llvm dev - Apr 2014 - [LLVMdev] InstCombine strips the inBounds attribute in GetElementPtr ConstantExpr

I can't upload my program due to confidentiality, but the problem is
obvious.

At lib/Analysis/ConstantFolding.cpp:646

  Constant *C = ConstantExpr::getGetElementPtr(Ops[0], NewIdxs);

  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {

    if (Constant *Folded = ConstantFoldConstantExpression(CE, TD, TLI))

      C = Folded;

  }

The generated ConstantExpr C doesn't inherit the inBounds attribute of the
original GEP.

FYI, the callstack is

#0  CastGEPIndices (Ops=..., ResultTy=0x2fb2850, TD=0x2fc8830,
TLI=0x2feb390)
    at ConstantFolding.cpp:623

#1  0x0000000001892dd1 in llvm::ConstantFoldInstOperands (Opcode=29,

    DestTy=0x2fb2850, Ops=..., TD=0x2fc8830, TLI=0x2feb390)

    at ConstantFolding.cpp:1040

#2  0x0000000001892647 in ConstantFoldConstantExpressionImpl (CE=0x2fcb0f8,

    TD=0x2fc8830, TLI=0x2feb390, FoldedOps=...) at ConstantFolding.cpp:931

#3  0x00000000018926d5 in llvm::ConstantFoldConstantExpression
(CE=0x2fcb0f8,
    TD=0x2fc8830, TLI=0x2feb390) at ConstantFolding.cpp:941

#4  0x0000000001892226 in llvm::ConstantFoldInstruction (I=0x2fcb208,

TD=0x2fc8830,

    TLI=0x2feb390) at ConstantFolding.cpp:883

#5  0x000000000171284e in AddReachableCodeToWorklist (BB=0x2fca970,
Visited=...,
    IC=..., DL=0x2fc8830, TLI=0x2feb390) at InstructionCombining.cpp:2283

#6  0x0000000001712e91 in llvm::InstCombiner::DoOneIteration
(this=0x2ff3be0,
F=...,

    Iteration=0) at InstructionCombining.cpp:2369

#7  0x0000000001713bf2 in llvm::InstCombiner::runOnFunction
(this=0x2ff3be0,
F=...)

    at InstructionCombining.cpp:2568

Jingyue
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On Mon, Apr 21, 2014 at 7:59 PM, Jingyue Wu <jingyue at google.com> wrote:
> I can't upload my program due to confidentiality, but the problem is
> obvious.

A reduced test case would still help, and should go into any fix to ensure
we don't regress here...
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Sure, Chandler. I was running late yesterday, and didn't get a chance to
reduce it.

Attached is a reduced test. One more thing I found is, to trigger this
issue, the base of the GEP mustn't be a GlobalVariable; otherwise, LLVM
will go to ConstantFold.cpp:2131 and generate an inbounds GEP ConstantExpr.

2127   // If all indices are known integers and normalized, we can do a
simple
2128   // check for the "inbounds" property.

2129   if (!Unknown && !inBounds &&

2130       isa<GlobalVariable>(C) && isInBoundsIndices(Idxs))

2131     return ConstantExpr::getInBoundsGetElementPtr(C, Idxs);


My attached test is reduced from a CUDA program that declares the global
array as __shared__. Therefore, the base of the GEP is an addrspacecast
from the global array.

One possible fix is to pass IsInBounds to ConstantFoldInstOperands and
CastGEPIndices, so that CastGEPIndices can inherit this attribute. Are
there better ways?

Let me know if you need more information.

Jingyue


On Mon, Apr 21, 2014 at 8:04 PM, Chandler Carruth <chandlerc at
google.com>wrote:
>
> On Mon, Apr 21, 2014 at 7:59 PM, Jingyue Wu <jingyue at google.com>
wrote:
>
>> I can't upload my program due to confidentiality, but the problem
is
>> obvious.
>
>
> A reduced test case would still help, and should go into any fix to ensure
> we don't regress here...
>
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