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

[Sanjay Patel via llvm-dev]

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
>>
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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