[CodeGen] Don't emit lifetime intrinsics for some local variables

Summary:
Current generation of lifetime intrinsics does not handle cases like:

```
  {
    char x;
  l1:
    bar(&x, 1);
  }
  goto l1;

```
We will get code like this:

```
  %x = alloca i8, align 1
  call void @llvm.lifetime.start(i64 1, i8* nonnull %x)
  br label %l1
l1:
  %call = call i32 @bar(i8* nonnull %x, i32 1)
  call void @llvm.lifetime.end(i64 1, i8* nonnull %x)
  br label %l1
```

So the second time bar was called for x which is marked as dead.
Lifetime markers here are misleading so it's better to remove them at all.
This type of bypasses are rare, e.g. code detects just 8 functions building
clang (2329 targets).

PR28267

Reviewers: eugenis

Subscribers: beanz, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D24693

llvm-svn: 285176

1 files changed, 11 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 6e48d14303b..7eaa6f62f0a 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1010,12 +1010,18 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
       bool IsMSCatchParam =
           D.isExceptionVariable() && getTarget().getCXXABI().isMicrosoft();
 
-      // Emit a lifetime intrinsic if meaningful.  There's no point
-      // in doing this if we don't have a valid insertion point (?).
+      // 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) {
-        uint64_t size = CGM.getDataLayout().getTypeAllocSize(allocaTy);
-        emission.SizeForLifetimeMarkers =
-          EmitLifetimeStart(size, address.getPointer());
+        // 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)) {
+          uint64_t size = CGM.getDataLayout().getTypeAllocSize(allocaTy);
+          emission.SizeForLifetimeMarkers =
+              EmitLifetimeStart(size, address.getPointer());
+        }
       } else {
         assert(!emission.useLifetimeMarkers());
       }