removed. Thanks, Ana. From: George Burgess IV [mailto:george.burgess.iv at gmail.com] Sent: Wednesday, January 14, 2015 10:31 AM To: Daniel Berlin Cc: Nick Lewycky; Ana Pazos; Jiangning Liu; LLVM Developers Mailing List Subject: Re: [LLVMdev] question about enabling cfl-aa and collecting a57 numbers Inline - George On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org> > wrote: On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at google.com <mailto:nlewycky at google.com> > wrote: On 13 January 2015 at 22:11, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org> > wrote: This is caused by CFLAA returning PartialAlias for a query that BasicAA can prove is NoAlias. One of them is wrong. Which one? CFL-AA. Right now it checks whether two things come out to be the same stratified info and have the same index, and if so, returns PartialAlias. This is wrong, because it never knows why two things got unified, only that they did. Therefore, the current check it does where it returns PartialAlias seems wrong. George, why does it return PartialAlias? It shouldn't -- unless I misinterpreted an earlier discussion, I was under the impression that we were testing with that line updated to return MayAlias. We do need to update that in LLVM though; thanks for the fix. IMHO, you can't ever prove any kind of must-alias or partialalias info with CFL-AA, unless i'm missing something. Agreed. I'm not sure from your description that this is a chaining issue. PartialAlias doesn't chain and isn't supposed to, it's a final answer just like NoAlias and MustAlias are. You return partial alias when you have proven that the accesses are overlapping, meaning that it is proven to neither be no alias nor must alias. Well, it's still chaining wrong, because it did not change on MayAlias, but you are right that it is not the issue i identified :) Anyway, updated patch attached. Nick I've attached a patch that fixes it, but truthfully, chaining seems somewhat badly designed IMHO. It should let you pass along the result you've gotten so far, so that you don't have CFLAA get PartialAlias, chain, and have BasicAA get MayAlias. (IE it should let it work strictly down the hierarchy). Note that this patch breaks the CFLAliasAnalysis tests because it no longer returns PartialAlias in some cases it is expecting it. I don't have time ATM to fix chaining completely, so if someone wants to shepherd this patch through, that would be appreciated. On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org> > wrote: Can you send me actual LLVM IR or a preprocessed source from using -E? I don't have a machine handy that has headers that target that arch. On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at dberlin.org <mailto:dberlin at dberlin.org> > wrote: Anything other than noalias or mustalias should be getting passed down the stack, so either that is not happening or CFL aa is giving better answers and something does worse with those better answers. I'll take a look this evening On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos at codeaurora.org <mailto:apazos at codeaurora.org> > wrote: Hi folks, Moving the discussion to llvm.dev. None of the changes we talked earlier help. Find attached the C source code that you can use to reproduce the issue. clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast -fno-math-errno test.c -S -o test.s -mllvm -debug-only=licm LICM hoisting to while.body.lr.ph <http://while.body.lr.ph> : %21 = load double** %arrayidx8, align 8, !tbaa !5 LICM hoisting to while.body.lr.ph <http://while.body.lr.ph> : %arrayidx72 = getelementptr inbounds double* %11, i64 1 LICM hoisting to while.body.lr.ph <http://while.body.lr.ph> : %arrayidx81 = getelementptr inbounds double* %11, i64 2 LICM hoisting to for.body.lr.ph <http://for.body.lr.ph> : %2 = ptrtoint i32* %Index to i64 clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast -fno-math-errno test.c -S -o test-cflaa.s -mllvm -use-cfl-aa -mllvm -debug-only=licm LICM hoisting to for.body.lr.ph <http://for.body.lr.ph> : %2 = ptrtoint i32* %Index to i64 Why CFL AA cannot allow hoisting this out: %21 = load double** %arrayidx8, align 8, !tbaa !5 Which leads to this extra load in assembly code: ldr x14, [x4, x9, lsl #3] Thanks! Ana. -----Original Message----- From: Hal Finkel [mailto:hfinkel at anl.gov <mailto:hfinkel at anl.gov> ] Sent: Tuesday, January 13, 2015 12:51 AM To: Chandler Carruth Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel Berlin; George Burgess IV Subject: Re: question about enabling cfl-aa and collecting a57 numbers ----- Original Message -----> From: "Chandler Carruth" <chandlerc at gmail.com <mailto:chandlerc at gmail.com> > > To: "Ana Pazos" <apazos at codeaurora.org <mailto:apazos at codeaurora.org> >, "Daniel Berlin" <dberlin at dberlin.org <mailto:dberlin at dberlin.org> >, "George Burgess IV" > <george.burgess.iv at gmail.com <mailto:george.burgess.iv at gmail.com> >, "Hal Finkel" <hfinkel at anl.gov <mailto:hfinkel at anl.gov> > > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> >, "Ana Pazos" > <apazos at quicinc.com <mailto:apazos at quicinc.com> > > Sent: Monday, January 12, 2015 6:43:44 PM > Subject: Re: question about enabling cfl-aa and collecting a57 numbers > > Not quite: > > > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos < apazos at codeaurora.org <mailto:apazos at codeaurora.org> > > wrote: > > Thanks George and Daniel, > > > > If the recommended order is “–basicaa –cfla-aa” it means we should fix > the trunk code that processes the flags -mllvm use-cfl-aa-in-codegen > and –mllvm use-cfl-aa, right? > > > > In Transforms/IPO/PassManagerBuilder.cpp and CodeGen/Passes.cpp we see > this sequence: > > > > if (UseCFLAA) > > PM.add(createCFLAliasAnalysisPass()); > > PM.add(createTypeBasedAliasAnalysisPass()); > > PM.add(createScopedNoAliasAAPass()); > > PM.add(createBasicAliasAnalysisPass()); > > > > So are you recommending changing to the sequence below instead? > > > Not quite: > > > addPass(createTypeBasedAliasAnalysisPass()); > > addPass(createScopedNoAliasAAPass()); > You want it here. > > addPass(createBasicAliasAnalysisPass()); > > if (UseCFLAA) > > addPass(createCFLAliasAnalysisPass()); > > > The way to think about this is as follows: > > > if TBAA or ScopedNoAlias *can* return results, you want them to. They > are reflecting a-priori knowledge of aliasing. > > > if both of those say "maybe", you want to ask CFL. If CFL says "maybe" > you want to ask "BasicAA".I could very well be misunderstanding something, but I recall that we add BasicAA last so that it will be queried first. We do this because of our BasicAA delegation hack: We want a MustAlias from BasicAA to override a NoAlias from TBAA, so that we can catch cases of basic local type punning. Thus, assuming CFL delegates properly, I fail to see how switching its order w.r.t. the metadata-based analysis changes anything. -Hal> > > It's possible that you could as a compile-time hack flip the last two > to your suggested ordering, but a) it should really only be a compile > time hack, and b) we haven't really tested that BasicAA actually > delegates rather than directly returning "maybe". > > > Thanks, > > Ana. > > > > From: Daniel Berlin [mailto: dberlin at dberlin.org <mailto:dberlin at dberlin.org> ] > Sent: Friday, January 09, 2015 6:41 PM > To: George Burgess IV; Hal Finkel > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu; chandlerc > > > Subject: Re: question