[PATCH] Emit a trap instruction for IR 'unreachable'

[Yaron Keren]

  I didn't use Phabricator correctly, hence the diff was split into two parts.
  Here is the complete diff, the combination of the preceding two.

Hi rnk, martell, vadimcn, Kai, nrieck, dblaikie, theraven, chandlerc,

http://reviews.llvm.org/D3417

CHANGE SINCE LAST DIFF
  http://reviews.llvm.org/D3417?vs=8632&id=8650#toc

Files:
  include/llvm/Target/TargetOptions.h
  lib/CodeGen/SelectionDAG/FastISel.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
  lib/Target/X86/X86TargetMachine.cpp

Index: include/llvm/Target/TargetOptions.h
===================================================================
--- include/llvm/Target/TargetOptions.h
+++ include/llvm/Target/TargetOptions.h
@@ -51,6 +51,7 @@
           EnableFastISel(false), PositionIndependentExecutable(false),
           UseInitArray(false),
           DisableIntegratedAS(false), CompressDebugSections(false),
+          TrapUnreachable(false),
           TrapFuncName(""), FloatABIType(FloatABI::Default),
           AllowFPOpFusion(FPOpFusion::Standard) {}
 
@@ -162,6 +163,9 @@
     /// 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 @@
     ARE_EQUAL(EnableFastISel) &&
     ARE_EQUAL(PositionIndependentExecutable) &&
     ARE_EQUAL(UseInitArray) &&
+    ARE_EQUAL(TrapUnreachable) &&
     ARE_EQUAL(TrapFuncName) &&
     ARE_EQUAL(FloatABIType) &&
     ARE_EQUAL(AllowFPOpFusion);
Index: lib/CodeGen/SelectionDAG/FastISel.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/FastISel.cpp
+++ lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1035,8 +1035,10 @@
   }
 
   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.
Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2765,6 +2765,11 @@
                           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();
Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
@@ -642,7 +642,7 @@
   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,
Index: lib/Target/X86/X86TargetMachine.cpp
===================================================================
--- lib/Target/X86/X86TargetMachine.cpp
+++ lib/Target/X86/X86TargetMachine.cpp
@@ -108,6 +108,13 @@
   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();
 }
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