Add a Windows EH preparation pass that zaps resumes

[rnk]

Summary

If the personality is not a recognized MSVC personality function, this
pass delegates to the dwarf EH preparation pass. This chaining supports
people on *-windows-itanium or *-windows-gnu targets.

Currently this recognizes some personalities used by MSVC and turns
resume instructions into traps to avoid link errors. Even if cleanups
are not used in the source program, LLVM requires the frontend to emit a
code path that resumes unwinding after an exception. Clang does this,
and we get unreachable resume instructions. PR20300 covers cleaning up
these unreachable calls to resume.

[majnemer]

I don't see an IR->IR test, could we have one for this transform?

lib/CodeGen/WinEHPrepare.cpp

28–29

I wouldn't indent WinEHPrepare: http://llvm.org/docs/CodingStandards.html#namespace-indentation

78

LPads.back() perhaps?

I bet you intended on using this LPads vector in the future, right now it seems pretty useless though.

92–93

Why do you create a call to @llvm.trap and an unreachable instruction?

101

Does it make sense to do this unconditionally, even if the function has MSVC personality?

105

Does it make sense to do this unconditionally, even if the function has MSVC personality?

[rnk]

Seems like the IR to IR test worked... I'm surprised TargetMachine isn't null, causing DwarfEHPrepare crashes... but I'll take what I can get.

lib/CodeGen/WinEHPrepare.cpp

28–29

True, thanks. This was cribbed from DwarfEHPrepare.

78

Sure. I am collecting the landing pads for future use.

92–93

unreachable is typically not ud2a, but literally "nothing" on non-windows platforms, but I guess unreachable is fine on Windows. We'll get one ud2a.

101

Yes, this is whole module finalization, so some functions could be MSVC and some could be GCC.

105

ditto

[rnk]

• Add IR to IR test

[majnemer]

LGTM.

include/llvm/CodeGen/Passes.h

/// MachineDominanaceFrontier - This pass is a machine dominators analysis pass.
   /// instructions for correctness.
   ///
   FunctionPass *createMachineVerifierPass(const std::string& Banner);
 
   /// createDwarfEHPass - This pass mulches exception handling code into a form
   /// adapted to code generation.  Required if using dwarf exception handling.
   FunctionPass *createDwarfEHPass(const TargetMachine *TM);
 
+  /// createWinEHPass - Prepares personality functions used by MSVC on Windows,
+  /// in addition to the Itanium LSDA based personalities.
+  FunctionPass *createWinEHPass(const TargetMachine *TM);
+
   /// createSjLjEHPreparePass - This pass adapts exception handling code to use
   /// the GCC-style builtin setjmp/longjmp (sjlj) to handling EH control flow.
   ///
   FunctionPass *createSjLjEHPreparePass(const TargetMachine *TM);
 
   /// LocalStackSlotAllocation - This pass assigns local frame indices to stack
   /// slots relative to one another and allocates base registers to access them
   /// when it is estimated by the target to be out of range of normal frame

lib/CodeGen/CMakeLists.txt

add_llvm_library(LLVMCodeGen
   TargetLoweringBase.cpp
   TargetLoweringObjectFileImpl.cpp
   TargetOptionsImpl.cpp
   TargetRegisterInfo.cpp
   TargetSchedule.cpp
   TwoAddressInstructionPass.cpp
   UnreachableBlockElim.cpp
   VirtRegMap.cpp
+  WinEHPrepare.cpp
   )
 
 add_dependencies(LLVMCodeGen intrinsics_gen)
 
 add_subdirectory(SelectionDAG)
 add_subdirectory(AsmPrinter)

lib/CodeGen/Passes.cpp

case ExceptionHandling::SjLj:
     // catch info can get misplaced when a selector ends up more than one block
     // removed from the parent invoke(s). This could happen when a landing
     // pad is shared by multiple invokes and is also a target of a normal
     // edge from elsewhere.
     addPass(createSjLjEHPreparePass(TM));
     // FALLTHROUGH
   case ExceptionHandling::DwarfCFI:
   case ExceptionHandling::ARM:
-  case ExceptionHandling::WinEH:
     addPass(createDwarfEHPass(TM));
     break;
+  case ExceptionHandling::WinEH:
+    addPass(createWinEHPass(TM));
+    break;
   case ExceptionHandling::None:
     addPass(createLowerInvokePass());
 
