[llvm-dev] Hitting assertion failure related to vectorization + instcombine

Wed Jul 27 16:54:58 PDT 2016

David, Sanjay: ping?

On Mon, Jul 25, 2016 at 11:07 AM, Hans Wennborg <hans at chromium.org> wrote:
> Sure. David, what do you think about merging this to 3.9?
>
> Sanjay: are you saying I'd just apply that diff to
> InstructionSimplify.cpp, not InstCombineSelect.cpp?
>
> On Fri, Jul 22, 2016 at 7:08 AM, Sanjay Patel <spatel at rotateright.com> wrote:
>> Hi Hans -
>>
>> Yes, I think this is a good patch for 3.9 (cc'ing David Majnemer as code
>> owner). The functional change was r276209. I made an NFC refactoring in
>> InstSimplify at r275911 that isn't in the branch, so I don't think the patch
>> will apply as-is. For safety, you could apply the one-line fix without
>> pulling the refactoring into the branch with this diff:
>>
>> -    unsigned BitWidth = Q.DL.getTypeSizeInBits(TrueVal->getType());
>> +    unsigned BitWidth =
>> +        Q.DL.getTypeSizeInBits(TrueVal->getType()->getScalarType());
>>
>>
>>
>>
>> On Fri, Jul 22, 2016 at 7:45 AM, Hans Wennborg <hans at chromium.org> wrote:
>>>
>>> Sanjay: let me know if this is something that will apply to 3.9.
>>>
>>> Thanks,
>>> Hans
>>>
>>> On Wed, Jul 20, 2016 at 5:59 PM, Sanjay Patel via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> > Quick update - the bug existed before I refactored that chunk in
>>> > InstSimplify with:
>>> > https://reviews.llvm.org/rL275911
>>> >
>>> > In fact, as discussed in https://reviews.llvm.org/D22537 - because we
>>> > have a
>>> > big pile of lazily copied code, the bug has an identical twin that
>>> > exists in
>>> > InstCombine. I swear I didn't touch that code...yet. :)
>>> >
>>> > define <2 x i32> @select_icmp_vec(<2 x i32> %x) {
>>> >   %cmp = icmp slt <2 x i32> %x, zeroinitializer
>>> >   %xor = xor <2 x i32> %x, <i32 2147483648, i32 2147483648>
>>> >   %x.xor = select <2 x i1> %cmp, <2 x i32> %x, <2 x i32> %xor
>>> >   ret <2 x i32> %x.xor
>>> > }
>>> >
>>> > $ ./opt -instcombine selvec.ll -S
>>> > Assertion failed: (BitWidth == RHS.BitWidth && "Comparison requires
>>> > equal
>>> > bit widths"), function operator==, file
>>> > /Users/spatel/myllvm/llvm/include/llvm/ADT/APInt.h, line 983.
>>> >
>>> > I should have a patch up for review shortly.
>>> >
>>> >
>>> > On Wed, Jul 20, 2016 at 2:03 PM, Sanjay Patel <spatel at rotateright.com>
>>> > wrote:
>>> >>
>>> >> Thanks for notifying me. Yes, this was a recent change. Taking a look
>>> >> now.
>>> >>
>>> >> On Wed, Jul 20, 2016 at 1:49 PM, Michael Kuperstein <mkuper at google.com>
>>> >> wrote:
>>> >>>
>>> >>> +Sanjay, who touched this last. :-)
>>> >>>
>>> >>> On Wed, Jul 20, 2016 at 12:44 PM, Ismail Badawi (ibadawi) via llvm-dev
>>> >>> <llvm-dev at lists.llvm.org> wrote:
>>> >>>>
>>> >>>> Hi folks,
>>> >>>>
>>> >>>> I'm hitting the below assertion failure when compiling this small
>>> >>>> piece
>>> >>>> of C code (repro.c, attached).
>>> >>>>
>>> >>>> My command line is:
>>> >>>>
>>> >>>> bin/clang --target=aarch64-linux-gnu -c -O2 repro.c
>>> >>>>
>>> >>>> clang is built from top of trunk as of this morning. It only happens
>>> >>>> at
>>> >>>> -O2, and it doesn't happen with the default target (x86_64). I tried
>>> >>>> to
>>> >>>> reproduce using just 'llc -O2' but didn't manage -- but I do have
>>> >>>> this
>>> >>>> reduced opt command line (repro.ll also attached, just generated from
>>> >>>> repro.c at -O0):
>>> >>>>
