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

Kevin Choi via llvm-dev
2016-07-20T21:44:37Z

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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