[CodeGen] Suppress emission of lifetime markers if a label has been seen

in the current lexical scope.

clang currently emits the lifetime.start marker of a variable when the
variable comes into scope even though a variable's lifetime starts at
the entry of the block with which it is associated, according to the C
standard. This normally doesn't cause any problems, but in the rare case
where a goto jumps backwards past the variable declaration to an earlier
point in the block (see the test case added to lifetime2.c), it can
cause mis-compilation.

To prevent such mis-compiles, this commit conservatively disables
emitting lifetime variables when a label has been seen in the current
block.

This problem was discussed on cfe-dev here: 
http://lists.llvm.org/pipermail/cfe-dev/2016-July/050066.html

rdar://problem/30153946

Differential Revision: https://reviews.llvm.org/D27680

llvm-svn: 293106

1 files changed, 11 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index 82ab5c3781e..8d7fd3f80ba 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -212,6 +212,13 @@ public:
   /// value. This is invalid iff the function has no return value.
   Address ReturnValue;
 
+  /// Return true if a label was seen in the current scope.
+  bool hasLabelBeenSeenInCurrentScope() const {
+    if (CurLexicalScope)
+      return CurLexicalScope->hasLabels();
+    return !LabelMap.empty();
+  }
+
   /// AllocaInsertPoint - This is an instruction in the entry block before which
   /// we prefer to insert allocas.
   llvm::AssertingVH<llvm::Instruction> AllocaInsertPt;
@@ -620,6 +627,10 @@ public:
         rescopeLabels();
     }
 
+    bool hasLabels() const {
+      return !Labels.empty();
+    }
+
     void rescopeLabels();
   };