Refines `getModRefInfo` for `Monotonic` and `Unordered` atomic
operations to perform a full alias evaluation instead of conservatively
returning `ModRef`.
This allows AA to return more precise results (e.g., `NoModRef`),
unblocking optimizations in passes like DSE/PRE when the pointers are
proven not to alias.
LIT testing:
1. Fixed the `XFAIL` test `@test9` in `DSE/atomic-todo.ll`, where the
improved AA now allows DSE to correctly remove a dead store.
2. Added test `@test9a` to the same file to ensure DSE is still blocked
when pointers may alias.
3. I am also reviewing tests in GVN/PRE to see where this change may be
beneficial. While all existing tests pass, I will update this PR with
any new GVN test cases in a follow-up commit.
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Co-authored-by: Jin Huang <jingold@google.com>
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