about enabling cfl-aa and collecting a57 numbers > > > Right. I would not try with -cfl-aa -basicaa as the order, it should > be the other way around. > > > If you discover performance regressions with *that*, that would be > highly interesting. > > > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV < > george.burgess.iv at gmail.com <mailto:george.burgess.iv at gmail.com> > wrote: > > > > Thanks for the notes and observations Ana! :) > > >> Hal, what do you think? Does it meant that cfl-aa is more > >> conservative (note I set the right order -cfa-aa followed by > >> -basicaa)? > Currently, yes. The implementation as it is is focused heavily on > being fast, and on trying to cover cases that BasicAA can’t easily > cover itself. If our end goal is to use this as a replacement for > BasicAA, there’s probably quite a few low-hanging fruit for accuracy > improvements, and we can start playing with sacrificing speed in > pursuit of greater accuracy. On the other hand, if the end goal is to > append this pass to the list of AA passes already being performed, > then I’d be interested in seeing the difference in perf between > `-basicaa` and `-basicaa -cfl-aa`, if any. > > WRT this issue in particular, I believe CFLAA answers conservatively > because: > A. it doesn’t take into account constant pointer offsets (i.e. it > considers a[0]..a[n] all as the same “address”) B. CFLAA is entirely > context insensitive > > IIRC, BasicAA is somewhat context sensitive, and it does take into > account constant pointer offsets, so it can be more accurate in this > case. That being said, I don’t have access to the SPEC code, so I can > only speculate. :) > > George > > > On Jan 9, 2015, at 4:25 PM, Hal Finkel < hfinkel at anl.gov <mailto:hfinkel at anl.gov> > wrote: > > > > Ana, > > > > Thanks so much for looking into this. I'm adding some additional > > relevant people to the CC line... > > > > -Hal > > > > ----- Original Message ----- > >> From: "Ana Pazos" < apazos at codeaurora.org <mailto:apazos at codeaurora.org> > > >> To: "Ana Pazos" < apazos at quicinc.com <mailto:apazos at quicinc.com> >, "Hal Finkel" < > >> hfinkel at anl.gov <mailto:hfinkel at anl.gov> >, "Jiangning Liu" < Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> > > >> Sent: Friday, January 9, 2015 3:12:04 PM > >> Subject: RE: question about enabling cfl-aa and collecting a57 > >> numbers > >> > >> Hi Hal and Jiangning, > >> > >> I started to look at the effect of cfl-aa on SPEC. > >> > >> The first observation is that I see more redundant loads not being > >> hoisted out of loops by LICM. > >> > >> Due to license restrictions, I cannot paste SPEC code here, but I > >> think you have access to it. > >> > >> I tried to simplify a top function in equake (quake.c smvp) to show > >> the issue. See the attached reduced test case. > >> > >> If you look at the the source code you will see something like: > >> for { > >> ...load v[i][0]... > >> while { > >> ...load v[i][0]... > >> store w[col][0].... > >> } > >> ...load w[i][0]... > >> store w[i][0]... > >> } > >> > >> It should be possible to avoid load &v[i][0], &v[i][1], &vi[i][2] > >> inside the while loop. > >> > >> In the simplified example that would be this instruction: > >> %21 = load double** %arrayidx8, align 8, !tbaa !5 > >> > >> And the pointer for the load instruction: > >> %arrayidx8 = getelementptr inbounds double** %v, i64 %idxprom > >> > >> The decision to hoist it fails at LICM.cpp:189: AliasSet &AS > >> CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod. > >> > >> With basicaa, only loads are in the alias set, but with cfl-aa you > >> find loads and stores, so isMod returns true. > >> > >> AliasSet with basicaa: > >> AliasSet[0x2b35aa0, 7] may alias, Ref Pointers: (double*** > >> %arrayidx32, 8), (double** %12, 8), (double** %arrayidx36, 8), > >> (double** %arrayidx8, 8), (double** %arrayidx68, 8), (double** > >> %arrayidx77, 8), (double** %arrayidx85, 8) > >> $5 = void > >> > >> AliasSet with cfa-aa: > >> AliasSet[0x2b37df0, 20] may alias, Mod/Ref Pointers: (i32* > >> %arrayidx30, 4), (double*** %arrayidx32, 8), (double** %12, 8), > >> (double* %13, 8), (double** %arrayidx36, 8), (double* %15, 8), > >> (double* %arrayidx42, 8), (double* %arrayidx45, 8), (double* > >> %arrayidx51, 8), (double* %arrayidx54, 8), (double** %arrayidx8, > >> 8), (double* %21, 8), (double** %arrayidx68, 8), (double* %23, 8), > >> (double* %arrayidx72, 8), (double** %arrayidx77, 8), (double* %26, > >> 8), (double* %arrayidx81, 8), (double** %arrayidx85, 8), (double* > >> %29, 8) > >> $8 = void > >> > >> You can reproduce it with the commands: > >> opt -O3 -S -cfl-aa -basicaa -licm -debug-only=licm reduce.ll -o out > >> > >> opt -O3 -S - -basicaa -licm -debug-only=licm reduce.ll -o out LICM > >> hoisting to while.body.lr.ph <http://while.body.lr.ph> : %21 = load double** %arrayidx8, > >> align 8, !tbaa !5 LICM hoisting to while.body.lr.ph <http://while.body.lr.ph> : %arrayidx72 > >> getelementptr inbounds double* %11, i64 1 LICM hoisting to > >> while.body.lr.ph <http://while.body.lr.ph> : %arrayidx81 = getelementptr inbounds double* > >> %11, i64 2 > >> > >> Hal, what do you think? Does it meant that cfl-aa is more > >> conservative (note I set the right order -cfa-aa followed by > >> -basicaa)? Could it be an issue with building the AliasSet tracker? > >> Any pointers on how to fix this? > >> > >> Thanks, > >> Ana. > >> > >> > >> > >> -----Original Message----- > >> From: Jiangning Liu [mailto: Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> ] > >> Sent: Wednesday, December 24, 2014 6:27 PM > >> To: Hal Finkel; Ana Pazos > >> Subject: RE: question about enabling cfl-aa and collecting a57 > >> numbers > >> > >> Hi, > >> > >> I saw big regressions on cortex-A57 for the following SPEC > >> benchmarks after using "-mllvm -use-cfl-aa ". > >> > >> spec.cpu2006.ref.462_libquantum 9.22% spec.cpu2000.ref.179_art > >> 5.32% > >> spec.cpu2000.ref.256_bzip2 4.91% > >> > >> Thanks, > >> -Jiangning > >> > >>> -----Original Message----- > >>> From: Hal Finkel [mailto: hfinkel at anl.gov <mailto:hfinkel at anl.gov> ] > >>> Sent: Wednesday, December 24, 2014 3:56 AM > >>> To: Ana Pazos > >>> Cc: Jiangning Liu > >>> Subject: Re: question about enabling cfl-aa and collecting a57 > >>> numbers > >>> > >>> ----- Original Message ----- > >>>> From: "Ana Pazos" < apazos at codeaurora.org <mailto:apazos at codeaurora.org> > > >>>> To: "Hal Finkel" < hfinkel at anl.gov <mailto:hfinkel at anl.gov> >, "Jiangning Liu" > >>>> < Jiangning.Liu at arm.com <mailto:Jiangning.Liu at arm.com> > > >>>> Sent: Monday, December 22, 2014 4:25:07 PM > >>>> Subject: question about enabling cfl-aa and collecting a57 > >>>> numbers > >>>> > >>>> > >>>> > >>>> > >>>> Hi Hal, > >>>> > >>>> > >>>> > >>>> Can you clarify to enable cfl-aa from clang we need both these > >>>> flags? > >>>> > >>>> > >>> > >>> Hi Ana, > >>> > >>> Thanks for following-up on this! You only need (-mllvm > >>> -use-cfl-aa) > >>> to > >>> use CFL AA during the main optimization pipeline, and just this is > >>> enough to reproduce the performance regressions as far as I know. > >>> You > >>> can also use CFL AA during code generation (-mllvm > >>> -use-cfl-aa-in-codegen), which matters only if your using AA at > >>> all for code generation (which AArch64 does only for the > >>> Cortex/A53). > >>> > >>> -Hal > >>> > >>>> > >>>> -mllvm -use-cfl-aa > >>>> > >>>> -mllvm -cfl-aa-in-codegen > >>>> > >>>> > >>>> > >>>> Hi Jiangning, > >>>> > >>>> Did you collect a57 perf results for SPEC 2000 and 2006 enabling > >>>> these two flags? > >>>> > >>>> Did you notice any correctness and performance regression? > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> Thanks, > >>>> > >>>> Ana. > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> Ana Pazos > >>>> Qualcomm Innovation Center, Inc. > >>>> The Qualcomm Innovation Center, Inc. is a member of the Code > >>>> Aurora Forum, a Linux Foundation Collaborative Project. > >>>> > >>>> > >>>> > >>>> > >>>> > >>>> > >>> > >>> -- > >>> Hal Finkel > >>> Assistant Computational Scientist > >>> Leadership Computing Facility > >>> Argonne National Laboratory > >> > >> > >> -- IMPORTANT NOTICE: The contents of this email and any attachments > >> are confidential and may also be privileged. If you are not the > >> intended recipient, please notify the sender immediately and do not > >> disclose the contents to any other person, use it for any purpose, > >> or store or copy the information in any medium. Thank you. > >> > >> ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1 > >> 9NJ, Registered in England & Wales, Company No: 2557590 ARM > >> Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1 > >> 9NJ, Registered in England & Wales, Company No: 2548782 > >> > > > > -- > > Hal Finkel > > Assistant Computational Scientist > > Leadership Computing Facility > > Argonne National Laboratory-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory _______________________________________________ LLVM Developers mailing list LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev <cflaafix.diff> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150114/30fe1f4d/attachment.html>
Daniel Berlin
2015-Jan-14 21:09 UTC
[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Oh, sorry, i didn't rebase it when i changed the fix, you would have had to apply the first on top of the second. Here is one against HEAD On Wed, Jan 14, 2015 at 12:32 PM, Ana Pazos <apazos at codeaurora.org> wrote:> Daniel, your patch does not apply cleanly. Are you on the tip? > > The code I see there is no line if (QueryResult == MayAlias|| QueryResult == PartialAlias) to be removed. > > Thanks, > > Ana. > > > > *From:* George Burgess IV [mailto:george.burgess.iv at gmail.com] > *Sent:* Wednesday, January 14, 2015 10:31 AM > *To:* Daniel Berlin > *Cc:* Nick Lewycky; Ana Pazos; Jiangning Liu; LLVM Developers Mailing List > *Subject:* Re: [LLVMdev] question about enabling cfl-aa and collecting > a57 numbers > > > > Inline > > - George > > > On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org> wrote: > > > > > > On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at google.com> > wrote: > > On 13 January 2015 at 22:11, Daniel Berlin <dberlin at dberlin.org> wrote: > > This is caused by CFLAA returning PartialAlias for a query that BasicAA > can prove is NoAlias. > > > > One of them is wrong. Which one? > > > > CFL-AA. > > > > Right now it checks whether two things come out to be the same stratified > info and have the same index, and if so, returns PartialAlias. This is > wrong, because it never knows why two things got unified, only that they > did. > > > > Therefore, the current check it does where it returns PartialAlias seems > wrong. > > > > George, why does it return PartialAlias? > > It shouldn't -- unless I misinterpreted an earlier discussion, I was under > the impression that we were testing with that line updated to return > MayAlias. We do need to update that in LLVM though; thanks for the fix. > > > > IMHO, you can't ever prove any kind of must-alias or partialalias info > with CFL-AA, unless i'm missing something. > > Agreed. > > > > > > I'm not sure from your description that this is a chaining issue. > PartialAlias doesn't chain and isn't supposed to, it's a final answer just > like NoAlias and MustAlias are. You return partial alias when you have > proven that the accesses are overlapping, meaning that it is proven to > neither be no alias nor must alias. > > Well, it's still chaining wrong, because it did not change on MayAlias, > but you are right that it is not the issue i identified :) > > > > Anyway, updated patch attached. > > > > > > Nick > > > > I've attached a patch that fixes it, but truthfully, chaining seems > somewhat badly designed IMHO. > > > > It should let you pass along the result you've gotten so far, so that you > don't have CFLAA get PartialAlias, chain, and have BasicAA get MayAlias. > > > > (IE it should let it work strictly down the hierarchy). > > > > Note that this patch breaks the CFLAliasAnalysis tests because it no > longer returns PartialAlias in some cases it is expecting it. > > > > I don't have time ATM to fix chaining completely, so if someone wants to > shepherd this patch through, that would be appreciated. > > > > > > > > > > > > On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at dberlin.org> > wrote: > > Can you send me actual LLVM IR or a preprocessed source from using -E? > I don't have a machine handy that has headers that target that arch. > > > > > > On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at dberlin.org> > wrote: > > Anything other than noalias or mustalias should be getting passed down the > stack, so either that is not happening or CFL aa is giving better answers > and something does worse with those better answers. I'll take a look this > evening > > On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos at codeaurora.org> wrote: > > Hi folks, > > Moving the discussion to llvm.dev. > > None of the changes we talked earlier help. > > Find attached the C source code that you can use to reproduce the issue. > > clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast > -fno-math-errno test.c -S -o test.s -mllvm -debug-only=licm > LICM hoisting to while.body.lr.ph: %21 = load double** %arrayidx8, > align 8, !tbaa !5 > LICM hoisting to while.body.lr.ph: %arrayidx72 = getelementptr inbounds > double* %11, i64 1 > LICM hoisting to while.body.lr.ph: %arrayidx81 = getelementptr inbounds > double* %11, i64 2 > LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64 > > clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast > -fno-math-errno test.c -S -o test-cflaa.s -mllvm -use-cfl-aa -mllvm > -debug-only=licm > LICM hoisting to for.body.lr.ph: %2 = ptrtoint i32* %Index to i64 > > Why CFL AA cannot allow hoisting this out: %21 = load double** > %arrayidx8, align 8, !tbaa !5 > > Which leads to this extra load in assembly code: ldr x14, [x4, x9, lsl > #3] > > Thanks! > Ana. > > -----Original Message----- > From: Hal Finkel [mailto:hfinkel at anl.gov] > Sent: Tuesday, January 13, 2015 12:51 AM > To: Chandler Carruth > Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel Berlin; George Burgess IV > Subject: Re: question about enabling cfl-aa and collecting a57 numbers > > ----- Original Message ----- > > From: "Chandler Carruth" <chandlerc at gmail.com> > > To: "Ana Pazos" <apazos at codeaurora.org>, "Daniel Berlin" < > dberlin at dberlin.org>, "George Burgess IV" > > <george.burgess.iv at gmail.com>, "Hal Finkel" <hfinkel at anl.gov> > > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com>, "Ana Pazos" > > <apazos at quicinc.com> > > Sent: Monday, January 12, 2015 6:43:44 PM > > Subject: Re: question about enabling cfl-aa and collecting a57 numbers > > > > Not quite: > > > > > > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos < apazos at codeaurora.org > > > wrote: > > > > Thanks George and Daniel, > > > > > > > > If the recommended order is “–basicaa –cfla-aa” it means we should fix > > the trunk code that processes the flags -mllvm use-cfl-aa-in-codegen > > and –mllvm use-cfl-aa, right? > > > > > > > > In Transforms/IPO/PassManagerBuilder.cpp and CodeGen/Passes.cpp we see > > this sequence: > > > > > > > > if (UseCFLAA) > > > > PM.add(createCFLAliasAnalysisPass()); > > > > PM.add(createTypeBasedAliasAnalysisPass()); > > > > PM.add(createScopedNoAliasAAPass()); > > > > PM.add(createBasicAliasAnalysisPass()); > > > > > > > > So are you recommending changing to the sequence below instead? > > > > > > Not quite: > > > > > > addPass(createTypeBasedAliasAnalysisPass()); > > > > addPass(createScopedNoAliasAAPass()); > > You want it here. > > > > addPass(createBasicAliasAnalysisPass()); > > > > if (UseCFLAA) > > > > addPass(createCFLAliasAnalysisPass()); > > > > > > The way to think about this is as follows: > > > > > > if TBAA or ScopedNoAlias *can* return results, you want them to. They > > are reflecting a-priori knowledge of aliasing. > > > > > > if both of those say "maybe", you want to ask CFL. If CFL says "maybe" > > you want to ask "BasicAA". > > I could very well be misunderstanding something, but I recall that we add > BasicAA last so that it will be queried first. We do this because of our > BasicAA delegation hack: We want a MustAlias from BasicAA to override a > NoAlias from TBAA, so that we can catch cases of basic local type punning. > Thus, assuming CFL delegates properly, I fail to see how switching its > order w.r.t. the metadata-based analysis changes anything. > > -Hal > > > > > > > It's possible that you could as a compile-time hack flip the last two > > to your suggested ordering, but a) it should really only be a compile > > time hack, and b) we haven't really tested that BasicAA actually > > delegates rather than directly returning "maybe". > > > > > > Thanks, > > > > Ana. > > > > > > > > From: Daniel Berlin [mailto: dberlin at dberlin.org ] > > Sent: Friday, January 09, 2015 6:41 PM > > To: George Burgess IV; Hal Finkel > > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu; chandlerc > > > > > > Subject: Re: question about enabling cfl-aa and collecting a57 numbers > > > > > > Right. I would not try with -cfl-aa -basicaa as the order, it should > > be the other way around. > > > > > > If you discover performance regressions with *that*, that would be > > highly interesting. > > > > > > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV < > > george.burgess.iv at gmail.com > wrote: > > > > > > > > Thanks for the notes and observations Ana! :) > > > > >> Hal, what do you think? Does it meant that cfl-aa is more > > >> conservative (note I set the right order -cfa-aa followed by > > >> -basicaa)? > > Currently, yes. The implementation as it is is focused heavily on > > being fast, and on trying to cover cases that BasicAA can’t easily > > cover itself. If our end goal is to use this as a replacement for > > BasicAA, there’s probably quite a few low-hanging fruit for accuracy > > improvements, and we can start playing with sacrificing speed in > > pursuit of greater accuracy. On the other hand, if the end goal is to > > append this pass to the list of AA passes already being performed, > > then I’d be interested in seeing the difference in perf between > > `-basicaa` and `-basicaa -cfl-aa`, if any. > > > > WRT this issue in particular, I believe CFLAA answers conservatively > > because: > > A. it doesn’t take into account constant pointer offsets (i.e. it > > considers a[0]..a[n] all as the same “address”) B. CFLAA is entirely > > context insensitive > > > > IIRC, BasicAA is somewhat context sensitive, and it does take into > > account constant pointer offsets, so it can be more accurate in this > > case. That being said, I don’t have access to the SPEC code, so I can > > only speculate. :) > > > > George > > > > > On Jan 9, 2015, at 4:25 PM, Hal Finkel < hfinkel at anl.gov > wrote: > > > > > > Ana, > > > > > > Thanks so much for looking into this. I'm adding some additional > > > relevant people to the CC line... > > > > > > -Hal > > > > > > ----- Original Message ----- > > >> From: "Ana Pazos" < apazos at codeaurora.org > > > >> To: "Ana Pazos" < apazos at quicinc.com >, "Hal Finkel" < > > >> hfinkel at anl.gov >, "Jiangning Liu" < Jiangning.Liu at arm.com > > > >> Sent: Friday, January 9, 2015 3:12:04 PM > > >> Subject: RE: question about enabling cfl-aa and collecting a57 > > >> numbers > > >> > > >> Hi Hal and Jiangning, > > >> > > >> I started to look at the effect of cfl-aa on SPEC. > > >> > > >> The first observation is that I see more redundant loads not being > > >> hoisted out of loops by LICM. > > >> > > >> Due to license restrictions, I cannot paste SPEC code here, but I > > >> think you have access to it. > > >> > > >> I tried to simplify a top function in equake (quake.c smvp) to show > > >> the issue. See the attached reduced test case. > > >> > > >> If you look at the the source code you will see something like: > > >> for { > > >> ...load v[i][0]... > > >> while { > > >> ...load v[i][0]... > > >> store w[col][0].... > > >> } > > >> ...load w[i][0]... > > >> store w[i][0]... > > >> } > > >> > > >> It should be possible to avoid load &v[i][0], &v[i][1], &vi[i][2] > > >> inside the while loop. > > >> > > >> In the simplified example that would be this instruction: > > >> %21 = load double** %arrayidx8, align 8, !tbaa !5 > > >> > > >> And the pointer for the load instruction: > > >> %arrayidx8 = getelementptr inbounds double** %v, i64 %idxprom > > >> > > >> The decision to hoist it fails at LICM.cpp:189: AliasSet &AS > > >> CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod. > > >> > > >> With basicaa, only loads are in the alias set, but with cfl-aa you > > >> find loads and stores, so isMod returns true. > > >> > > >> AliasSet with basicaa: > > >> AliasSet[0x2b35aa0, 7] may alias, Ref Pointers: (double*** > > >> %arrayidx32, 8), (double** %12, 8), (double** %arrayidx36, 8), > > >> (double** %arrayidx8, 8), (double** %arrayidx68, 8), (double** > > >> %arrayidx77, 8), (double** %arrayidx85, 8) > > >> $5 = void > > >> > > >> AliasSet with cfa-aa: > > >> AliasSet[0x2b37df0, 20] may alias, Mod/Ref Pointers: (i32* > > >> %arrayidx30, 4), (double*** %arrayidx32, 8), (double** %12, 8), > > >> (double* %13, 8), (double** %arrayidx36, 8), (double* %15, 8), > > >> (double* %arrayidx42, 8), (double* %arrayidx45, 8), (double* > > >> %arrayidx51, 8), (double* %arrayidx54, 8), (double** %arrayidx8, > > >> 8), (double* %21, 8), (double** %arrayidx68, 8), (double* %23, 8), > > >> (double* %arrayidx72, 8), (double** %arrayidx77, 8), (double* %26, > > >> 8), (double* %arrayidx81, 8), (double** %arrayidx85, 8), (double* > > >> %29, 8) > > >> $8 = void > > >> > > >> You can reproduce it with the commands: > > >> opt -O3 -S -cfl-aa -basicaa -licm -debug-only=licm reduce.ll -o out > > >> > > >> opt -O3 -S - -basicaa -licm -debug-only=licm reduce.ll -o out LICM > > >> hoisting to while.body.lr.ph : %21 = load double** %arrayidx8, > > >> align 8, !tbaa !5 LICM hoisting to while.body.lr.ph : %arrayidx72 > > >> getelementptr inbounds double* %11, i64 1 LICM hoisting to > > >> while.body.lr.ph : %arrayidx81 = getelementptr inbounds double* > > >> %11, i64 2 > > >> > > >> Hal, what do you think? Does it meant that cfl-aa is more > > >> conservative (note I set the right order -cfa-aa followed by > > >> -basicaa)? Could it be an issue with building the AliasSet tracker? > > >> Any pointers on how to fix this? > > >> > > >> Thanks, > > >> Ana. > > >> > > >> > > >> > > >> -----Original Message----- > > >> From: Jiangning Liu [mailto: Jiangning.Liu at arm.com ] > > >> Sent: Wednesday, December 24, 2014 6:27 PM > > >> To: Hal Finkel; Ana Pazos > > >> Subject: RE: question about enabling cfl-aa and collecting a57 > > >> numbers > > >> > > >> Hi, > > >> > > >> I saw big regressions on cortex-A57 for the following SPEC > > >> benchmarks after using "-mllvm -use-cfl-aa ". > > >> > > >> spec.cpu2006.ref.462_libquantum 9.22% spec.cpu2000.ref.179_art > > >> 5.32% > > >> spec.cpu2000.ref.256_bzip2 4.91% > > >> > > >> Thanks, > > >> -Jiangning > > >> > > >>> -----Original Message----- > > >>> From: Hal Finkel [mailto: hfinkel at anl.gov ] > > >>> Sent: Wednesday, December 24, 2014 3:56 AM > > >>> To: Ana Pazos > > >>> Cc: Jiangning Liu > > >>> Subject: Re: question about enabling cfl-aa and collecting a57 > > >>> numbers > > >>> > > >>> ----- Original Message ----- > > >>>> From: "Ana Pazos" < apazos at codeaurora.org > > > >>>> To: "Hal Finkel" < hfinkel at anl.gov >, "Jiangning Liu" > > >>>> < Jiangning.Liu at arm.com > > > >>>> Sent: Monday, December 22, 2014 4:25:07 PM > > >>>> Subject: question about enabling cfl-aa and collecting a57 > > >>>> numbers > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> Hi Hal, > > >>>> > > >>>> > > >>>> > > >>>> Can you clarify to enable cfl-aa from clang we need both these > > >>>> flags? > > >>>> > > >>>> > > >>> > > >>> Hi Ana, > > >>> > > >>> Thanks for following-up on this! You only need (-mllvm > > >>> -use-cfl-aa) > > >>> to > > >>> use CFL AA during the main optimization pipeline, and just this is > > >>> enough to reproduce the performance regressions as far as I know. > > >>> You > > >>> can also use CFL AA during code generation (-mllvm > > >>> -use-cfl-aa-in-codegen), which matters only if your using AA at > > >>> all for code generation (which AArch64 does only for the > > >>> Cortex/A53). > > >>> > > >>> -Hal > > >>> > > >>>> > > >>>> -mllvm -use-cfl-aa > > >>>> > > >>>> -mllvm -cfl-aa-in-codegen > > >>>> > > >>>> > > >>>> > > >>>> Hi Jiangning, > > >>>> > > >>>> Did you collect a57 perf results for SPEC 2000 and 2006 enabling > > >>>> these two flags? > > >>>> > > >>>> Did you notice any correctness and performance regression? > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> Thanks, > > >>>> > > >>>> Ana. > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> Ana Pazos > > >>>> Qualcomm Innovation Center, Inc. > > >>>> The Qualcomm Innovation Center, Inc. is a member of the Code > > >>>> Aurora Forum, a Linux Foundation Collaborative Project. > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> > > >>> > > >>> -- > > >>> Hal Finkel > > >>> Assistant Computational Scientist > > >>> Leadership Computing Facility > > >>> Argonne National Laboratory > > >> > > >> > > >> -- IMPORTANT NOTICE: The contents of this email and any attachments > > >> are confidential and may also be privileged. If you are not the > > >> intended recipient, please notify the sender immediately and do not > > >> disclose the contents to any other person, use it for any purpose, > > >> or store or copy the information in any medium. Thank you. > > >> > > >> ARM Limited, Registered office 110 Fulbourn Road, Cambridge CB1 > > >> 9NJ, Registered in England & Wales, Company No: 2557590 ARM > > >> Holdings plc, Registered office 110 Fulbourn Road, Cambridge CB1 > > >> 9NJ, Registered in England & Wales, Company No: 2548782 > > >> > > > > > > -- > > > Hal Finkel > > > Assistant Computational Scientist > > > Leadership Computing Facility > > > Argonne National Laboratory > > -- > Hal Finkel > Assistant Computational Scientist > Leadership Computing Facility > Argonne National Laboratory > > > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > > > <cflaafix.diff> > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150114/b9e2ab45/attachment.html> -------------- next part -------------- diff --git a/lib/Analysis/CFLAliasAnalysis.cpp b/lib/Analysis/CFLAliasAnalysis.cpp index 5f1b3d3..0fa8c6d 100644 --- a/lib/Analysis/CFLAliasAnalysis.cpp +++ b/lib/Analysis/CFLAliasAnalysis.cpp @@ -227,10 +227,13 @@ public: // Comparisons between global variables and other constants should be // handled by BasicAA. if (isa<Constant>(LocA.Ptr) && isa<Constant>(LocB.Ptr)) { - return MayAlias; + return AliasAnalysis::alias(LocA, LocB); } + AliasResult QueryResult = query(LocA, LocB); + if (QueryResult == MayAlias) + return AliasAnalysis::alias(LocA, LocB); - return query(LocA, LocB); + return QueryResult; } void initializePass() override { InitializeAliasAnalysis(this); } @@ -993,7 +996,7 @@ CFLAliasAnalysis::query(const AliasAnalysis::Location &LocA, auto SetB = *MaybeB; if (SetA.Index == SetB.Index) - return AliasAnalysis::PartialAlias; + return AliasAnalysis::MayAlias; auto AttrsA = Sets.getLink(SetA.Index).Attrs; auto AttrsB = Sets.getLink(SetB.Index).Attrs; diff --git a/lib/CodeGen/Passes.cpp b/lib/CodeGen/Passes.cpp index a5d3210..b2e42e7 100644 --- a/lib/CodeGen/Passes.cpp +++ b/lib/CodeGen/Passes.cpp @@ -400,10 +400,10 @@ void TargetPassConfig::addIRPasses() { // Add TypeBasedAliasAnalysis before BasicAliasAnalysis so that // BasicAliasAnalysis wins if they disagree. This is intended to help // support "obvious" type-punning idioms. - if (UseCFLAA) - addPass(createCFLAliasAnalysisPass()); addPass(createTypeBasedAliasAnalysisPass()); addPass(createScopedNoAliasAAPass()); + if (UseCFLAA) + addPass(createCFLAliasAnalysisPass()); addPass(createBasicAliasAnalysisPass()); // Before running any passes, run the verifier to determine if the input
George Burgess IV
2015-Jan-15 03:13 UTC
[LLVMdev] question about enabling cfl-aa and collecting a57 numbers
Returning PartialAlias if SetA.Index == SetB.Index was an artifact of the
assumption that we would be queried last. Clearly, this assumption is now
incorrect — I should have at least explicitly noted this in the code; it was my
mistake. :)
XFAIL for now SGTM — I start classes again next week, and am booked until then,
so I’ll see what I can get done once I reach college. (I’ll have quite a bit
more free time than last semester)
So that we’re clear, for your code, CFLAA should (now) output %A MayAlias
%Index. It will respond with this for either of the following reasons:
- The StratifiedSets themselves are {%mul26, %1, %0} “below” {%arrayidx22,
%arrayidx25, %A, %Index}.
The fmul causes the union between the (previously separate) {%1} “below"
{%arrayidx25, %A}, and {%0} “below” {%arrayidx22, %Index} sets.
- %A and %Index are both arguments, and the following code compiles/has defined
behavior with -fno-strict-aliasing:
void* ptr = new double[3];
test((double***)ptr, (int*)ptr))
// (A potential future improvement, naturally, would be to make use of the
noalias attribute on args.)
> On Jan 14, 2015, at 3:23 PM, Daniel Berlin <dberlin at dberlin.org>
wrote:
>
> Yeah, it was this (which you mentioned) *and* the fact that if query()
returned MayAlias in alias(), we did not delegate there either.
>
>
> I had assumed the PartialAlias was intentional because you have a test for
it explicitly test/Analysis/CFLAliasAnalysis/must-and-partial.ll
>
> Note that i actually agree that those cases in must-and-partial are
partialalias/etc.
