[SampleFDO] Skip PreLink ICP for better profile quality of MonoLTO PostLink

[wenlei]

Summary

For ThinLTO, PreLink ICP is skipped to favor better profile annotation during LTO PostLink. This change applies the same tweak for MonoLTO. Note that PreLink ICP not only makes PostLink profile annotation harder, it is also uncoordinated with PostLink ICP so duplicated ICP could happen.

[wenlei]

update

[wenlei]

update

[wenlei]

This is a change we've been using for some time internally, sending it up prompted by @wmi's ICP improvements. I don't have numbers but thought it could be helpful in general and it also aligns MonoLTO with ThinLTO.

[wmi]

LGTM.

llvm/lib/Passes/PassBuilder.cpp

PassBuilder::buildModuleSimplificationPipeline(OptimizationLevel Level,
   if (LoadSampleProfile) {
     // Annotate sample profile right after early FPM to ensure freshness of
     // the debug info.
     MPM.addPass(SampleProfileLoaderPass(PGOOpt->ProfileFile,
                                         PGOOpt->ProfileRemappingFile, Phase));
     // Cache ProfileSummaryAnalysis once to avoid the potential need to insert
     // RequireAnalysisPass for PSI before subsequent non-module passes.
     MPM.addPass(RequireAnalysisPass<ProfileSummaryAnalysis, Module>());
-    // Do not invoke ICP in the ThinLTOPrelink phase as it makes it hard
-    // for the profile annotation to be accurate in the ThinLTO backend.
-    if (Phase != ThinOrFullLTOPhase::ThinLTOPreLink)
+    // Do not invoke ICP in the LTOPrelink phase as it makes it hard
+    // for the profile annotation to be accurate in the LTO backend.
+    if (Phase != ThinOrFullLTOPhase::ThinLTOPreLink &&
+        Phase != ThinOrFullLTOPhase::FullLTOPreLink)
       // We perform early indirect call promotion here, before globalopt.
       // This is important for the ThinLTO backend phase because otherwise
       // imported available_externally functions look unreferenced and are
       // removed.
-      MPM.addPass(PGOIndirectCallPromotion(
-          Phase == ThinOrFullLTOPhase::ThinLTOPostLink, true /* SamplePGO */));
+      MPM.addPass(
+          PGOIndirectCallPromotion(true /* IsInLTO */, true /* SamplePGO */));
   }
 
   if (AttributorRun & AttributorRunOption::MODULE)
     MPM.addPass(AttributorPass());
 
   // Lower type metadata and the type.test intrinsic in the ThinLTO
   // post link pipeline after ICP. This is to enable usage of the type
   // tests in ICP sequences.