[llvm] [DWARFLinker] Make synthetic type names deterministic under threading (PR #209553)

[Jonas Devlieghere via llvm-commits]

https://github.com/JDevlieghere updated https://github.com/llvm/llvm-project/pull/209553

>From 37bd5cd6a12e3d8384c65b1761ed19a80c55156e Mon Sep 17 00:00:00 2001
From: Jonas Devlieghere <jonas at devlieghere.com>
Date: Tue, 14 Jul 2026 10:05:23 -0700
Subject: [PATCH] [DWARFLinker] Make synthetic type names deterministic under
 threading

The parallel DWARF linker assigns type names concurrently across
compile units. SyntheticTypeNameBuilder::addDIETypeName composes a
type's name from its referenced types (e.g. template arguments).

For a referenced type it either appends that type's fully-qualified
pool key, when the type has already been assigned a type entry, or
recomputes the name inline. The inline path only prepended the parent
scope when AssignNameToTypeDescriptor was set, so an embedded
reference came out fully-qualified when the referenced type happened
to be assigned already and unqualified otherwise.

Which branch runs depends on the order type names are assigned, and
that order races during the parallel assignment phase. The same type
then gets different synthetic names across runs, is entered into the
type pool under different keys, and no longer deduplicates
deterministically.

Two threaded runs of the same input:

  ...EventInstance:()<{1}CMEvent.{1}detail.{F}Group:()<...>>
  ...EventInstance:()<{F}Group:()<...>>

Always qualify with the parent scope regardless of
AssignNameToTypeDescriptor so the inline path agrees with the stored
key. Assignment to the type pool stays gated on the flag.

This removes essentially all of the run-to-run type-unit divergence
on a large threaded link.
---
 .../DWARFLinker/Parallel/SyntheticTypeNameBuilder.cpp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/DWARFLinker/Parallel/SyntheticTypeNameBuilder.cpp b/llvm/lib/DWARFLinker/Parallel/SyntheticTypeNameBuilder.cpp
index b5be5d498ed3e..c3d8a2489de9a 100644
--- a/llvm/lib/DWARFLinker/Parallel/SyntheticTypeNameBuilder.cpp
+++ b/llvm/lib/DWARFLinker/Parallel/SyntheticTypeNameBuilder.cpp
@@ -405,10 +405,13 @@ Error SyntheticTypeNameBuilder::addDIETypeName(
   // Check if DIE already has a name.
   if (!TypeEntryPtr) {
     size_t NameStart = SyntheticName.size();
-    if (AssignNameToTypeDescriptor) {
-      if (Error Err = addParentName(*UnitEntryPair))
-        return Err;
-    }
+    // Prepend the parent scope so this name matches the key the type is
+    // stored under in the pool (the getKey() branch below). Otherwise the
+    // same type gets different names depending on whether it already has a
+    // pool entry, which races under parallel assignment and breaks
+    // deterministic deduplication.
+    if (Error Err = addParentName(*UnitEntryPair))
+      return Err;
     addTypePrefix(UnitEntryPair->DieEntry);
 
     if (ChildIndex) {