> You may even be able to prove them in CFL-AA if you had "never touched
arguments in this set", "never touched globals in this set", and
all variables in the set were SSA variables.
> But otherwise, ...
>
> For example, you are also adding edges between arguments.
>
> So in this case:
>
> ; Function Attrs: nounwind
> define void @test(double*** nocapture readonly %A, i32* nocapture readonly
%Index) #0 {
> entry:
> br i1 undef, label %for.body.lr.ph <http://for.body.lr.ph/>, label
%for.end
>
> for.body.lr.ph <http://for.body.lr.ph/>:
; preds = %entry
> br label %for.body
>
> for.body: ; preds = %for.body,
%for.body.lr.ph <http://for.body.lr.ph/>
> %arrayidx22 = getelementptr inbounds double* %Index, i64 2
> %0 = load double* %arrayidx22, align 8, !tbaa !1
> %arrayidx25 = getelementptr inbounds double* %A, i64 2
> %1 = load double* %arrayidx25, align 8, !tbaa !1
> %mul26 = fmul double %0, %1
> br i1 undef, label %for.end, label %for.body
>
> for.end: ; preds = %for.body,
%entry
> ret void
> }
>
> It believes %Index PartialAlias %A, which is clearly wrong :)
>
>
> If you have any time to do the work necessary to make partialalias work for
the other cases, let me know.
> Otherwise, i'd say we disable it and XFAIL the testcases for now.
>
>
>
> On Wed, Jan 14, 2015 at 10:30 AM, George Burgess IV <george.burgess.iv
at gmail.com <mailto:george.burgess.iv at gmail.com>> wrote:
> Inline
>
> - George
>
> On Jan 14, 2015, at 10:49 AM, Daniel Berlin <dberlin at dberlin.org
<mailto:dberlin at dberlin.org>> wrote:
>
>>
>>
>> On Tue, Jan 13, 2015 at 11:26 PM, Nick Lewycky <nlewycky at
google.com <mailto:nlewycky at google.com>> wrote:
>> On 13 January 2015 at 22:11, Daniel Berlin <dberlin at dberlin.org
<mailto:dberlin at dberlin.org>> wrote:
>> This is caused by CFLAA returning PartialAlias for a query that BasicAA
can prove is NoAlias.
>>
>> One of them is wrong. Which one?
>>
>> CFL-AA.
>>
>> Right now it checks whether two things come out to be the same
stratified info and have the same index, and if so, returns PartialAlias. This
is wrong, because it never knows why two things got unified, only that they did.
>>
>> Therefore, the current check it does where it returns PartialAlias
seems wrong.
>>
>> George, why does it return PartialAlias?
> It shouldn't -- unless I misinterpreted an earlier discussion, I was
under the impression that we were testing with that line updated to return
MayAlias. We do need to update that in LLVM though; thanks for the fix.
>
>> IMHO, you can't ever prove any kind of must-alias or partialalias
info with CFL-AA, unless i'm missing something.
> Agreed.
>
>>
>> I'm not sure from your description that this is a chaining issue.
PartialAlias doesn't chain and isn't supposed to, it's a final
answer just like NoAlias and MustAlias are. You return partial alias when you
have proven that the accesses are overlapping, meaning that it is proven to
neither be no alias nor must alias.
>> Well, it's still chaining wrong, because it did not change on
MayAlias, but you are right that it is not the issue i identified :)
>>
>> Anyway, updated patch attached.
>>
>>
>> Nick
>>
>> I've attached a patch that fixes it, but truthfully, chaining seems
somewhat badly designed IMHO.
>>
>> It should let you pass along the result you've gotten so far, so
that you don't have CFLAA get PartialAlias, chain, and have BasicAA get
MayAlias.
>>
>> (IE it should let it work strictly down the hierarchy).
>>
>> Note that this patch breaks the CFLAliasAnalysis tests because it no
longer returns PartialAlias in some cases it is expecting it.
>>
>> I don't have time ATM to fix chaining completely, so if someone
wants to shepherd this patch through, that would be appreciated.
>>
>>
>>
>>
>>
>> On Tue, Jan 13, 2015 at 8:58 PM, Daniel Berlin <dberlin at
dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>> Can you send me actual LLVM IR or a preprocessed source from using -E?
>> I don't have a machine handy that has headers that target that
arch.
>>
>>
>> On Tue Jan 13 2015 at 4:33:29 PM Daniel Berlin <dberlin at
dberlin.org <mailto:dberlin at dberlin.org>> wrote:
>> Anything other than noalias or mustalias should be getting passed down
the stack, so either that is not happening or CFL aa is giving better answers
and something does worse with those better answers. I'll take a look this
evening
>>
>> On Jan 13, 2015 3:58 PM, "Ana Pazos" <apazos at
codeaurora.org <mailto:apazos at codeaurora.org>> wrote:
>> Hi folks,
>>
>> Moving the discussion to llvm.dev.
>>
>> None of the changes we talked earlier help.
>>
>> Find attached the C source code that you can use to reproduce the
issue.
>>
>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast
-fno-math-errno test.c -S -o test.s -mllvm -debug-only=licm
>> LICM hoisting to while.body.lr.ph <http://while.body.lr.ph/>:
%21 = load double** %arrayidx8, align 8, !tbaa !5
>> LICM hoisting to while.body.lr.ph <http://while.body.lr.ph/>:
%arrayidx72 = getelementptr inbounds double* %11, i64 1
>> LICM hoisting to while.body.lr.ph <http://while.body.lr.ph/>:
%arrayidx81 = getelementptr inbounds double* %11, i64 2
>> LICM hoisting to for.body.lr.ph <http://for.body.lr.ph/>: %2 =
ptrtoint i32* %Index to i64
>>
>> clang --target=aarch64-linux-gnu -c -mcpu=cortex-a57 -Ofast
-fno-math-errno test.c -S -o test-cflaa.s -mllvm -use-cfl-aa -mllvm
-debug-only=licm
>> LICM hoisting to for.body.lr.ph <http://for.body.lr.ph/>: %2 =
ptrtoint i32* %Index to i64
>>
>> Why CFL AA cannot allow hoisting this out: %21 = load double**
%arrayidx8, align 8, !tbaa !5
>>
>> Which leads to this extra load in assembly code: ldr x14, [x4, x9,
lsl #3]
>>
>> Thanks!
>> Ana.
>>
>> -----Original Message-----
>> From: Hal Finkel [mailto:hfinkel at anl.gov <mailto:hfinkel at
anl.gov>]
>> Sent: Tuesday, January 13, 2015 12:51 AM
>> To: Chandler Carruth
>> Cc: Jiangning Liu; Pazos, Ana; Ana Pazos; Daniel Berlin; George Burgess
IV
>> Subject: Re: question about enabling cfl-aa and collecting a57 numbers
>>
>> ----- Original Message -----
>> > From: "Chandler Carruth" <chandlerc at gmail.com
<mailto:chandlerc at gmail.com>>
>> > To: "Ana Pazos" <apazos at codeaurora.org
<mailto:apazos at codeaurora.org>>, "Daniel Berlin"
<dberlin at dberlin.org <mailto:dberlin at dberlin.org>>,
"George Burgess IV"
>> > <george.burgess.iv at gmail.com <mailto:george.burgess.iv at
gmail.com>>, "Hal Finkel" <hfinkel at anl.gov
<mailto:hfinkel at anl.gov>>
>> > Cc: "Jiangning Liu" <Jiangning.Liu at arm.com
<mailto:Jiangning.Liu at arm.com>>, "Ana Pazos"
>> > <apazos at quicinc.com <mailto:apazos at quicinc.com>>
>> > Sent: Monday, January 12, 2015 6:43:44 PM
>> > Subject: Re: question about enabling cfl-aa and collecting a57
numbers
>> >
>> > Not quite:
>> >
>> >
>> > On Mon Jan 12 2015 at 4:27:30 PM Ana Pazos < apazos at
codeaurora.org <mailto:apazos at codeaurora.org> >
>> > wrote:
>> >
>> > Thanks George and Daniel,
>> >
>> >
>> >
>> > If the recommended order is “–basicaa –cfla-aa” it means we should
fix
>> > the trunk code that processes the flags -mllvm
use-cfl-aa-in-codegen
>> > and –mllvm use-cfl-aa, right?