>>> >>>> bin/opt -instcombine -licm -simplifycfg -instcombine -loop-rotate
>>> >>>> -loop-vectorize -instcombine < repro.ll
>>> >>>>
>>> >>>> The failure is:
>>> >>>>
>>> >>>> opt: /scratch/1/ismail/llvm-upstream/include/llvm/ADT/APInt.h:983:
>>> >>>> bool
>>> >>>> llvm::APInt::operator==(const llvm::APInt&) const: Assertion
>>> >>>> `BitWidth ==
>>> >>>> RHS.BitWidth && "Comparison requires equal bit widths"' failed.
>>> >>>> ...snip...
>>> >>>> #8 0x00000000013a8553 llvm::APInt::operator==(llvm::APInt const&)
>>> >>>> const
>>> >>>> /scratch/1/ismail/llvm-upstream/include/llvm/ADT/APInt.h:984:0
>>> >>>> #9 0x0000000001f875b0 simplifySelectBitTest(llvm::Value*,
>>> >>>> llvm::Value*,
>>> >>>> llvm::Value*, llvm::APInt const*, bool)
>>> >>>>
>>> >>>> /scratch/1/ismail/llvm-upstream/lib/Analysis/InstructionSimplify.cpp:3388:0
>>> >>>> #10 0x0000000001f87ad5 simplifySelectWithICmpCond(llvm::Value*,
>>> >>>> llvm::Value*, llvm::Value*, (anonymous namespace)::Query const&,
>>> >>>> unsigned
>>> >>>> int)
>>> >>>>
>>> >>>> /scratch/1/ismail/llvm-upstream/lib/Analysis/InstructionSimplify.cpp:3434:0
>>> >>>> #11 0x0000000001f87f6a SimplifySelectInst(llvm::Value*, llvm::Value*,
>>> >>>> llvm::Value*, (anonymous namespace)::Query const&, unsigned int)
>>> >>>>
>>> >>>> /scratch/1/ismail/llvm-upstream/lib/Analysis/InstructionSimplify.cpp:3515:0
>>> >>>> #12 0x0000000001f87fe3 llvm::SimplifySelectInst(llvm::Value*,
>>> >>>> llvm::Value*, llvm::Value*, llvm::DataLayout const&,
>>> >>>> llvm::TargetLibraryInfo
>>> >>>> const*, llvm::DominatorTree const*, llvm::AssumptionCache*,
>>> >>>> llvm::Instruction const*)
>>> >>>>
>>> >>>> /scratch/1/ismail/llvm-upstream/lib/Analysis/InstructionSimplify.cpp:3528:0
>>> >>>> ...snip...
>>> >>>> Stack dump:
>>> >>>> 0.  Program arguments: debugbuild/bin/opt -instcombine -licm
>>> >>>> -simplifycfg -instcombine -loop-rotate -loop-vectorize -instcombine
>>> >>>> 1.  Running pass 'Function Pass Manager' on module '<stdin>'.
>>> >>>> 2.  Running pass 'Combine redundant instructions' on function
>>> >>>> '@strsave'
>>> >>>>
>>> >>>> ---
>>> >>>>
>>> >>>> Looking at the code, the issue is with this line:
>>> >>>>
>>> >>>>   if (TrueVal == X && match(FalseVal, m_And(m_Specific(X),
>>> >>>> m_APInt(C)))
>>> >>>> &&
>>> >>>>         *Y == ~*C)
>>> >>>>
>>> >>>> In this case Y is a 128-bit APInt, and so is the value that C is
>>> >>>> extracted from, but the m_APInt matcher has code that calls
>>> >>>> getSplatValue()
>>> >>>> if the matched expression has vector type. So C ends up as an 8-bit
>>> >>>> value,
>>> >>>> triggering the assertion failure on the call to ==.
>>> >>>>
>>> >>>> The issue is clear but I'm not sure what the correct fix should be --
>>> >>>> whether this code is just not meant to work with vector types, or
>>> >>>> whether
>>> >>>> the call to getSplatValue() doesn't belong in the matcher, or whether
>>> >>>> there
>>> >>>> should be a corresponding call to getSplatValue() on the caller side,
>>> >>>> or
>>> >>>> something else.
>>> >>>>
>>> >>>> Any help from someone with more context would be appreciated.
>>> >>>>
>>> >>>> Thanks,
>>> >>>> Ismail
>>> >>>>
>>> >>>>
>>> >>>> _______________________________________________
>>> >>>> LLVM Developers mailing list
>>> >>>> llvm-dev at lists.llvm.org
>>> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >>>>
>>> >>>
>>> >>
>>> >
>>> >
>>> > _______________________________________________
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>>> >
>>
>>