[PATCH] D60659: [InstCombine] Eliminate stores to constant memory

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: apilipenko, skatkov, fedor.sergeev.
Herald added subscribers: bollu, mcrosier.
Herald added a project: LLVM.

If we have a store to a piece of memory which is known constant, then we know the store must be storing back the same value.  As a result, the store (or memset, or memmove) must either be down a dead path, or a noop.  In either case, it is valid to simply remove the store.

The motivating case for this involves a memmove to a buffer which is constant down a path which is dynamically dead.

Note that I'm choosing to implement the less aggressive of two possible semantics here.  We could simply say that the store *is undefined*, and prune the path.  That seemed a bit too likely to break real code in subtle ways, so I decide to just remove the store itself.

(I'm going to rebase the tests shortly, so you can ignore the spurious diffs.  I'll rebase once done.)


Repository:
  rL LLVM

https://reviews.llvm.org/D60659

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
  test/Transforms/InstCombine/gep-custom-dl.ll
  test/Transforms/InstCombine/getelementptr.ll
  test/Transforms/InstCombine/memcpy.ll
  test/Transforms/InstCombine/memmove.ll
  test/Transforms/InstCombine/memset.ll
  test/Transforms/InstCombine/store.ll

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