>> >
>> >
>> >
>> > In Transforms/IPO/PassManagerBuilder.cpp and CodeGen/Passes.cpp we
see
>> > this sequence:
>> >
>> >
>> >
>> > if (UseCFLAA)
>> >
>> > PM.add(createCFLAliasAnalysisPass());
>> >
>> > PM.add(createTypeBasedAliasAnalysisPass());
>> >
>> > PM.add(createScopedNoAliasAAPass());
>> >
>> > PM.add(createBasicAliasAnalysisPass());
>> >
>> >
>> >
>> > So are you recommending changing to the sequence below instead?
>> >
>> >
>> > Not quite:
>> >
>> >
>> > addPass(createTypeBasedAliasAnalysisPass());
>> >
>> > addPass(createScopedNoAliasAAPass());
>> > You want it here.
>> >
>> > addPass(createBasicAliasAnalysisPass());
>> >
>> > if (UseCFLAA)
>> >
>> > addPass(createCFLAliasAnalysisPass());
>> >
>> >
>> > The way to think about this is as follows:
>> >
>> >
>> > if TBAA or ScopedNoAlias *can* return results, you want them to.
They
>> > are reflecting a-priori knowledge of aliasing.
>> >
>> >
>> > if both of those say "maybe", you want to ask CFL. If
CFL says "maybe"
>> > you want to ask "BasicAA".
>>
>> I could very well be misunderstanding something, but I recall that we
add BasicAA last so that it will be queried first. We do this because of our
BasicAA delegation hack: We want a MustAlias from BasicAA to override a NoAlias
from TBAA, so that we can catch cases of basic local type punning. Thus,
assuming CFL delegates properly, I fail to see how switching its order w.r.t.
the metadata-based analysis changes anything.
>>
>> -Hal
>>
>> >
>> >
>> > It's possible that you could as a compile-time hack flip the
last two
>> > to your suggested ordering, but a) it should really only be a
compile
>> > time hack, and b) we haven't really tested that BasicAA
actually
>> > delegates rather than directly returning "maybe".
>> >
>> >
>> > Thanks,
>> >
>> > Ana.
>> >
>> >
>> >
>> > From: Daniel Berlin [mailto: dberlin at dberlin.org
<mailto:dberlin at dberlin.org> ]
>> > Sent: Friday, January 09, 2015 6:41 PM
>> > To: George Burgess IV; Hal Finkel
>> > Cc: Ana Pazos; Pazos, Ana; Jiangning Liu; chandlerc
>> >
>> >
>> > Subject: Re: question about enabling cfl-aa and collecting a57
numbers
>> >
>> >
>> > Right. I would not try with -cfl-aa -basicaa as the order, it
should
>> > be the other way around.
>> >
>> >
>> > If you discover performance regressions with *that*, that would be
>> > highly interesting.
>> >
>> >
>> > On Fri Jan 09 2015 at 6:32:28 PM George Burgess IV <
>> > george.burgess.iv at gmail.com <mailto:george.burgess.iv at
gmail.com> > wrote:
>> >
>> >
>> >
>> > Thanks for the notes and observations Ana! :)
>> >
>> > >> Hal, what do you think? Does it meant that cfl-aa is more
>> > >> conservative (note I set the right order -cfa-aa followed
by
>> > >> -basicaa)?
>> > Currently, yes. The implementation as it is is focused heavily on
>> > being fast, and on trying to cover cases that BasicAA can’t easily
>> > cover itself. If our end goal is to use this as a replacement for
>> > BasicAA, there’s probably quite a few low-hanging fruit for
accuracy
>> > improvements, and we can start playing with sacrificing speed in
>> > pursuit of greater accuracy. On the other hand, if the end goal is
to
>> > append this pass to the list of AA passes already being performed,
>> > then I’d be interested in seeing the difference in perf between
>> > `-basicaa` and `-basicaa -cfl-aa`, if any.
>> >
>> > WRT this issue in particular, I believe CFLAA answers
conservatively
>> > because:
>> > A. it doesn’t take into account constant pointer offsets (i.e. it
>> > considers a[0]..a[n] all as the same “address”) B. CFLAA is
entirely
>> > context insensitive
>> >
>> > IIRC, BasicAA is somewhat context sensitive, and it does take into
>> > account constant pointer offsets, so it can be more accurate in
this
>> > case. That being said, I don’t have access to the SPEC code, so I
can
>> > only speculate. :)
>> >
>> > George
>> >
>> > > On Jan 9, 2015, at 4:25 PM, Hal Finkel < hfinkel at
anl.gov <mailto:hfinkel at anl.gov> > wrote:
>> > >
>> > > Ana,
>> > >
>> > > Thanks so much for looking into this. I'm adding some
additional
>> > > relevant people to the CC line...
>> > >
>> > > -Hal
>> > >
>> > > ----- Original Message -----
>> > >> From: "Ana Pazos" < apazos at codeaurora.org
<mailto:apazos at codeaurora.org> >
>> > >> To: "Ana Pazos" < apazos at quicinc.com
<mailto:apazos at quicinc.com> >, "Hal Finkel" <
>> > >> hfinkel at anl.gov <mailto:hfinkel at anl.gov>
>, "Jiangning Liu" < Jiangning.Liu at arm.com
<mailto:Jiangning.Liu at arm.com> >
>> > >> Sent: Friday, January 9, 2015 3:12:04 PM
>> > >> Subject: RE: question about enabling cfl-aa and
collecting a57
>> > >> numbers
>> > >>
>> > >> Hi Hal and Jiangning,
>> > >>
>> > >> I started to look at the effect of cfl-aa on SPEC.
>> > >>
>> > >> The first observation is that I see more redundant loads
not being
>> > >> hoisted out of loops by LICM.
>> > >>
>> > >> Due to license restrictions, I cannot paste SPEC code
here, but I
>> > >> think you have access to it.
>> > >>
>> > >> I tried to simplify a top function in equake (quake.c
smvp) to show
>> > >> the issue. See the attached reduced test case.
>> > >>
>> > >> If you look at the the source code you will see something
like:
>> > >> for {
>> > >> ...load v[i][0]...
>> > >> while {
>> > >> ...load v[i][0]...
>> > >> store w[col][0]....
>> > >> }
>> > >> ...load w[i][0]...
>> > >> store w[i][0]...
>> > >> }
>> > >>
>> > >> It should be possible to avoid load &v[i][0],
&v[i][1], &vi[i][2]
>> > >> inside the while loop.
>> > >>
>> > >> In the simplified example that would be this instruction:
>> > >> %21 = load double** %arrayidx8, align 8, !tbaa !5
>> > >>
>> > >> And the pointer for the load instruction:
>> > >> %arrayidx8 = getelementptr inbounds double** %v, i64
%idxprom
>> > >>
>> > >> The decision to hoist it fails at LICM.cpp:189: AliasSet
&AS >> > >> CurAST->getAliasSetForPointer(V, Size,
AAInfo).isMod.
>> > >>
>> > >> With basicaa, only loads are in the alias set, but with
cfl-aa you
>> > >> find loads and stores, so isMod returns true.
