[PATCH] D149446: [Pipelines] Don't skip GlobalDCE in ThinLTO pre-link

[Nikita Popov via Phabricator via llvm-commits]

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GlobalDCE will only remove functions with available externally linkage if they are unreferenced. As such, I don't believe there is any problem with running this pass as part of the ThinLTO pre-link pipeline. It will only remove functions that are completely dead in that module, and I don't think there is any benefit to keeping them around for the post-link phase.

There is no compile-time impact from the additional pass.

This is a followup to one of the side discussions in D146776 <https://reviews.llvm.org/D146776>.


https://reviews.llvm.org/D149446

Files:
  llvm/lib/Passes/PassBuilderPipelines.cpp
  llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll


Index: llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
@@ -153,6 +153,7 @@
 ; CHECK-O-NEXT: Running pass: DeadArgumentEliminationPass
 ; CHECK-O-NEXT: Running pass: CoroCleanupPass
 ; CHECK-O-NEXT: Running pass: GlobalOptPass
+; CHECK-O-NEXT: Running pass: GlobalDCEPass
 ; CHECK-O-NEXT: Running pass: AnnotationRemarksPass on foo
 ; CHECK-O-NEXT: Running pass: CanonicalizeAliasesPass
 ; CHECK-O-NEXT: Running pass: NameAnonGlobalPass
Index: llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
@@ -189,6 +189,7 @@
 ; CHECK-O-NEXT: Running pass: CoroCleanupPass
 ; CHECK-O-NEXT: Running pass: GlobalOptPass
 ; CHECK-O-NEXT: Running analysis: TargetLibraryAnalysis on bar
+; CHECK-O-NEXT: Running pass: GlobalDCEPass
 ; CHECK-EXT: Running pass: {{.*}}::Bye
 ; CHECK-O-NEXT: Running pass: AnnotationRemarksPass on foo
 ; CHECK-O-NEXT: Running pass: CanonicalizeAliasesPass
Index: llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
===================================================================
--- llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
+++ llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
@@ -163,6 +163,7 @@
 ; CHECK-O-NEXT: Running pass: DeadArgumentEliminationPass
 ; CHECK-O-NEXT: Running pass: CoroCleanupPass
 ; CHECK-O-NEXT: Running pass: GlobalOptPass
+; CHECK-O-NEXT: Running pass: GlobalDCEPass
 ; CHECK-EXT: Running pass: {{.*}}::Bye
 ; CHECK-EP-OPT-EARLY-NEXT: Running pass: NoOpModulePass
 ; CHECK-EP-OPT-LAST-NEXT: Running pass: NoOpModulePass
Index: llvm/lib/Passes/PassBuilderPipelines.cpp
===================================================================
--- llvm/lib/Passes/PassBuilderPipelines.cpp
+++ llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1111,11 +1111,7 @@
 
   // Optimize globals now that functions are fully simplified.
   MPM.addPass(GlobalOptPass());
-
-  // Remove dead code, except in the ThinLTO pre-link pipeline where we may want
-  // to keep available_externally functions.
-  if (Phase != ThinOrFullLTOPhase::ThinLTOPreLink)
-    MPM.addPass(GlobalDCEPass());
+  MPM.addPass(GlobalDCEPass());
 
   if (EnableMemProfiler && Phase != ThinOrFullLTOPhase::ThinLTOPreLink) {
     MPM.addPass(createModuleToFunctionPassAdaptor(MemProfilerPass()));


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