[PATCH] D20665: Claim NoAlias if two GEPs index different fields of the same struct

Daniel Berlin via llvm-commits llvm-commits at lists.llvm.org
Wed Jun 1 20:52:35 PDT 2016

On Wed, Jun 1, 2016 at 8:35 PM, Ahmed Bougacha <ahmed.bougacha at gmail.com>

> Clang already has this: -fstrict-aliasing, which enables struct-path
> aware TBAA, via !tbaa metadata.

There are plenty of cases it does not generate good tbaa or struct path
tbaa. :)

> That TBAA isn't sufficient is intentional though: you're probably
> hitting the return PartialAlias at the end of BasicAAResult::aliasGEP
> (PR9971).

This is a different case, and easily resolvable with info that says it is
part of a union or not.

At least, GCC does not pretend to okay non-explicit union accesses.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
and https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14319

> -Ahmed
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