[llvm] 3060ee0 - [Pipelines] Don't skip GlobalDCE in ThinLTO pre-link

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2023-05-15T14:58:24+02:00
New Revision: 3060ee0c6adaf3015e39c90a0929029b8d368522

URL: https://github.com/llvm/llvm-project/commit/3060ee0c6adaf3015e39c90a0929029b8d368522
DIFF: https://github.com/llvm/llvm-project/commit/3060ee0c6adaf3015e39c90a0929029b8d368522.diff

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

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.

Differential Revision: https://reviews.llvm.org/D149446

Added: 
    

Modified: 
    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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index 66b49da572075..a7dfa95dadc2b 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1113,11 +1113,7 @@ PassBuilder::buildModuleSimplificationPipeline(OptimizationLevel Level,
 
   // 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()));

diff  --git a/llvm/test/Other/new-pm-thinlto-prelink-defaults.ll b/llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
index 252b68fab469a..4216cf90d2036 100644
--- a/llvm/test/Other/new-pm-thinlto-prelink-defaults.ll
+++ b/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

diff  --git a/llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll b/llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
index fa0ec4eeeaebe..e8b81b7383960 100644
--- a/llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
+++ b/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

diff  --git a/llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll b/llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
index a3784aee11bcd..80ddd1174ee94 100644
--- a/llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
+++ b/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