llvm dev - Jul 2016 - Hitting assertion failure related to vectorization + instcombine

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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+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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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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