[NPM] Support -fmerge-functions

[aeubanks]

Summary

I tried to put it in the same place in the pipeline as the legacy PM.

Fixes PR48399.

[asbirlea]

Thank you for adding this support.

[nikic]

llvm/lib/Passes/PassBuilder.cpp

1776

In the legacy PM this is placed after the AlwaysInlinerPass rather than before it: https://github.com/llvm/llvm-project/blob/57db6d20c6dadbc129f0bf842b2028e2716b67f6/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp#L536-L547

[aeubanks]

move -O0 MergeFunctions after inlining
add to LTO pipeline (matches legacy PM)

[nikic]

LGTM

clang/lib/CodeGen/BackendUtil.cpp

void EmitAssemblyHelper::EmitAssemblyWithNewPassManager(
 
   PipelineTuningOptions PTO;
   PTO.LoopUnrolling = CodeGenOpts.UnrollLoops;
   // For historical reasons, loop interleaving is set to mirror setting for loop
   // unrolling.
   PTO.LoopInterleaving = CodeGenOpts.UnrollLoops;
   PTO.LoopVectorization = CodeGenOpts.VectorizeLoop;
   PTO.SLPVectorization = CodeGenOpts.VectorizeSLP;
+  PTO.MergeFunctions = CodeGenOpts.MergeFunctions;
   // Only enable CGProfilePass when using integrated assembler, since
   // non-integrated assemblers don't recognize .cgprofile section.
   PTO.CallGraphProfile = !CodeGenOpts.DisableIntegratedAS;
   PTO.Coroutines = LangOpts.Coroutines;
 
   PassInstrumentationCallbacks PIC;
   StandardInstrumentations SI(CodeGenOpts.DebugPassManager);
   SI.registerCallbacks(PIC);

clang/test/CodeGenCXX/merge-functions.cpp

 // REQUIRES: x86-registered-target
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -O0 -fno-experimental-new-pass-manager -fmerge-functions -emit-llvm -o - -x c++ < %s | FileCheck %s -implicit-check-not=_ZN1A1gEiPi
 // RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -O1 -fno-experimental-new-pass-manager -fmerge-functions -emit-llvm -o - -x c++ < %s | FileCheck %s -implicit-check-not=_ZN1A1gEiPi
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -O0 -fexperimental-new-pass-manager -fmerge-functions -emit-llvm -o - -x c++ < %s | FileCheck %s -implicit-check-not=_ZN1A1gEiPi
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -O1 -fexperimental-new-pass-manager -fmerge-functions -emit-llvm -o - -x c++ < %s | FileCheck %s -implicit-check-not=_ZN1A1gEiPi
 
 // Basic functionality test. Function merging doesn't kick in on functions that
 // are too simple.
 
 struct A {
   virtual int f(int x, int *p) { return x ? *p : 1; }
   virtual int g(int x, int *p) { return x ? *p : 1; }
 } a;
 
 // CHECK: define {{.*}} @_ZN1A1fEiPi

llvm/include/llvm/Passes/PassBuilder.h

public:
 
   /// Tuning option to disable promotion to scalars in LICM with MemorySSA, if
   /// the number of access is too large.
   unsigned LicmMssaNoAccForPromotionCap;
 
   /// Tuning option to enable/disable call graph profile. Its default value is
   /// that of the flag: `-enable-npm-call-graph-profile`.
   bool CallGraphProfile;
+
+  /// Tuning option to enable/disable function merging. Its default value is
+  /// false.
+  bool MergeFunctions;
 };
 
 /// This class provides access to building LLVM's passes.
 ///
 /// Its members provide the baseline state available to passes during their
 /// construction. The \c PassRegistry.def file specifies how to construct all
 /// of the built-in passes, and those may reference these members during
 /// construction.

llvm/lib/Passes/PassBuilder.cpp

PipelineTuningOptions::PipelineTuningOptions() {
   LoopVectorization = true;
   SLPVectorization = false;
   LoopUnrolling = true;
   ForgetAllSCEVInLoopUnroll = ForgetSCEVInLoopUnroll;
   Coroutines = false;
   LicmMssaOptCap = SetLicmMssaOptCap;
   LicmMssaNoAccForPromotionCap = SetLicmMssaNoAccForPromotionCap;
   CallGraphProfile = true;
+  MergeFunctions = false;
 }
 
 extern cl::opt<bool> EnableConstraintElimination;
 extern cl::opt<bool> EnableGVNHoist;
 extern cl::opt<bool> EnableGVNSink;
 extern cl::opt<bool> EnableHotColdSplit;
 extern cl::opt<bool> EnableOrderFileInstrumentation;
 extern cl::opt<bool> EnableCHR;
PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   OptimizePM.addPass(AlignmentFromAssumptionsPass());
 
   // Split out cold code. Splitting is done late to avoid hiding context from
   // other optimizations and inadvertently regressing performance. The tradeoff
   // is that this has a higher code size cost than splitting early.
   if (EnableHotColdSplit && !LTOPreLink)
     MPM.addPass(HotColdSplittingPass());
 
+  // Merge functions if requested.
+  if (PTO.MergeFunctions)
+    MPM.addPass(MergeFunctionsPass());
+
   // LoopSink pass sinks instructions hoisted by LICM, which serves as a
   // canonicalization pass that enables other optimizations. As a result,
   // LoopSink pass needs to be a very late IR pass to avoid undoing LICM
   // result too early.
   OptimizePM.addPass(LoopSinkPass());
 
   // And finally clean up LCSSA form before generating code.
   OptimizePM.addPass(InstSimplifyPass());
ModulePassManager PassBuilder::buildO0DefaultPipeline(OptimizationLevel Level,
 
   if (PGOOpt && (PGOOpt->Action == PGOOptions::IRInstr ||
                  PGOOpt->Action == PGOOptions::IRUse))
     addPGOInstrPassesForO0(
         MPM,
         /* RunProfileGen */ (PGOOpt->Action == PGOOptions::IRInstr),
         /* IsCS */ false, PGOOpt->ProfileFile, PGOOpt->ProfileRemappingFile);
 
+  if (PTO.MergeFunctions)
+    MPM.addPass(MergeFunctionsPass());
+
   for (auto &C : PipelineStartEPCallbacks)
     C(MPM, Level);
   for (auto &C : PipelineEarlySimplificationEPCallbacks)
     C(MPM, Level);
 
   // Build a minimal pipeline based on the semantics required by LLVM,
   // which is just that always inlining occurs. Further, disable generating
   // lifetime intrinsics to avoid enabling further optimizations during