The LifetimeExtendedCleanupHeader is carefully fit into 32 bytes,
meaning that cleanups on the LifetimeExtendedCleanupStack are *always*
allocated at a misaligned address and cause undefined behaviour.
There are two ways to solve this - add padding after the header when
we allocated our cleanups, or just simplify the header and let it use
64 bits in the first place. I've opted for the latter, and added a
static assert to avoid the issue in the future.
This does waste some space in the 32-bit case. It wouldn't be too hard
to make the size of this struct dependent on the host machine if
that's an issue. WDYT?
commit 1f1d41a60f5106e9e89fbc917bfc0826b5caad02
Author: Justin Bogner <m...@justinbogner.com>
Date: Mon Jun 22 16:02:17 2015 -0700
CodeGen: Resize LifetimeExtendedCleanupHeader to avoid alignment issues
The LifetimeExtendedCleanupHeader is carefully fit into 32 bytes,
meaning that cleanups on the LifetimeExtendedCleanupStack are *always*
allocated at a misaligned address and cause undefined behaviour.
There are two ways to solve this - add padding after the header when
we allocated our cleanups, or just simplify the header and let it use
64 bits in the first place. I've opted for the latter, and added a
static assert to avoid the issue in the future.
diff --git a/lib/CodeGen/CodeGenFunction.h b/lib/CodeGen/CodeGenFunction.h
index 45475d1..0891b5d 100644
--- a/lib/CodeGen/CodeGenFunction.h
+++ b/lib/CodeGen/CodeGenFunction.h
@@ -283,12 +283,12 @@ public:
/// Header for data within LifetimeExtendedCleanupStack.
struct LifetimeExtendedCleanupHeader {
/// The size of the following cleanup object.
- unsigned Size : 29;
+ unsigned Size;
/// The kind of cleanup to push: a value from the CleanupKind enumeration.
- unsigned Kind : 3;
+ CleanupKind Kind;
- size_t getSize() const { return size_t(Size); }
- CleanupKind getKind() const { return static_cast<CleanupKind>(Kind); }
+ size_t getSize() const { return Size; }
+ CleanupKind getKind() const { return Kind; }
};
/// i32s containing the indexes of the cleanup destinations.
@@ -388,6 +388,8 @@ public:
LifetimeExtendedCleanupStack.resize(
LifetimeExtendedCleanupStack.size() + sizeof(Header) + Header.Size);
+ static_assert(sizeof(Header) % llvm::AlignOf<T>::Alignment == 0,
+ "Cleanup will be allocated on misaligned address");
char *Buffer = &LifetimeExtendedCleanupStack[OldSize];
new (Buffer) LifetimeExtendedCleanupHeader(Header);
new (Buffer + sizeof(Header)) T(A...);
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