[PATCH] D50233: [InstCombine] Transform str(n)cmp to memcmp

[Evgenii Stepanov via Phabricator via llvm-commits]

eugenis added subscribers: rsmith, eugenis.
eugenis added a comment.

There are concerns about this optimization expressed in

  https://github.com/google/sanitizers/issues/993

Specifically, it can transform

  strcmp("abc\0...undef...undef...", "foobar")

into a similar memcmp() which would observe uninitialized data at the end of the first string.

I'm not a 100% sure that it is undefined behavior because there are no indeterminate values in the code at all, and memcmp() as a standard library function does not have to follow the same rules (technically). @rsmith  But it upsets MemorySanitizer, and for a good reason IMHO.

This optimization should be either limited to functions w/o sanitize_memory attribute, or disabled completely.


Repository:
  rL LLVM

https://reviews.llvm.org/D50233