[PATCH] D53342: [SimplifyLibCalls] Mark known arguments with nonnull

[Dávid Bolvanský via Phabricator via llvm-commits]

xbolva00 added a comment.

Anway if libc tags with nonnull, like int memcmp(void * nonnull dst,  void * nonnull src, size_t n) this applies to function signature, not to specific callsites so this fix will not work, indeed. We need drop nonnull from signatures in BuildLibCalls.

I think we just doing shitty workarounds and gcc 4.9 optimizes it fully since ptr cannot be null even if size is 0.
And we have sanitizer check to do that: 
https://reviews.llvm.org/D9673?id=25492

Jakub Jelinek at GCC Bugzilla:

C says e.g. for memchr:
"The memchr function locates the first occurrence of c (converted to an unsigned
char) in the initial n characters (each interpreted as unsigned char) of the object pointed to by s."
and elsewhere:
"If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function."
As memchr first argument must point to an object and NULL does not point to any object, NULL is not a valid value.  It is similar to memcpy (NULL, NULL, 0) or memset (NULL, 0, 0) etc. being invalid.”

Sanitizer warns so I dont know why se should permit UB.


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https://reviews.llvm.org/D53342