[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, 15 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 0a88b23..b7c1743 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1022,11 +1022,21 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
       // Emit a lifetime intrinsic if meaningful. There's no point in doing this
       // if we don't have a valid insertion point (?).
       if (HaveInsertPoint() && !IsMSCatchParam) {
-        // goto or switch-case statements can break lifetime into several
-        // regions which need more efforts to handle them correctly. PR28267
-        // This is rare case, but it's better just omit intrinsics than have
-        // them incorrectly placed.
-        if (!Bypasses.IsBypassed(&D)) {
+        // If there's a jump into the lifetime of this variable, its lifetime
+        // gets broken up into several regions in IR, which requires more work
+        // to handle correctly. For now, just omit the intrinsics; this is a
+        // rare case, and it's better to just be conservatively correct.
+        // PR28267.
+        //
+        // We have to do this in all language modes if there's a jump past the
+        // declaration. We also have to do it in C if there's a jump to an
+        // earlier point in the current block because non-VLA lifetimes begin as
+        // soon as the containing block is entered, not when its variables
+        // actually come into scope; suppressing the lifetime annotations
+        // completely in this case is unnecessarily pessimistic, but again, this
+        // is rare.
+        if (!Bypasses.IsBypassed(&D) &&
+            !(!getLangOpts().CPlusPlus && hasLabelBeenSeenInCurrentScope())) {
           uint64_t size = CGM.getDataLayout().getTypeAllocSize(allocaTy);
           emission.SizeForLifetimeMarkers =
               EmitLifetimeStart(size, address.getPointer());