[PATCH] D123626: [InstCombine] Fold memrchr calls with constant size of zero or one.

Wed Apr 13 03:39:21 PDT 2022

xbolva00 added inline comments.

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Comment at: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp:873
+  Value *Size = CI->getArgOperand(2);
+  annotateNonNullAndDereferenceable(CI, 0, Size, DL);
+  Value *CharVal = CI->getArgOperand(1);
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nikic wrote:
> nikic wrote:
> > I'm not entirely sure this is correct. This is definitely incorrect for memchr() and I submitted D123665 to fix that. memrchr() is a GNU extension, so I'm not sure whether the same rules apply there. I guess one could argue that memrchr() should require a fully dereferenceable object, as otherwise the base pointer will already point outside the object.
> As a data point, sanitizer interception for these functions is implemented as follows: https://github.com/llvm/llvm-project/blob/ee44896cf42d3360f7e0229c360258c92262a47b/compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc#L984-L1021 So while the interceptor does not assume full dereferenceability for memchr(), it does assume so for memrchr(). So your current implementation would be consistent with sanitizers.
Any sanitizer developer here?

It would be great to know why there is a such differenr behaviour.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123626/new/

https://reviews.llvm.org/D123626