[PATCH] D148343: [Pipelines] Add LoopSink and DivRemPairs to LTO post-link pipeline

[Nikita Popov via Phabricator via llvm-commits]

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As pointed out in D148010 <https://reviews.llvm.org/D148010>, these passes are missing from the LTO post-link pipeline. They are present in the pre-link pipeline, but LoopSink is completely useless there (it will always be fully undone by LICM post-link) and DivRemPairs is mostly useless (I believe most of what it does will be undone by InstCombine).

I've not added RelLookupTableConverterPass, because it's also disabled in the LTO pre-link pipeline, with a comment that there is an unresolved issue with full LTO.

Compile-time impact of the extra passes is minimal: http://llvm-compile-time-tracker.com/compare.php?from=09ba7b605327812cfcec4f3c01d7fc232dee651d&to=52d546b4fe89ad66011ca9e3a8f2f6dfe4b4ac2f&stat=instructions:u Of course, LoopSink will have a larger impact in PGO builds.


https://reviews.llvm.org/D148343

Files:
  llvm/lib/Passes/PassBuilderPipelines.cpp
  llvm/test/Other/new-pm-lto-defaults.ll


Index: llvm/test/Other/new-pm-lto-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-lto-defaults.ll
+++ llvm/test/Other/new-pm-lto-defaults.ll
@@ -135,6 +135,8 @@
 ; CHECK-O23SZ-NEXT: Running pass: JumpThreadingPass on foo
 ; CHECK-O23SZ-NEXT: Running pass: LowerTypeTestsPass
 ; CHECK-O-NEXT: Running pass: LowerTypeTestsPass
+; CHECK-O23SZ-NEXT: Running pass: LoopSink
+; CHECK-O23SZ-NEXT: Running pass: DivRemPairs
 ; CHECK-O23SZ-NEXT: Running pass: SimplifyCFGPass
 ; CHECK-O23SZ-NEXT: Running pass: EliminateAvailableExternallyPass
 ; CHECK-O23SZ-NEXT: Running pass: GlobalDCEPass
Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1838,10 +1838,24 @@
     MPM.addPass(HotColdSplittingPass());
 
   // Add late LTO optimization passes.
+  FunctionPassManager LateFPM;
+
+  // LoopSink pass sinks instructions hoisted by LICM, which serves as a
+  // canonicalization pass that enables other optimizations. As a result,
+  // LoopSink pass needs to be a very late IR pass to avoid undoing LICM
+  // result too early.
+  LateFPM.addPass(LoopSinkPass());
+
+  // This hoists/decomposes div/rem ops. It should run after other sink/hoist
+  // passes to avoid re-sinking, but before SimplifyCFG because it can allow
+  // flattening of blocks.
+  LateFPM.addPass(DivRemPairsPass());
+
   // Delete basic blocks, which optimization passes may have killed.
-  MPM.addPass(createModuleToFunctionPassAdaptor(SimplifyCFGPass(
+  LateFPM.addPass(SimplifyCFGPass(
       SimplifyCFGOptions().convertSwitchRangeToICmp(true).hoistCommonInsts(
-          true))));
+          true)));
+  MPM.addPass(createModuleToFunctionPassAdaptor(std::move(LateFPM)));
 
   // Drop bodies of available eternally objects to improve GlobalDCE.
   MPM.addPass(EliminateAvailableExternallyPass());


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