     // The lower invoke pass may create unreachable code. Remove it.
     addPass(createUnreachableBlockEliminationPass());
     break;
   }
 }

lib/CodeGen/WinEHPrepare.cpp

+//===-- WinEHPrepare - Prepare exception handling for code generation ---===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass lowers LLVM IR exception handling into something closer to what the
+// backend wants. It snifs the personality function to see which kind of
+// preparation is necessary. If the personality function uses the Itanium LSDA,
+// this pass delegates to the DWARF EH preparation pass.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/Analysis/LibCallSemantics.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/Pass.h"
+#include "llvm/Target/TargetLowering.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "winehprepare"
+
+namespace {
+  class WinEHPrepare : public FunctionPass {
+    const TargetMachine *TM;
+    FunctionPass *DwarfPrepare;
+
+  public:
+    static char ID; // Pass identification, replacement for typeid.
+    WinEHPrepare(const TargetMachine *TM)
+        : FunctionPass(ID), TM(TM), DwarfPrepare(createDwarfEHPass(TM)) {
+      initializeDominatorTreeWrapperPassPass(*PassRegistry::getPassRegistry());
+    }
+
+    bool runOnFunction(Function &Fn) override;
+
+    bool doFinalization(Module &M) override;
+
+    void getAnalysisUsage(AnalysisUsage &AU) const override;
+
+    const char *getPassName() const override {
+      return "Windows exception handling preparation";
+    }
+  };
+} // end anonymous namespace
+
+char WinEHPrepare::ID = 0;
+
+FunctionPass *llvm::createWinEHPass(const TargetMachine *TM) {
+  return new WinEHPrepare(TM);
+}
+
+static bool isMSVCPersonality(EHPersonality Pers) {
+  return Pers == EHPersonality::MSVC_Win64SEH ||
+         Pers == EHPersonality::MSVC_CXX;
+}
+
+bool WinEHPrepare::runOnFunction(Function &Fn) {
+  SmallVector<LandingPadInst *, 4> LPads;
+  SmallVector<ResumeInst *, 4> Resumes;
+  for (BasicBlock &BB : Fn) {
+    if (auto *LP = BB.getLandingPadInst())
+      LPads.push_back(LP);
+    if (auto *Resume = dyn_cast<ResumeInst>(BB.getTerminator()))
+      Resumes.push_back(Resume);
+  }
+
+  // No need to prepare functions that lack landing pads.
+  if (LPads.empty())
+    return false;
+
+  // Classify the personality to see what kind of preparation we need.
+  EHPersonality Pers = ClassifyEHPersonality(LPads[0]->getPersonalityFn());
+
+  // Delegate through to the DWARF pass if this is unrecognized.
+  if (!isMSVCPersonality(Pers))
+    return DwarfPrepare->runOnFunction(Fn);
+
+  // FIXME: Cleanups are unimplemented. Replace them with calls to @llvm.trap.
+  if (Resumes.empty())
+    return false;
+
+  Function *Trap =
+      Intrinsic::getDeclaration(Fn.getParent(), Intrinsic::trap, None);
+  for (ResumeInst *Resume : Resumes) {
+    IRBuilder<> Builder(Resume);
+    Builder.CreateCall(Trap);
+    Builder.CreateUnreachable();
+    Resume->eraseFromParent();
+  }
+
+  return true;
+}
+
+bool WinEHPrepare::doFinalization(Module &M) {
+  return DwarfPrepare->doFinalization(M);
+}
+
+void WinEHPrepare::getAnalysisUsage(AnalysisUsage &AU) const {
+  DwarfPrepare->getAnalysisUsage(AU);
+}

test/CodeGen/X86/seh-safe-div.ll

 
 ; CHECK: cmpl $2, %[[sel]]
 
 ; CHECK: # %handler1
 ; CHECK: callq puts
 ; CHECK: movl $-2, [[rloc]]
 ; CHECK: jmp .LBB0_7
 
-; FIXME: EH preparation should not call _Unwind_Resume.
-; CHECK: callq _Unwind_Resume
+; FIXME: EH preparation should eliminate the 'resume' instr and we should not do
+; the previous 'cmp;jeq'.
 ; CHECK: ud2
 
 ; CHECK: .seh_handlerdata
 ; CHECK: .long 2
 ; CHECK: .long .Ltmp0@IMGREL
 ; CHECK: .long .Ltmp1@IMGREL+1
 ; CHECK: .long safe_div_filt0@IMGREL
 ; CHECK: .long .Ltmp3@IMGREL