[llvm] ThinLTO: Add flag to print uselistorder in bitcode writer pass (PR #133230)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm updated https://github.com/llvm/llvm-project/pull/133230

>From f323086aa09676f496557be563ead90e88238d41 Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Thu, 27 Mar 2025 17:00:57 +0700
Subject: [PATCH] ThinLTO: Add flag to print uselistorder in bitcode writer
 pass

This is needed in llvm-reduce to avoid perturbing the uselistorder in
intermediate steps. Really llvm-reduce wants pure serialization with
no dependency on the pass manager. There are other optimizations mixed
in to the serialization here depending on metadata in the module, which
is also bad.

Part of #63621
---
 .../Transforms/IPO/ThinLTOBitcodeWriter.h     |  7 +++++--
 .../Transforms/IPO/ThinLTOBitcodeWriter.cpp   | 18 ++++++++++-------
 .../Bitcode/thinlto-preserve-uselistorder.ll  | 20 +++++++++++++++++++
 llvm/tools/opt/NewPMDriver.cpp                |  3 ++-
 4 files changed, 38 insertions(+), 10 deletions(-)
 create mode 100644 llvm/test/Bitcode/thinlto-preserve-uselistorder.ll

diff --git a/llvm/include/llvm/Transforms/IPO/ThinLTOBitcodeWriter.h b/llvm/include/llvm/Transforms/IPO/ThinLTOBitcodeWriter.h
index 9bcb01c9dbe43..b8fed10404dfa 100644
--- a/llvm/include/llvm/Transforms/IPO/ThinLTOBitcodeWriter.h
+++ b/llvm/include/llvm/Transforms/IPO/ThinLTOBitcodeWriter.h
@@ -26,12 +26,15 @@ class ThinLTOBitcodeWriterPass
     : public PassInfoMixin<ThinLTOBitcodeWriterPass> {
   raw_ostream &OS;
   raw_ostream *ThinLinkOS;
+  const bool ShouldPreserveUseListOrder;
 
 public:
   // Writes bitcode to OS. Also write thin link file to ThinLinkOS, if
   // it's not nullptr.
-  ThinLTOBitcodeWriterPass(raw_ostream &OS, raw_ostream *ThinLinkOS)
-      : OS(OS), ThinLinkOS(ThinLinkOS) {}
+  ThinLTOBitcodeWriterPass(raw_ostream &OS, raw_ostream *ThinLinkOS,
+                           bool ShouldPreserveUseListOrder = false)
+      : OS(OS), ThinLinkOS(ThinLinkOS),
+        ShouldPreserveUseListOrder(ShouldPreserveUseListOrder) {}
 
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
 
diff --git a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
index cd0e412bdf353..c0bb641f23ed6 100644
--- a/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
+++ b/llvm/lib/Transforms/IPO/ThinLTOBitcodeWriter.cpp
@@ -274,7 +274,8 @@ static bool enableUnifiedLTO(Module &M) {
 // regular LTO bitcode file to OS.
 void splitAndWriteThinLTOBitcode(
     raw_ostream &OS, raw_ostream *ThinLinkOS,
-    function_ref<AAResults &(Function &)> AARGetter, Module &M) {
+    function_ref<AAResults &(Function &)> AARGetter, Module &M,
+    const bool ShouldPreserveUseListOrder) {
   std::string ModuleId = getUniqueModuleId(&M);
   if (ModuleId.empty()) {
     assert(!enableUnifiedLTO(M));
@@ -487,9 +488,9 @@ void splitAndWriteThinLTOBitcode(
   // be used in the backends, and use that in the minimized bitcode
   // produced for the full link.
   ModuleHash ModHash = {{0}};
-  W.writeModule(M, /*ShouldPreserveUseListOrder=*/false, &Index,
+  W.writeModule(M, ShouldPreserveUseListOrder, &Index,
                 /*GenerateHash=*/true, &ModHash);
-  W.writeModule(*MergedM, /*ShouldPreserveUseListOrder=*/false, &MergedMIndex);
+  W.writeModule(*MergedM, ShouldPreserveUseListOrder, &MergedMIndex);
   W.writeSymtab();
   W.writeStrtab();
   OS << Buffer;
@@ -530,13 +531,15 @@ bool hasTypeMetadata(Module &M) {
 