>> > >>
>> > >> AliasSet with basicaa:
>> > >> AliasSet[0x2b35aa0, 7] may alias, Ref Pointers:
(double***
>> > >> %arrayidx32, 8), (double** %12, 8), (double**
%arrayidx36, 8),
>> > >> (double** %arrayidx8, 8), (double** %arrayidx68, 8),
(double**
>> > >> %arrayidx77, 8), (double** %arrayidx85, 8)
>> > >> $5 = void
>> > >>
>> > >> AliasSet with cfa-aa:
>> > >> AliasSet[0x2b37df0, 20] may alias, Mod/Ref Pointers:
(i32*
>> > >> %arrayidx30, 4), (double*** %arrayidx32, 8), (double**
%12, 8),
>> > >> (double* %13, 8), (double** %arrayidx36, 8), (double*
%15, 8),
>> > >> (double* %arrayidx42, 8), (double* %arrayidx45, 8),
(double*
>> > >> %arrayidx51, 8), (double* %arrayidx54, 8), (double**
%arrayidx8,
>> > >> 8), (double* %21, 8), (double** %arrayidx68, 8), (double*
%23, 8),
>> > >> (double* %arrayidx72, 8), (double** %arrayidx77, 8),
(double* %26,
>> > >> 8), (double* %arrayidx81, 8), (double** %arrayidx85, 8),
(double*
>> > >> %29, 8)
>> > >> $8 = void
>> > >>
>> > >> You can reproduce it with the commands:
>> > >> opt -O3 -S -cfl-aa -basicaa -licm -debug-only=licm
reduce.ll -o out
>> > >>
>> > >> opt -O3 -S - -basicaa -licm -debug-only=licm reduce.ll -o
out LICM
>> > >> hoisting to while.body.lr.ph
<http://while.body.lr.ph/> : %21 = load double** %arrayidx8,
>> > >> align 8, !tbaa !5 LICM hoisting to while.body.lr.ph
<http://while.body.lr.ph/> : %arrayidx72 >> > >>
getelementptr inbounds double* %11, i64 1 LICM hoisting to
>> > >> while.body.lr.ph <http://while.body.lr.ph/> :
%arrayidx81 = getelementptr inbounds double*
>> > >> %11, i64 2
>> > >>
>> > >> Hal, what do you think? Does it meant that cfl-aa is more
>> > >> conservative (note I set the right order -cfa-aa followed
by
>> > >> -basicaa)? Could it be an issue with building the
AliasSet tracker?
>> > >> Any pointers on how to fix this?
>> > >>
>> > >> Thanks,
>> > >> Ana.
>> > >>
>> > >>
>> > >>
>> > >> -----Original Message-----
>> > >> From: Jiangning Liu [mailto: Jiangning.Liu at arm.com
<mailto:Jiangning.Liu at arm.com> ]
>> > >> Sent: Wednesday, December 24, 2014 6:27 PM
>> > >> To: Hal Finkel; Ana Pazos
>> > >> Subject: RE: question about enabling cfl-aa and
collecting a57
>> > >> numbers
>> > >>
>> > >> Hi,
>> > >>
>> > >> I saw big regressions on cortex-A57 for the following
SPEC
>> > >> benchmarks after using "-mllvm -use-cfl-aa ".
>> > >>
>> > >> spec.cpu2006.ref.462_libquantum 9.22%
spec.cpu2000.ref.179_art
>> > >> 5.32%
>> > >> spec.cpu2000.ref.256_bzip2 4.91%
>> > >>
>> > >> Thanks,
>> > >> -Jiangning
>> > >>
>> > >>> -----Original Message-----
>> > >>> From: Hal Finkel [mailto: hfinkel at anl.gov
<mailto:hfinkel at anl.gov> ]
>> > >>> Sent: Wednesday, December 24, 2014 3:56 AM
>> > >>> To: Ana Pazos
>> > >>> Cc: Jiangning Liu
>> > >>> Subject: Re: question about enabling cfl-aa and
collecting a57
>> > >>> numbers
>> > >>>
>> > >>> ----- Original Message -----
>> > >>>> From: "Ana Pazos" < apazos at
codeaurora.org <mailto:apazos at codeaurora.org> >
>> > >>>> To: "Hal Finkel" < hfinkel at
anl.gov <mailto:hfinkel at anl.gov> >, "Jiangning Liu"
>> > >>>> < Jiangning.Liu at arm.com
<mailto:Jiangning.Liu at arm.com> >
>> > >>>> Sent: Monday, December 22, 2014 4:25:07 PM
>> > >>>> Subject: question about enabling cfl-aa and
collecting a57
>> > >>>> numbers
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>> Hi Hal,
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>> Can you clarify to enable cfl-aa from clang we
need both these
>> > >>>> flags?
>> > >>>>
>> > >>>>
>> > >>>
>> > >>> Hi Ana,
>> > >>>
>> > >>> Thanks for following-up on this! You only need
(-mllvm
>> > >>> -use-cfl-aa)
>> > >>> to
>> > >>> use CFL AA during the main optimization pipeline, and
just this is
>> > >>> enough to reproduce the performance regressions as
far as I know.
>> > >>> You
>> > >>> can also use CFL AA during code generation (-mllvm
>> > >>> -use-cfl-aa-in-codegen), which matters only if your
using AA at
>> > >>> all for code generation (which AArch64 does only for
the
>> > >>> Cortex/A53).
>> > >>>
>> > >>> -Hal
>> > >>>
>> > >>>>
>> > >>>> -mllvm -use-cfl-aa
>> > >>>>
>> > >>>> -mllvm -cfl-aa-in-codegen
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>> Hi Jiangning,
>> > >>>>
>> > >>>> Did you collect a57 perf results for SPEC 2000
and 2006 enabling
>> > >>>> these two flags?
>> > >>>>
>> > >>>> Did you notice any correctness and performance
regression?
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>> Thanks,
>> > >>>>
>> > >>>> Ana.
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>> Ana Pazos
>> > >>>> Qualcomm Innovation Center, Inc.
>> > >>>> The Qualcomm Innovation Center, Inc. is a member
of the Code
>> > >>>> Aurora Forum, a Linux Foundation Collaborative
Project.
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>>
>> > >>>
>> > >>> --
>> > >>> Hal Finkel
>> > >>> Assistant Computational Scientist
>> > >>> Leadership Computing Facility
>> > >>> Argonne National Laboratory
>> > >>
>> > >>
>> > >> -- IMPORTANT NOTICE: The contents of this email and any
attachments
>> > >> are confidential and may also be privileged. If you are
not the
>> > >> intended recipient, please notify the sender immediately
and do not
>> > >> disclose the contents to any other person, use it for any
purpose,
>> > >> or store or copy the information in any medium. Thank
you.
>> > >>
>> > >> ARM Limited, Registered office 110 Fulbourn Road,
Cambridge CB1
>> > >> 9NJ, Registered in England & Wales, Company No:
2557590 ARM
>> > >> Holdings plc, Registered office 110 Fulbourn Road,
Cambridge CB1
>> > >> 9NJ, Registered in England & Wales, Company No:
2548782
>> > >>
>> > >
>> > > --
>> > > Hal Finkel
>> > > Assistant Computational Scientist
>> > > Leadership Computing Facility
>> > > Argonne National Laboratory
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu>
http://llvm.cs.uiuc.edu <http://llvm.cs.uiuc.edu/>
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>>
>>
>>
>> <cflaafix.diff>
>
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Apparently Analagous Threads
- [LLVMdev] question about enabling cfl-aa and collecting a57 numbers
- [LLVMdev] question about enabling cfl-aa and collecting a57 numbers
- [LLVMdev] question about enabling cfl-aa and collecting a57 numbers
- [LLVMdev] question about enabling cfl-aa and collecting a57 numbers
- [LLVMdev] question about enabling cfl-aa and collecting a57 numbers