Fix a latent bug where, after emitting an expression statement, we would

delete the block we began emitting into if it had no predecessors.  We never
want to do this, because there are several valid cases during statement
emission where an existing block has no known predecessors but will acquire
some later.  The case in my test case doesn't inherently fall into this 
category, because we could safely emit the case-range code before the statement
body, but there are examples with labels that can't be fallen into 
that would also demonstrate this bug.

rdar://problem/8837067



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123303 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 24 insertions, 8 deletions

diff --git a/lib/CodeGen/CGStmt.cpp b/lib/CodeGen/CGStmt.cpp
index dbef09f63a..b0278c7d02 100644
--- a/lib/CodeGen/CGStmt.cpp
+++ b/lib/CodeGen/CGStmt.cpp
@@ -89,18 +89,34 @@ void CodeGenFunction::EmitStmt(const Stmt *S) {
 #define EXPR(Type, Base) \
   case Stmt::Type##Class:
 #include "clang/AST/StmtNodes.inc"
+  {
+    // Remember the block we came in on.
+    llvm::BasicBlock *incoming = Builder.GetInsertBlock();
+    assert(incoming && "expression emission must have an insertion point");
+
     EmitIgnoredExpr(cast<Expr>(S));
 
-    // 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();
-      }
+    llvm::BasicBlock *outgoing = Builder.GetInsertBlock();
+    assert(outgoing && "expression emission cleared block!");
+
+    // The expression emitters assume (reasonably!) that the insertion
+    // point is always set.  To maintain that, the call-emission code
+    // for noreturn functions has to enter a new block with no
+    // predecessors.  We want to kill that block and mark the current
+    // insertion point unreachable in the common case of a call like
+    // "exit();".  Since expression emission doesn't otherwise create
+    // blocks with no predecessors, we can just test for that.
+    // However, we must be careful not to do this to our incoming
+    // block, because *statement* emission does sometimes create
+    // reachable blocks which will have no predecessors until later in
+    // the function.  This occurs with, e.g., labels that are not
+    // reachable by fallthrough.
+    if (incoming != outgoing && outgoing->use_empty()) {
+      outgoing->eraseFromParent();
+      Builder.ClearInsertionPoint();
     }
     break;
+  }
 
   case Stmt::IndirectGotoStmtClass:
     EmitIndirectGotoStmt(cast<IndirectGotoStmt>(*S)); break;