 bool writeThinLTOBitcode(raw_ostream &OS, raw_ostream *ThinLinkOS,
                          function_ref<AAResults &(Function &)> AARGetter,
-                         Module &M, const ModuleSummaryIndex *Index) {
+                         Module &M, const ModuleSummaryIndex *Index,
+                         const bool ShouldPreserveUseListOrder) {
   std::unique_ptr<ModuleSummaryIndex> NewIndex = nullptr;
   // See if this module has any type metadata. If so, we try to split it
   // or at least promote type ids to enable WPD.
   if (hasTypeMetadata(M)) {
     if (enableSplitLTOUnit(M)) {
-      splitAndWriteThinLTOBitcode(OS, ThinLinkOS, AARGetter, M);
+      splitAndWriteThinLTOBitcode(OS, ThinLinkOS, AARGetter, M,
+                                  ShouldPreserveUseListOrder);
       return true;
     }
     // Promote type ids as needed for index-based WPD.
@@ -564,7 +567,7 @@ bool writeThinLTOBitcode(raw_ostream &OS, raw_ostream *ThinLinkOS,
   // be used in the backends, and use that in the minimized bitcode
   // produced for the full link.
   ModuleHash ModHash = {{0}};
-  WriteBitcodeToFile(M, OS, /*ShouldPreserveUseListOrder=*/false, Index,
+  WriteBitcodeToFile(M, OS, ShouldPreserveUseListOrder, Index,
                      /*GenerateHash=*/true, &ModHash);
   // If a minimized bitcode module was requested for the thin link, only
   // the information that is needed by thin link will be written in the
@@ -591,7 +594,8 @@ llvm::ThinLTOBitcodeWriterPass::run(Module &M, ModuleAnalysisManager &AM) {
       [&FAM](Function &F) -> AAResults & {
         return FAM.getResult<AAManager>(F);
       },
-      M, &AM.getResult<ModuleSummaryIndexAnalysis>(M));
+      M, &AM.getResult<ModuleSummaryIndexAnalysis>(M),
+      ShouldPreserveUseListOrder);
 
   return Changed ? PreservedAnalyses::none() : PreservedAnalyses::all();
 }
diff --git a/llvm/test/Bitcode/thinlto-preserve-uselistorder.ll b/llvm/test/Bitcode/thinlto-preserve-uselistorder.ll
new file mode 100644
index 0000000000000..bdbef151f876e
--- /dev/null
+++ b/llvm/test/Bitcode/thinlto-preserve-uselistorder.ll
@@ -0,0 +1,20 @@
+; Check that thin lto bitcode respects preserve-bc-uselistorder
+; RUN: opt --preserve-bc-uselistorder --thinlto-bc --thinlto-split-lto-unit < %s | llvm-dis --preserve-ll-uselistorder | FileCheck %s
+
+; CHECK: uselistorder ptr @g, { 3, 2, 1, 0 }
+
+ at g = external global i32
+
+define void @func1() {
+  load i32, ptr @g
+  load i32, ptr @g
+  ret void
+}
+
+define void @func2() {
+  load i32, ptr @g
+  load i32, ptr @g
+  ret void
+}
+
+uselistorder ptr @g, { 3, 2, 1, 0 }
diff --git a/llvm/tools/opt/NewPMDriver.cpp b/llvm/tools/opt/NewPMDriver.cpp
index b0840bb5b392f..cff77bbd78ca3 100644
--- a/llvm/tools/opt/NewPMDriver.cpp
+++ b/llvm/tools/opt/NewPMDriver.cpp
@@ -517,7 +517,8 @@ bool llvm::runPassPipeline(
     break;
   case OK_OutputThinLTOBitcode:
     MPM.addPass(ThinLTOBitcodeWriterPass(
-        Out->os(), ThinLTOLinkOut ? &ThinLTOLinkOut->os() : nullptr));
+        Out->os(), ThinLTOLinkOut ? &ThinLTOLinkOut->os() : nullptr,
+        ShouldPreserveBitcodeUseListOrder));
     break;
   }