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

Fri Jul 22 06:45:32 PDT 2016

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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