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

[Michael Kuperstein via llvm-dev]

+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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>
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