From 44addbd4e1fbf754092185cd74c08a1e283df80f Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov@redhat.com>
Date: Fri, 17 Jul 2026 09:51:47 +0200
Subject: [PATCH] [LTO] Sort DefinedGlobals in LTO cache key (#210025)

Sort the DefindeGlobals by GUID when computing the LTO cache key, to
avoid making the cache key dependent on the insertion order. This fixes
large compile-time regressions in rust incremental builds.

Alternatively one could make collectDefinedGVSummariesPerModule() work
on the sortedRange(), but as that is also used in other places, it
probably makes sense to sort locally.

The issue was introduced in 760bb06cb3cd98832f1e4e7a4eaebd98b66d2bdd.

(cherry picked from commit 90e31c46d005ae0a7f5968c38a9310bee499923c)
---
 llvm/lib/LTO/LTO.cpp                          | 11 +++++---
 .../X86/Inputs/cache-defined-globals-order.ll |  6 +++++
 .../X86/cache-defined-globals-order.ll        | 27 +++++++++++++++++++
 3 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/ThinLTO/X86/Inputs/cache-defined-globals-order.ll
 create mode 100644 llvm/test/ThinLTO/X86/cache-defined-globals-order.ll

diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index b0fd1d77a2d66..c8c5b0880819a 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -304,9 +304,14 @@ std::string llvm::computeLTOCacheKey(
     }
   };
 
-  // Include the hash for the linkage type to reflect internalization and weak
-  // resolution, and collect any used type identifier resolutions.
-  for (auto &GS : DefinedGlobals) {
+  // Sort the defined globals by GUID to be independent of the insertion order,
+  // which may depend on the order that modules are added.
+  SmallVector<std::pair<GlobalValue::GUID, GlobalValueSummary *>>
+      SortedDefinedGlobals(DefinedGlobals.begin(), DefinedGlobals.end());
+  llvm::sort(SortedDefinedGlobals, llvm::less_first());
+  for (auto &GS : SortedDefinedGlobals) {
+    // Include the hash for the linkage type to reflect internalization and weak
+    // resolution, and collect any used type identifier resolutions.
     GlobalValue::LinkageTypes Linkage = GS.second->linkage();
     Hasher.update(
         ArrayRef<uint8_t>((const uint8_t *)&Linkage, sizeof(Linkage)));
diff --git a/llvm/test/ThinLTO/X86/Inputs/cache-defined-globals-order.ll b/llvm/test/ThinLTO/X86/Inputs/cache-defined-globals-order.ll
new file mode 100644
index 0000000000000..5a527c9bae924
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/Inputs/cache-defined-globals-order.ll
@@ -0,0 +1,6 @@
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define linkonce_odr void @shared29() {
+  ret void
+}
diff --git a/llvm/test/ThinLTO/X86/cache-defined-globals-order.ll b/llvm/test/ThinLTO/X86/cache-defined-globals-order.ll
new file mode 100644
index 0000000000000..84f71f20f9e6d
--- /dev/null
+++ b/llvm/test/ThinLTO/X86/cache-defined-globals-order.ll
@@ -0,0 +1,27 @@
+; RUN: rm -rf %t && mkdir -p %t
+; RUN: opt -module-hash -module-summary %s -o %t/t.bc
+; RUN: opt -module-hash -module-summary %S/Inputs/cache-defined-globals-order.ll -o %t/a.bc
+
+; Tests that the LTO cache key is insensitive to the order of the modules.
+; The linkonce_odr function @shared29 is defined in both t.bc and a.bc, so
+; it gets a different position in the combined index depending on which
+; module is processed first, which results in a different insertion order
+; for DefinedGlobals.
+
+; RUN: llvm-lto2 run -cache-dir %t/cache -o %t.o %t/t.bc %t/a.bc -r=%t/t.bc,main,plx -r=%t/t.bc,shared29,plx -r=%t/a.bc,shared29,lx
+; RUN: ls %t/cache | count 2
+
+; RUN: llvm-lto2 run -cache-dir %t/cache -o %t.o %t/a.bc %t/t.bc -r=%t/t.bc,main,plx -r=%t/t.bc,shared29,plx -r=%t/a.bc,shared29,lx
+; RUN: ls %t/cache | count 2
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @main() {
+  call void @shared29()
+  ret void
+}
+
+define linkonce_odr void @shared29() {
+  ret void
+}