4 files changed, 26 insertions, 15 deletions
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 13b097cfc60..f97edb4ef09 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1569,10 +1569,8 @@ bool FastISel::selectOperator(const User *I, unsigned Opcode) {
   }
 
   case Instruction::Unreachable:
-    if (TM.Options.TrapUnreachable)
-      return fastEmit_(MVT::Other, MVT::Other, ISD::TRAP) != 0;
-    else
-      return true;
+    // Nothing to emit.
+    return true;
 
   case Instruction::Alloca:
     // FunctionLowering has the static-sized case covered.
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 72af2760bb7..a48de572113 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2205,10 +2205,7 @@ 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::visitUnreachable(const UnreachableInst &I) {}
 
 void SelectionDAGBuilder::visitFSub(const User &I) {
   // -0.0 - X --> fneg
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 6fd04733e01..0ed196a4acb 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2945,6 +2945,20 @@ static SDValue getMOVL(SelectionDAG &DAG, SDLoc dl, EVT VT, SDValue V1,
   return DAG.getVectorShuffle(VT, dl, V1, V2, &Mask[0]);
 }
 
+/// Check if the fall through instruction after a call site is unreachable.
+/// FIXME: This will fail if there are interesting non-code generating IR
+/// instructions between the call and the unreachable (lifetime.end). In
+/// practice, this should be rare because optimizations like to delete non-call
+/// code before unreachable.
+static bool isCallFollowedByUnreachable(ImmutableCallSite CS) {
+  const Instruction *NextInst;
+  if (auto *II = dyn_cast<InvokeInst>(CS.getInstruction()))
+    NextInst = II->getNormalDest()->getFirstNonPHIOrDbg();
+  else
+    NextInst = CS.getInstruction()->getNextNode();
+  return isa<UnreachableInst>(NextInst);
+}
+
 SDValue
 X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
                              SmallVectorImpl<SDValue> &InVals) const {
@@ -3450,6 +3464,15 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     InFlag = Chain.getValue(1);
   }
 
+  if (Subtarget->isTargetWin64() && CLI.CS) {
+    // Look for a call followed by unreachable. On Win64, we need to ensure that
+    // the call does not accidentally fall through to something that looks like
+    // an epilogue. We do this by inserting a DEBUGTRAP, which lowers to int3,
+    // which is what MSVC emits after noreturn calls.
+    if (isCallFollowedByUnreachable(*CLI.CS))
+      Chain = DAG.getNode(ISD::DEBUGTRAP, dl, MVT::Other, Chain);
+  }
+
   // Handle result values, copying them out of physregs into vregs that we
   // return.
   return LowerCallResult(Chain, InFlag, CallConv, isVarArg,
diff --git a/llvm/lib/Target/X86/X86TargetMachine.cpp b/llvm/lib/Target/X86/X86TargetMachine.cpp
index 2e869eb7c3c..d318244e0f7 100644
--- a/llvm/lib/Target/X86/X86TargetMachine.cpp
+++ b/llvm/lib/Target/X86/X86TargetMachine.cpp
@@ -110,13 +110,6 @@ X86TargetMachine::X86TargetMachine(const Target &T, const Triple &TT,
                         OL),
       TLOF(createTLOF(getTargetTriple())),
       Subtarget(TT, CPU, FS, *this, Options.StackAlignmentOverride) {
-  // 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;
-
   // By default (and when -ffast-math is on), enable estimate codegen for
   // everything except scalar division. By default, use 1 refinement step for
   // all operations. Defaults may be overridden by using command-line options.