[llvm-dev] load instruction erroneously removed by GVN v2

Kevin Choi via llvm-dev
2016-07-20T23:58:51Z

I think I've narrowed it to a failure point:
I am seeing that after
[0x1a5b080] %35 = call i16* @PAL_wcsncat(i16* %7, i16* %33, i64 %34), !dbg
!20064

is inlined into main, the alias of load and the preceding store no longer
are aliased. I verified this via inserting modulePrinterPass right after
inliningPass.

I am wondering if this is because pointer aliasing is incorrectly set up on
call to PAL_wcsncat or something is wrong in the inliner. I will see if I
can pull the pointer aliases on the callsite.

-Kevin

On 20 July 2016 at 14:44, Kevin Choi <code.kchoi at gmail.com> wrote:

> before inlining
> all 20005
>
> after inlining
>   somewhere here changed made it NoAlias
> after Global Variable Optimizer
>    20014
> 20373  20255
> 20372  20254
>
>
> before GVN
>    19993
> 20011  19991
> 20010  20030
>
> It appears that TBAA metadata certainly changed after inlining and
> subsequent passes. I have attached the .bc file. I think I will try to dump
> out more TBAA metadata between passes. The method in interest is @main.
>
>
>
> On 20 July 2016 at 11:32, Daniel Berlin <dberlin at dberlin.org>
wrote:
>
>>
>>
>> On Wed, Jul 20, 2016 at 11:24 AM, Kevin Choi <code.kchoi at
gmail.com>
>> wrote:
>>
>>> Thanks for quick reply Daniel,
>>>
>>> I tried to make a simple C testcase, but could not reproduce the
same
>>> condition with output from Clang.
>>>
>>
>> even if you have a .ll file you can share, that would be helpful.
>>
>>
>>> I suppose I could modify the C code to make it look similar with
TBAA's;
>>> I may be able to provide this by eod.
>>>
>>> > store %ptr above the load.
>>>
>>> My mistake; I was referring to the store $lcssa in bb2. Looking at
the C
>>> source code, it should definitely alias with %dest. I will try and
see if I
>>> can find out why it thinks there is no local dependence.
>>>
>>
>> Yes, i would start by seeing why the getdependency call is returning no
>> dep :)
>>
>> At a glance from your file, the load and that store are tagged with
>> different TBAA tags.
>> THat does not necessarily mean they don't alias, i'd have to
see the TBAA
>> tree to say for sure.
>> The TBAA rule is that a tag aliases all of it's descendants and
ancestors
>> in the tree.
>>
>> So if the tree look like this:
>>
>> !something
>>     /            \
>> !20010      !20030
>>
>> It will say no alias
>> but if it looks like
>>
>> !something
>>      |
>> !20010
>>     |
>> !20030
>>
>> TBAA will say they alias (something else may still say no-alias for
other
>> reasons, of course)
>>
>>
>>> Thanks,
>>> Kevin
>>>
>>> On 20 July 2016 at 11:06, Daniel Berlin <dberlin at
dberlin.org> wrote:
>>>
>>>>
>>>>
>>>> On Wed, Jul 20, 2016 at 9:56 AM, Kevin Choi via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hello to whom this may concern,
>>>>>
>>>>> Versioned this as I saw identical title before. I'm
compiling a clang
>>>>> project where I'm seeing GVN mess up and replace a load
with a wrong def
>>>>> value.
>>>>>
>>>>
>>>>
>>>> Do you have a c testcase or a .ll file i can actually compile?
>>>>
>>>>
>>>>> I am using LLVM-3.5, but the problem has been observed upto
3.8.
>>>>> To illustrate the problem,
>>>>>
>>>>> define i32 @main
>>>>> scalar.ph:
>>>>>     <initialize [80 x i16] %dest>
>>>>> ...
>>>>> preheader:
>>>>> %index=0
>>>>> br test, loop1, bb2
>>>>> loop1:
>>>>>   ... write to %dest in increasing index // ptr-based while
loop
>>>>>   %ptr++;
>>>>>   br test, loop1, bb2
>>>>> bb2:
>>>>>   %lcssa = phi [%ptr, loop1], [%ptr, preheader]
>>>>>   store i16 0, i16* %lcssa !dbg !20094 !tbaa 20030  //
write null byte
>>>>> at end
>>>>>   %76 = getelementptr inbounds [80 x i16]* %dest, i64 0,
i64 7, !dbg
>>>>> !20095 // load addr of null byte (7th index)
>>>>>   %77 = load i16* %76, align 2, !dbg !20095, !tbaa !20010
>>>>>   %78 = icmp eq i16 %77, 0, !dbg !20095
>>>>>   br i1 %78, label %80, label %79, !dbg !20095
>>>>>
>>>>> GVN calls processNonLocalLoad() on "%77 =
load..." and replaces it
>>>>> with init value from scalar.ph.
>>>>>
>>>>>
>>>>
>>>>
>>>>> bb2:
>>>>> %lcssa = phi [%ptr, loop1], [%ptr, preheader]
>>>>> store i16 0, i16* %lcssa, !dbg !20094, !tbaa !20030
>>>>> %76 = getelementptr inbounds [80 x i16]* %dest, i64 0, i64
7, !dbg
>>>>> !20095
>>>>> br i1 icmp eq (i16 trunc (i128 lshr (i128 bitcast (<8 x
i16> <i16 120,
>>>>> i16 120, i16 120, i16 120, i16 120, i16 120, i16 120, i16
120> to i128),
>>>>> i128 96) to i16), i16 0), label %78, label %77, !dbg !20095
// simplifies
>>>>> to "icmp eq (i16 120, i16 0) --> false"
>>>>>
>>>>> I first suspected problem might be TBAA; invoking clang
with
>>>>> -fno-strict-aliasing makes the test pass (similarly, opt
with -basicaa does
>>>>> not make GVN transform the load). When I look at the C/C++
source code, I
>>>>> cannot find any type-based aliasing violations from
eyeballing.
>>>>>
>>>>
>>>> Can you excerpt the actual source code?
>>>>
>>>>>
>>>>> I started looking at the aliasing and landed at
>>>>> getNonLocalPointerDepFromBB(), in which the worklist
algorithm to find
>>>>> MemDep reported the result from the init block, ignoring
all the kills
>>>>> after it. I did see one of the parm was SkipFirstBlock and
this appears to
>>>>> ignore the store %ptr above the load. Is there a reason why
it skips first
>>>>> block?
>>>>>
>>>>
>>>>
>>>> Yes. It is looking for *non-local* dependences.  The block it
should
>>>> skip is bb2, not loop1.
>>>> There is another call for local dependences.  If you ask for
local
>>>> dependences, then non-local ones, as GVN does, there is no
point in having
>>>> it go and look at the local ones again :)
>>>>
>>>> Look at how GVN calls this:
>>>> MemDepResult Dep = MD->getDependency(L);
>>>>
>>>>    // If it is defined in another block, try harder.
>>>>    if (Dep.isNonLocal())
>>>>      return processNonLocalLoad(L);
>>>>
>>>> (IE get local dependence, if there is none, go looking at
non-local
>>>> dependences).
>>>>
>>>>
>>>> The list of non-local dependences it includes should definitely
include
>>>> the one from block "loop1", and if it is ignoring it,
it is most likely
>>>> thinks it does not alias for some reason.
>>>>
>>>>
>>>
>>
>
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