Patch by Vadim Chugunov

Win64 stack unwinder gets confused when execution flow "falls through" after
a call to 'noreturn' function. This fixes the "missing epilogue" problem by 
emitting a trap instruction for IR 'unreachable' on x86_x64-pc-windows.

A secondary use for it would be for anyone wanting to make double-sure that
'noreturn' functions, indeed, do not return.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206684 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetOptions.h

@@ -51,6 +51,7 @@ namespace llvm {
           EnableFastISel(false), PositionIndependentExecutable(false),
           UseInitArray(false),
           DisableIntegratedAS(false), CompressDebugSections(false),
+          TrapUnreachable(false),
           TrapFuncName(""), FloatABIType(FloatABI::Default),
           AllowFPOpFusion(FPOpFusion::Standard) {}
 
@@ -162,6 +163,9 @@ namespace llvm {
     /// Compress DWARF debug sections.
     unsigned CompressDebugSections : 1;
 
+    /// Emit target-specific trap instruction for 'unreachable' IR instructions.
+    unsigned TrapUnreachable : 1;
+
     /// getTrapFunctionName - If this returns a non-empty string, this means
     /// isel should lower Intrinsic::trap to a call to the specified function
     /// name instead of an ISD::TRAP node.
@@ -216,6 +220,7 @@ inline bool operator==(const TargetOptions &LHS,
     ARE_EQUAL(EnableFastISel) &&
     ARE_EQUAL(PositionIndependentExecutable) &&
     ARE_EQUAL(UseInitArray) &&
+    ARE_EQUAL(TrapUnreachable) &&
     ARE_EQUAL(TrapFuncName) &&
     ARE_EQUAL(FloatABIType) &&
     ARE_EQUAL(AllowFPOpFusion);

lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1041,8 +1041,10 @@ FastISel::SelectOperator(const User *I, unsigned Opcode) {
   }
 
   case Instruction::Unreachable:
-    // Nothing to emit.
-    return true;
+    if (TM.Options.TrapUnreachable)
+      return FastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
+    else
+      return true;
 
   case Instruction::Alloca:
     // FunctionLowering has the static-sized case covered.

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -2765,6 +2765,11 @@ void SelectionDAGBuilder::visitIndirectBr(const IndirectBrInst &I) {
                           getValue(I.getAddress())));
 }
 
+void SelectionDAGBuilder::visitUnreachable(const UnreachableInst &I) {
+  if (DAG.getTarget().Options.TrapUnreachable)
+    DAG.setRoot(DAG.getNode(ISD::TRAP, getCurSDLoc(), MVT::Other, DAG.getRoot()));
+}
+
 void SelectionDAGBuilder::visitFSub(const User &I) {
   // -0.0 - X --> fneg
   Type *Ty = I.getType();

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h

@@ -642,7 +642,7 @@ private:
   void visitBr(const BranchInst &I);
   void visitSwitch(const SwitchInst &I);
   void visitIndirectBr(const IndirectBrInst &I);
-  void visitUnreachable(const UnreachableInst &I) { /* noop */ }
+  void visitUnreachable(const UnreachableInst &I);
 
   // Helpers for visitSwitch
   bool handleSmallSwitchRange(CaseRec& CR,

lib/Target/X86/X86TargetMachine.cpp

@@ -108,6 +108,13 @@ X86TargetMachine::X86TargetMachine(const Target &T, StringRef TT,
   if (Options.FloatABIType == FloatABI::Default)
     this->Options.FloatABIType = FloatABI::Hard;
 
+  // Windows stack unwinder gets confused when execution flow "falls through"
+  // after a call to 'noreturn' function.
+  // To prevent that, we emit a trap for 'unreachable' IR instructions.
+  // (which on X86, happens to be the 'ud2' instruction)
+  if (Subtarget.isTargetWin64())
+    this->Options.TrapUnreachable = true;
+
   initAsmInfo();
 }
 

test/CodeGen/X86/br-fold.ll

@@ -1,7 +1,19 @@
-; RUN: llc -march=x86-64 < %s | FileCheck %s
+; RUN: llc -mtriple=x86_64-apple-darwin < %s | FileCheck  -check-prefix=X64_DARWIN %s
+; RUN: llc -mtriple=x86_64-pc-linux < %s | FileCheck  -check-prefix=X64_LINUX %s
+; RUN: llc -mtriple=x86_64-pc-windows < %s | FileCheck  -check-prefix=X64_WINDOWS %s
+; RUN: llc -mtriple=x86_64-pc-windows-gnu < %s | FileCheck  -check-prefix=X64_WINDOWS_GNU %s
 
-; CHECK: orq
-; CHECK-NEXT: %bb8.i329
+; X64_DARWIN: orq
+; X64_DARWIN-NEXT: %bb8.i329
+
+; X64_LINUX: orq %rax, %rcx
+; X64_LINUX-NEXT: %bb8.i329
+
+; X64_WINDOWS: orq %rax, %rcx
+; X64_WINDOWS-NEXT: ud2
+
+; X64_WINDOWS_GNU: orq %rax, %rcx
+; X64_WINDOWS_GNU-NEXT: ud2
 
 @_ZN11xercesc_2_513SchemaSymbols21fgURI_SCHEMAFORSCHEMAE = external constant [33 x i16], align 32 ; <[33 x i16]*> [#uses=1]
 @_ZN11xercesc_2_56XMLUni16fgNotationStringE = external constant [9 x i16], align 16 ; <[9 x i16]*> [#uses=1]