[PATCH] D86967: [PassManager] Move load/store motion pass after DSE in LTO pipeline.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: spatel, xbolva00, lebedev.ri, asbirlea.
Herald added subscribers: dexonsmith, george.burgess.iv, hiraditya, inglorion, Prazek.
Herald added a project: LLVM.
fhahn requested review of this revision.

As far as I am aware, the placement of MergedLoadStoreMotion in the
pipeline is not heavily tuned currently. It seems to not matter much if
we do it after DSE in the LTO pipeline (no binary changes for -O3 -flto
on MultiSource/SPEC2000/SPEC2006). Moving it after DSE however has a
major benefit: MemorySSA is constructed by LICM and is consumed by DSE,
so if MergedLoadStoreMotion happens after DSE, we do not need to
preserve MemorySSA in it.

If there are any concerns with this move, I can also update
MergedLoadStoreMotion to preserve MemorySSA.

This patch together with D86651 <https://reviews.llvm.org/D86651> (preserve MemSSA in MemCpyOpt) and
D86534 <https://reviews.llvm.org/D86534> (preserve MemSSA in GVN) are the remaining patches to bring down
compile-time for DSE + MemorySSA to the levels outlined in
http://lists.llvm.org/pipermail/llvm-dev/2020-August/144417.html

Once they land, we should be able to start with flipping the switch on
enabling DSE + MmeorySSA.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D86967

Files:
  llvm/lib/Transforms/IPO/PassManagerBuilder.cpp


Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -1011,13 +1011,13 @@
   PM.add(createGlobalsAAWrapperPass()); // IP alias analysis.
 
   PM.add(createLICMPass(LicmMssaOptCap, LicmMssaNoAccForPromotionCap));
-  PM.add(createMergedLoadStoreMotionPass()); // Merge ld/st in diamonds.
   PM.add(NewGVN ? createNewGVNPass()
                 : createGVNPass(DisableGVNLoadPRE)); // Remove redundancies.
   PM.add(createMemCpyOptPass());            // Remove dead memcpys.
 
   // Nuke dead stores.
   PM.add(createDeadStoreEliminationPass());
+  PM.add(createMergedLoadStoreMotionPass()); // Merge ld/st in diamonds.
 
   // More loops are countable; try to optimize them.
   PM.add(createIndVarSimplifyPass());


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