[hot-cold-split] Only perform splitting in ThinLTO backend post-link

[tejohnson]

Summary

Fix the new PM to only perform hot cold splitting once during ThinLTO,
by skipping it in the pre-link phase.

This was already fixed in the old PM by the move of the hot cold split
pass later (after the early return when PrepareForThinLTO) by r344869.

[vsk]

Thank you! For completeness, it might help to have a test for the case 'Phase = PostLink'. Happy to leave it to you to decide.

[tejohnson]

In D53611#1273295, @vsk wrote:

Thank you! For completeness, it might help to have a test for the case 'Phase = PostLink'. Happy to leave it to you to decide.

I did initially, but it caused a large number of perturbations of the expected sequence since hot cold splitting invalidated a bunch of analysis passes, so I decided just to test the change being made here.

lib/Passes/PassBuilder.cpp

PassBuilder::buildModuleSimplificationPipeline(OptimizationLevel Level,
   if (Level == O3)
     MainCGPipeline.addPass(ArgumentPromotionPass());
 
   // Lastly, add the core function simplification pipeline nested inside the
   // CGSCC walk.
   MainCGPipeline.addPass(createCGSCCToFunctionPassAdaptor(
       buildFunctionSimplificationPipeline(Level, Phase, DebugLogging)));
 
-  if (EnableHotColdSplit)
+  // We only want to do hot cold splitting once for ThinLTO, during the
+  // post-link ThinLTO.
+  if (EnableHotColdSplit && Phase != ThinLTOPhase::PreLink)
     MPM.addPass(HotColdSplittingPass());
 
   for (auto &C : CGSCCOptimizerLateEPCallbacks)
     C(MainCGPipeline, Level);
 
   // We wrap the CGSCC pipeline in a devirtualization repeater. This will try
   // to detect when we devirtualize indirect calls and iterate the SCC passes
   // in that case to try and catch knock-on inlining or function attrs

test/Other/new-pm-thinlto-defaults.ll

 ; RUN:     -passes='thinlto-pre-link<Os>,name-anon-globals' -S %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=CHECK-O,CHECK-Os,CHECK-PRELINK-O,CHECK-PRELINK-O-NODIS,CHECK-PRELINK-Os
 ; RUN: opt -disable-verify -debug-pass-manager \
 ; RUN:     -passes='thinlto-pre-link<Oz>,name-anon-globals' -S %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=CHECK-O,CHECK-Oz,CHECK-PRELINK-O,CHECK-PRELINK-O-NODIS,CHECK-PRELINK-Oz
 ; RUN: opt -disable-verify -debug-pass-manager -new-pm-debug-info-for-profiling \
 ; RUN:     -passes='thinlto-pre-link<O2>,name-anon-globals' -S  %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=CHECK-DIS,CHECK-O,CHECK-O2,CHECK-PRELINK-O,CHECK-PRELINK-O2
+; Enabling the hot-cold-split pass should not affect the ThinLTO pre-link
+; RUN: opt -disable-verify -debug-pass-manager \
+; RUN:     -passes='thinlto-pre-link<O2>,name-anon-globals' -hot-cold-split -S  %s 2>&1 \
+; RUN:     | FileCheck %s --check-prefixes=CHECK-O,CHECK-O2,CHECK-PRELINK-O,CHECK-PRELINK-O-NODIS,CHECK-PRELINK-O2
 ;
 ; Postlink pipelines:
 ; RUN: opt -disable-verify -debug-pass-manager \
 ; RUN:     -passes='thinlto<O1>' -S %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=CHECK-O,CHECK-O1,CHECK-POSTLINK-O,CHECK-POSTLINK-O1
 ; RUN: opt -disable-verify -debug-pass-manager \
 ; RUN:     -passes='thinlto<O2>' -S  %s 2>&1 \
 ; RUN:     | FileCheck %s --check-prefixes=CHECK-O,CHECK-O2,CHECK-POSTLINK-O,CHECK-POSTLINK-O2