IR: Update references to temporaries before deleting

During `MDNode::deleteTemporary()`, call `replaceAllUsesWith(nullptr)`
to update all tracking references to `nullptr`.

This fixes PR22280, where inverted destruction order between tracking
references and the temporaries themselves caused a use-after-free in
`LLParser`.

An alternative fix would be to add an assertion that there are no users,
and continue to fix inverted destruction order in clients (like
`LLParser`), but instead I decided to make getting-teardown-right easy.
(If someone disagrees let me know.)

llvm-svn: 226866

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index 6c8f71ffa99..fac9268f786 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -788,6 +788,7 @@ GenericDwarfNode *GenericDwarfNode::getImpl(LLVMContext &Context, unsigned Tag,
 
 void MDNode::deleteTemporary(MDNode *N) {
   assert(N->isTemporary() && "Expected temporary node");
+  N->replaceAllUsesWith(nullptr);
   N->deleteAsSubclass();
 }