Detect when the current generation point is unreachable after emitting

expressions.
 - This generally catches the important case of noreturn functions.

 - With the last two changes, we are down to 152 unreachable blocks emitted on
   403.gcc, vs the 1805 we started with.

llvm-svn: 76364

clang/lib/CodeGen/CGStmt.cpp

@@ -68,10 +68,19 @@ void CodeGenFunction::EmitStmt(const Stmt *S) {
   default:
     // Must be an expression in a stmt context.  Emit the value (to get
     // side-effects) and ignore the result.
-    if (const Expr *E = dyn_cast<Expr>(S)) {
-      EmitAnyExpr(E, 0, false, true);
-    } else {
+    if (!isa<Expr>(S))
       ErrorUnsupported(S, "statement");
+
+    EmitAnyExpr(cast<Expr>(S), 0, false, true);
+    
+    // Expression emitters don't handle unreachable blocks yet, so look for one
+    // explicitly here. This handles the common case of a call to a noreturn
+    // function.
+    if (llvm::BasicBlock *CurBB = Builder.GetInsertBlock()) {
+      if (CurBB->empty() && CurBB->use_empty()) {
+        CurBB->eraseFromParent();
+        Builder.ClearInsertionPoint();
+      }
     }
     break;
   case Stmt::IndirectGotoStmtClass:  

clang/test/CodeGen/unreachable.c

@@ -1,6 +1,7 @@
 // RUN: clang-cc -emit-llvm -o %t %s &&
 // RUN: grep '@unreachable' %t | count 0
 
+extern void abort() __attribute__((noreturn));
 extern int unreachable();
 
 int f0() {
@@ -24,3 +25,13 @@ int f2(int i) {
   a = i + 1;
   return a;
 }
+
+int f3(int i) {
+  if (i) {
+    return 0;
+  } else {
+    abort();
+  }
+  unreachable();
+  return 3;
+}