[CodeGen][DwarfEHPrepare] Preserve Dominator Tree

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

Differential Revision: https://reviews.llvm.org/D95548

Author: LebedevRI

llvm/lib/CodeGen/DwarfEHPrepare.cpp

@@ -153,7 +153,7 @@ size_t DwarfEHPrepare::pruneUnreachableResumes(
       BasicBlock *BB = RI->getParent();
       new UnreachableInst(Ctx, RI);
       RI->eraseFromParent();
-      simplifyCFG(BB, *TTI, RequireAndPreserveDomTree ? DTU : nullptr);
+      simplifyCFG(BB, *TTI, DTU);
     }
   }
   Resumes.resize(ResumesLeft);
@@ -242,33 +242,23 @@ bool DwarfEHPrepare::InsertUnwindResumeCalls() {
   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)
@@ -295,8 +285,11 @@ class DwarfEHPrepareLegacyPass : public FunctionPass {
     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) {
-      DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+      if (!DT)
+        DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
       TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     }
     return prepareDwarfEH(OptLevel, RewindFunction, F, TLI, DT, TTI);
@@ -308,9 +301,8 @@ class DwarfEHPrepareLegacyPass : public FunctionPass {
     if (OptLevel != CodeGenOpt::None) {
       AU.addRequired<DominatorTreeWrapperPass>();
       AU.addRequired<TargetTransformInfoWrapperPass>();
-      if (RequireAndPreserveDomTree)
-        AU.addPreserved<DominatorTreeWrapperPass>();
     }
+    AU.addPreserved<DominatorTreeWrapperPass>();
   }
 
   StringRef getPassName() const override {

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

@@ -55,7 +55,6 @@
 ; 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

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

@@ -69,7 +69,6 @@
 ; 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