[DwarfEHPrepare] Preserve Dominator Tree

[lebedev.ri]

Summary

Now that D94827 has flipped the switch, and SimplifyCFG is officially marked
as production-ready regarding Dominator Tree preservation,
we can update this user pass to also preserve Dominator Tree.

This is a geomean compile-time win of -0.05%..-0.08%.
https://llvm-compile-time-tracker.com/compare.php?from=51a25846c198cff00abad0936f975167357afa6f&to=082499aac236a5c141e50a9e77870d5be2de5f0b&stat=instructions

llvm/lib/CodeGen/DwarfEHPrepare.cpp

size_t DwarfEHPrepare::pruneUnreachableResumes(
   for (size_t I = 0, E = Resumes.size(); I < E; ++I) {
     ResumeInst *RI = Resumes[I];
     if (ResumeReachable[I]) {
       Resumes[ResumesLeft++] = RI;
     } else {
       BasicBlock *BB = RI->getParent();
       new UnreachableInst(Ctx, RI);
       RI->eraseFromParent();
-      simplifyCFG(BB, *TTI, RequireAndPreserveDomTree ? DTU : nullptr);
+      simplifyCFG(BB, *TTI, DTU);
     }
   }
   Resumes.resize(ResumesLeft);
   return ResumesLeft;
 }
 
 bool DwarfEHPrepare::InsertUnwindResumeCalls() {
   SmallVector<ResumeInst *, 16> Resumes;
bool DwarfEHPrepare::InsertUnwindResumeCalls() {
   // Call the function.
   CallInst *CI = CallInst::Create(RewindFunction, PN, "", UnwindBB);
   CI->setCallingConv(TLI.getLibcallCallingConv(RTLIB::UNWIND_RESUME));
 
   // We never expect _Unwind_Resume to return.
   CI->setDoesNotReturn();
   new UnreachableInst(Ctx, UnwindBB);
 
-  if (DTU && RequireAndPreserveDomTree)
+  if (DTU)
     DTU->applyUpdates(Updates);
 
   return true;
 }
 
 bool DwarfEHPrepare::run() {
-  assert(((OptLevel == CodeGenOpt::None || !RequireAndPreserveDomTree) ||
-          (DTU &&
-           DTU->getDomTree().verify(DominatorTree::VerificationLevel::Full))) &&
-         "Original domtree is invalid?");
-
   bool Changed = InsertUnwindResumeCalls();
 
-  assert(((OptLevel == CodeGenOpt::None || !RequireAndPreserveDomTree) ||
-          (DTU &&
-           DTU->getDomTree().verify(DominatorTree::VerificationLevel::Full))) &&
-         "Original domtree is invalid?");
-
   return Changed;
 }
 
 static bool prepareDwarfEH(CodeGenOpt::Level OptLevel,
                            FunctionCallee &RewindFunction, Function &F,
                            const TargetLowering &TLI, DominatorTree *DT,
                            const TargetTransformInfo *TTI) {
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
 
   return DwarfEHPrepare(OptLevel, RewindFunction, F, TLI, DT ? &DTU : nullptr,
                         TTI)
       .run();
 }
 
 namespace {
 
DwarfEHPrepareLegacyPass(CodeGenOpt::Level OptLevel = CodeGenOpt::Default)
       : FunctionPass(ID), OptLevel(OptLevel) {}
 
   bool runOnFunction(Function &F) override {
     const TargetMachine &TM =
         getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
     const TargetLowering &TLI = *TM.getSubtargetImpl(F)->getTargetLowering();
     DominatorTree *DT = nullptr;
     const TargetTransformInfo *TTI = nullptr;
+    if (auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>())
+      DT = &DTWP->getDomTree();
     if (OptLevel != CodeGenOpt::None) {
+      if (!DT)
         DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     }
     return prepareDwarfEH(OptLevel, RewindFunction, F, TLI, DT, TTI);
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.addRequired<TargetPassConfig>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
     if (OptLevel != CodeGenOpt::None) {
       AU.addRequired<DominatorTreeWrapperPass>();
       AU.addRequired<TargetTransformInfoWrapperPass>();
-      if (RequireAndPreserveDomTree)
-        AU.addPreserved<DominatorTreeWrapperPass>();
     }
+    AU.addPreserved<DominatorTreeWrapperPass>();
   }
 
   StringRef getPassName() const override {
     return "Exception handling preparation";
   }
 };
 
 } // end anonymous namespace

llvm/test/CodeGen/ARM/O3-pipeline.ll

 ; CHECK-NEXT:      Dominator Tree Construction
 ; CHECK-NEXT:      Natural Loop Information
 ; CHECK-NEXT:      CodeGen Prepare
 ; CHECK-NEXT:    Rewrite Symbols
 ; CHECK-NEXT:    FunctionPass Manager
 ; CHECK-NEXT:      Dominator Tree Construction
 ; CHECK-NEXT:      Exception handling preparation
 ; CHECK-NEXT:      Merge internal globals
-; CHECK-NEXT:      Dominator Tree Construction
 ; CHECK-NEXT:      Natural Loop Information
 ; CHECK-NEXT:      Scalar Evolution Analysis
 ; CHECK-NEXT:      Lazy Branch Probability Analysis
 ; CHECK-NEXT:      Lazy Block Frequency Analysis
 ; CHECK-NEXT:      Optimization Remark Emitter
 ; CHECK-NEXT:      Hardware Loop Insertion
 ; CHECK-NEXT:      Scalar Evolution Analysis
 ; CHECK-NEXT:      Loop Pass Manager

llvm/test/CodeGen/X86/opt-pipeline.ll

 ; CHECK-NEXT:       CodeGen Prepare
 ; CHECK-NEXT:     Rewrite Symbols
 ; CHECK-NEXT:     FunctionPass Manager
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Exception handling preparation
 ; CHECK-NEXT:       Safe Stack instrumentation pass
 ; CHECK-NEXT:       Insert stack protectors
 ; CHECK-NEXT:       Module Verifier
-; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Basic Alias Analysis (stateless AA impl)
 ; CHECK-NEXT:       Function Alias Analysis Results
 ; CHECK-NEXT:       Natural Loop Information
 ; CHECK-NEXT:       Post-Dominator Tree Construction
 ; CHECK-NEXT:       Branch Probability Analysis
 ; CHECK-NEXT:       Lazy Branch Probability Analysis
 ; CHECK-NEXT:       Lazy Block Frequency Analysis
 ; CHECK-NEXT:       X86 DAG->DAG Instruction Selection