[ThinLTO] Fix a crash in lazy loading of Metadata

This is a revised version of D41474.

When the debug location is parsed in BitcodeReader::parseFunction, the
scope and inlinedAt MDNodes are obtained via MDLoader->getMDNodeFwdRefOrNull(),
which will create a forward ref if they were not yet loaded.
Specifically, if one of these MDNodes is in the module level metadata
block, and this is during ThinLTO importing, that metadata block is
lazily loaded.

Most places in that invoke getMDNodeFwdRefOrNull have a corresponding call
to resolveForwardRefsAndPlaceholders which will take care of resolving them.
E.g. places that call getMetadataFwdRefOrLoad, or at the end of parsing a
function-level metadata block, or at the end of the initial lazy load of
module level metadata in order to handle invocations of getMDNodeFwdRefOrNull
for named metadata and global object attachments. However, the calls for
the scope/inlinedAt of debug locations are not backed by any such call to
resolveForwardRefsAndPlaceholders.

To fix this, change the scope and inlinedAt parsing to instead use
getMetadataFwdRefOrLoad, which will ensure the forward refs to lazily
loaded metadata are resolved.

Fixes PR35472.

llvm-svn: 346891

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
index 3b1d54f82cb..5f5167c9a2e 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -3548,12 +3548,14 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
 
       MDNode *Scope = nullptr, *IA = nullptr;
       if (ScopeID) {
-        Scope = MDLoader->getMDNodeFwdRefOrNull(ScopeID - 1);
+        Scope = dyn_cast_or_null<MDNode>(
+            MDLoader->getMetadataFwdRefOrLoad(ScopeID - 1));
         if (!Scope)
           return error("Invalid record");
       }
       if (IAID) {
-        IA = MDLoader->getMDNodeFwdRefOrNull(IAID - 1);
+        IA = dyn_cast_or_null<MDNode>(
+            MDLoader->getMetadataFwdRefOrLoad(IAID - 1));
         if (!IA)
           return error("Invalid record");
       }