[PassBuilder] Add RelLookupTableConverterPass to LTO (#124053)

[PassBuilder] Add RelLookupTableConverterPass to LTO

This patch adds RelLookupTableConverterPass into the LTO
post-link optimization pass pipeline. This optimization
converts lookup tables to relative lookup tables to make
them PIC-friendly, which is already included in the non-LTO
pass pipeline. This patch adds this optimization to the
post-link optimization pipeline to discover more
opportunities in the LTO context.

Author: gulfemsavrun

llvm/docs/ReleaseNotes.md

@@ -133,6 +133,8 @@ Changes to TableGen
 Changes to Interprocedural Optimizations
 ----------------------------------------
 
+* Added RelLookupTableConverterPass to LTO post-link pass pipeline.
+
 Changes to the AArch64 Backend
 ------------------------------
 

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -1594,9 +1594,7 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
     MPM.addPass(CGProfilePass(LTOPhase == ThinOrFullLTOPhase::FullLTOPostLink ||
                               LTOPhase == ThinOrFullLTOPhase::ThinLTOPostLink));
 
-  // TODO: Relative look table converter pass caused an issue when full lto is
-  // enabled. See https://reviews.llvm.org/D94355 for more details.
-  // Until the issue fixed, disable this pass during pre-linking phase.
+  // RelLookupTableConverterPass runs later in LTO post-link pipeline.
   if (!LTOPreLink)
     MPM.addPass(RelLookupTableConverterPass());
 
@@ -2121,6 +2119,8 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   if (PTO.MergeFunctions)
     MPM.addPass(MergeFunctionsPass());
 
+  MPM.addPass(RelLookupTableConverterPass());
+
   if (PTO.CallGraphProfile)
     MPM.addPass(CGProfilePass(/*InLTOPostLink=*/true));
 

llvm/test/Other/new-pm-lto-defaults.ll

@@ -146,6 +146,7 @@
 ; CHECK-O23SZ-NEXT: Running pass: SimplifyCFGPass
 ; CHECK-O23SZ-NEXT: Running pass: EliminateAvailableExternallyPass
 ; CHECK-O23SZ-NEXT: Running pass: GlobalDCEPass
+; CHECK-O23SZ-NEXT: Running pass: RelLookupTableConverterPass
 ; CHECK-O23SZ-NEXT: Running pass: CGProfilePass
 ; CHECK-EP-NEXT: Running pass: NoOpModulePass
 ; CHECK-O-NEXT: Running pass: AnnotationRemarksPass on foo