[DwarfEHPrepare] Don't prune unreachable resumes at optnone

Disable pruning of unreachable resumes in the DwarfEHPrepare pass
at optnone. While I expect the pruning itself to be essentially free,
this does require a dominator tree calculation, that is not used for
anything else. Saving this DT construction makes for a 0.4% O0
compile-time improvement.

Differential Revision: https://reviews.llvm.org/D80400

include/llvm/CodeGen/Passes.h

@@ -14,6 +14,7 @@
 #ifndef LLVM_CODEGEN_PASSES_H
 #define LLVM_CODEGEN_PASSES_H
 
+#include "llvm/Support/CodeGen.h"
 #include <functional>
 #include <string>
 
@@ -340,7 +341,7 @@ namespace llvm {
 
   /// createDwarfEHPass - This pass mulches exception handling code into a form
   /// adapted to code generation.  Required if using dwarf exception handling.
-  FunctionPass *createDwarfEHPass();
+  FunctionPass *createDwarfEHPass(CodeGenOpt::Level OptLevel);
 
   /// createWinEHPass - Prepares personality functions used by MSVC on Windows,
   /// in addition to the Itanium LSDA based personalities.

lib/CodeGen/DwarfEHPrepare.cpp

@@ -48,6 +48,7 @@ namespace {
     // RewindFunction - _Unwind_Resume or the target equivalent.
     FunctionCallee RewindFunction = nullptr;
 
+    CodeGenOpt::Level OptLevel;
     DominatorTree *DT = nullptr;
     const TargetLowering *TLI = nullptr;
 
@@ -61,7 +62,8 @@ namespace {
   public:
     static char ID; // Pass identification, replacement for typeid.
 
-    DwarfEHPrepare() : FunctionPass(ID) {}
+    DwarfEHPrepare(CodeGenOpt::Level OptLevel = CodeGenOpt::Default)
+      : FunctionPass(ID), OptLevel(OptLevel) {}
 
     bool runOnFunction(Function &Fn) override;
 
@@ -89,12 +91,15 @@ INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
 INITIALIZE_PASS_END(DwarfEHPrepare, DEBUG_TYPE,
                     "Prepare DWARF exceptions", false, false)
 
-FunctionPass *llvm::createDwarfEHPass() { return new DwarfEHPrepare(); }
+FunctionPass *llvm::createDwarfEHPass(CodeGenOpt::Level OptLevel) {
+  return new DwarfEHPrepare(OptLevel);
+}
 
 void DwarfEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addRequired<TargetPassConfig>();
   AU.addRequired<TargetTransformInfoWrapperPass>();
-  AU.addRequired<DominatorTreeWrapperPass>();
+  if (OptLevel != CodeGenOpt::None)
+    AU.addRequired<DominatorTreeWrapperPass>();
 }
 
 /// GetExceptionObject - Return the exception object from the value passed into
@@ -202,7 +207,10 @@ bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
 
   LLVMContext &Ctx = Fn.getContext();
 
-  size_t ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
+  size_t ResumesLeft = Resumes.size();
+  if (OptLevel != CodeGenOpt::None)
+    ResumesLeft = pruneUnreachableResumes(Fn, Resumes, CleanupLPads);
+
   if (ResumesLeft == 0)
     return true; // We pruned them all.
 
@@ -259,7 +267,8 @@ bool DwarfEHPrepare::InsertUnwindResumeCalls(Function &Fn) {
 bool DwarfEHPrepare::runOnFunction(Function &Fn) {
   const TargetMachine &TM =
       getAnalysis<TargetPassConfig>().getTM<TargetMachine>();
-  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+  DT = OptLevel != CodeGenOpt::None
+      ? &getAnalysis<DominatorTreeWrapperPass>().getDomTree() : nullptr;
   TLI = TM.getSubtargetImpl(Fn)->getTargetLowering();
   bool Changed = InsertUnwindResumeCalls(Fn);
   DT = nullptr;

lib/CodeGen/TargetPassConfig.cpp

@@ -729,14 +729,14 @@ void TargetPassConfig::addPassesToHandleExceptions() {
     LLVM_FALLTHROUGH;
   case ExceptionHandling::DwarfCFI:
   case ExceptionHandling::ARM:
-    addPass(createDwarfEHPass());
+    addPass(createDwarfEHPass(getOptLevel()));
     break;
   case ExceptionHandling::WinEH:
     // We support using both GCC-style and MSVC-style exceptions on Windows, so
     // add both preparation passes. Each pass will only actually run if it
     // recognizes the personality function.
     addPass(createWinEHPass());
-    addPass(createDwarfEHPass());
+    addPass(createDwarfEHPass(getOptLevel()));
     break;
   case ExceptionHandling::Wasm:
     // Wasm EH uses Windows EH instructions, but it does not need to demote PHIs

test/CodeGen/AArch64/O0-pipeline.ll

@@ -27,7 +27,6 @@
 ; CHECK-NEXT:       AArch64 Stack Tagging
 ; 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

test/CodeGen/X86/O0-pipeline.ll

@@ -29,7 +29,6 @@
 ; CHECK-NEXT:       Expand indirectbr instructions
 ; 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