[LLVMdev] Question Regarding Sign-Overflow

[Jingyue Wu]

Sorry, the old title didn't make sense.


> Hi,
>
> I have doubts on the following transformation in InstCombineAddSub.cpp. Is
> it always safe to preserve NSW/NUW in this case?
>
>   // If this is a 'B = x-(-A)', change to B = x+A.  This preserves NSW/NUW.  if (Value <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/include/llvm/IR/Value.h&ct=xref_jump_to_def&cl=GROK&l=69&gsn=Value> * <https://cs.corp.google.com/#piper///depot/google3/GENERATED/figments/cpp/PointerTo/start-with-ll/llvm/class-Value.cc&ct=xref_jump_to_def&cl=GROK&l=3&gsn=*>V <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_usages&gs=cpp:llvm::class-InstCombiner::visitSub(llvm::BinaryOperator%2520&)::V at google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp:48204%257Cdef&l=1470&gsn=V> = dyn_castNegVal <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp&ct=xref_jump_to_def&cl=GROK&l=611&gsn=dyn_castNegVal>(Op1 <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1456&gsn=Op1>)) {    BinaryOperator <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/include/llvm/IR/InstrTypes.h&ct=xref_jump_to_def&cl=GROK&l=138&gsn=BinaryOperator> * <https://cs.corp.google.com/#piper///depot/google3/GENERATED/figments/cpp/PointerTo/start-with-ll/llvm/class-BinaryOperator.cc&ct=xref_jump_to_def&cl=GROK&l=3&gsn=*>Res <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_usages&gs=cpp:llvm::class-InstCombiner::visitSub(llvm::BinaryOperator%2520&)::Res at google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp:48251%257Cdef&l=1471&gsn=Res> = BinaryOperator <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/include/llvm/IR/InstrTypes.h&ct=xref_jump_to_def&cl=GROK&l=138&gsn=BinaryOperator>::CreateAdd(Op0 <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1456&gsn=Op0>, V <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1470&gsn=V>);    Res <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1471&gsn=Res>->setHasNoSignedWrap <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/IR/Instructions.cpp&ct=xref_jump_to_def&cl=GROK&l=2013&gsn=setHasNoSignedWrap>(I <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1455&gsn=I>.hasNoSignedWrap <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/IR/Instructions.cpp&ct=xref_jump_to_def&cl=GROK&l=2025&gsn=hasNoSignedWrap>());    Res <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1471&gsn=Res>->setHasNoUnsignedWrap <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/IR/Instructions.cpp&ct=xref_jump_to_def&cl=GROK&l=2009&gsn=setHasNoUnsignedWrap>(I <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1455&gsn=I>.hasNoUnsignedWrap <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/IR/Instructions.cpp&ct=xref_jump_to_def&cl=GROK&l=2021&gsn=hasNoUnsignedWrap>());    return Res <https://cs.corp.google.com/#piper///depot/google3/third_party/llvm/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp&ct=xref_jump_to_def&cl=GROK&l=1471&gsn=Res>;  }
>
>
> Suppose A = x = (i4 -8) (the minimum signed integer of i4). The first
> minus in x - (-A) does not sign-overflow, but x + A sign-overflows.
>
> Thanks,
> Jingyue
>
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