[PATCH] D72182: [WIP][DSE] Add basic cross-block dse pass using MemorySSA

[Tyker via Phabricator via llvm-commits]

Tyker created this revision.
Tyker added reviewers: dmgreen, rnk, efriedma.
Herald added subscribers: llvm-commits, asbirlea, jfb, george.burgess.iv, hiraditya, Prazek.
Herald added a project: LLVM.

This patch add a cross-block dead store elimination pass that eliminate stores.
This pass work by detecting if a store is observable. this detection has 2 phases. the first phase is a search through the MemorySSA graph for loads or end of function. if on all path in the memssa graph hit a store before they hit a load. if the first phase didn't find any use we passe to the second phase which check if an optimization barrier (throw, fence ...) is reachable between the "dead" store and a killing store. and that all killing store together post-dominate the "dead" store. this pass also has a special case for non-escaping pointers on which we can consider end of function as a killer.

todo in the patch:

- improve the test suite.

todo probably in separate patchs:

- measure and tune for compile time impact.
- fix the many todos in code.
- handle builtin / library functions.
- deal with store merging.

todo maybe:

- improve the pass to deal with partial overwrites.

for a meta build of check-llvm + check-clang here is the stats:

- sum of dse.* transformation old: 145k this: 202k
- sum of dse.NumFastStores   old: 98k this: 174k

this patch was inspired by D40480 <https://reviews.llvm.org/D40480>
i wasn't planning to post it for review before i improved test but i saw D72146 <https://reviews.llvm.org/D72146> and we probably should probably try to collaborate.


https://reviews.llvm.org/D72182

Files:
  llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp
  llvm/test/Transforms/DeadStoreElimination/multiblock.ll
  llvm/test/Transforms/DeadStoreElimination/non-escaping-ptr.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D72182.236086.patch
Type: text/x-patch
Size: 99048 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200103/9604a16f/attachment-0001.bin>