[CGCilk] Fix code generation of unreachable _Cilk_sync statements. Fi…

…xes issue llvm#93.

clang/lib/CodeGen/CGCilk.cpp

@@ -431,11 +431,13 @@ llvm::Instruction *CodeGenFunction::EmitSyncRegionStart() {
 void CodeGenFunction::EmitCilkSyncStmt(const CilkSyncStmt &S) {
   llvm::BasicBlock *ContinueBlock = createBasicBlock("sync.continue");
 
-  // If this code is reachable then emit a stop point (if generating
-  // debug info). We have to do this ourselves because we are on the
-  // "simple" statement path.
-  if (HaveInsertPoint())
-    EmitStopPoint(&S);
+  // Check if we are generating unreachable code.
+  if (!HaveInsertPoint())
+    // We don't need to generate actual code.
+    return;
+
+  // Generate a stoppoint if we are emitting debug info.
+  EmitStopPoint(&S);
 
   EnsureSyncRegion();
 

clang/test/Cilk/unreachable-sync.cpp

@@ -0,0 +1,18 @@
+// Check that a sync is not inserted when clang recognizes that it is not reachable.
+//
+// RUN: %clang_cc1 -fopencilk -ftapir=none -triple x86_64-unknown-linux-gnu -std=c++11 -emit-llvm %s -o - | FileCheck %s
+int create() {
+  return 1;
+  _Cilk_sync;
+}
+
+// CHECK: define dso_local {{.*}}i32 @_Z6createv()
+// CHECK: {
+// CHECK: ret i32 1
+// CHECK-NOT: sync
+// CHECK: }
+
+int main([[maybe_unused]] int argc, char *argv[]) {
+  int e = create();
+  return 0;
+}