Hitting assertion failure related to vectorization + instcombine

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#1

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 ‘’.
  2. Running pass ‘Combine redundant instructions’ on function ‘@strsave’

repro.c (275 Bytes)

ATT00001.htm (215 Bytes)

repro.ll (3.45 KB)

ATT00002.htm (233 Bytes)

#2

+Sanjay, who touched this last. :slight_smile:

#3

Thanks for notifying me. Yes, this was a recent change. Taking a look now.

#4

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. :slight_smile:

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.

#5

Sanjay: let me know if this is something that will apply to 3.9.

Thanks,
Hans

#6

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());

#7

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?

#8

David, Sanjay: ping?

#9

LGTM

#10

Back-ported to 3.9 in r276986.

Thanks,
Hans