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

Author: arsenm

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);
 

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
@@ -590,7 +593,8 @@ 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();
 }

llvm/test/Bitcode/thinlto-preserve-uselistorder.ll

@@ -0,0 +1,25 @@
+; Check that thin lto bitcode respects preserve-bc-uselistorder
+
+; Test without -thin-link-bitcode-file
+; RUN: opt --preserve-bc-uselistorder --thinlto-bc --thinlto-split-lto-unit < %s | llvm-dis --preserve-ll-uselistorder | FileCheck %s
+
+; Test with -thin-link-bitcode-file
+; RUN: opt --preserve-bc-uselistorder --thinlto-bc --thinlto-split-lto-unit -thin-link-bitcode-file=%t.thinlink.bc < %s | llvm-dis --preserve-ll-uselistorder | FileCheck %s
+
+; CHECK: uselistorder ptr @g, { 3, 2, 1, 0 }
+
+@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 }

llvm/tools/opt/NewPMDriver.cpp

@@ -519,7 +519,8 @@ bool llvm::runPassPipeline(
     break;
   case OK_OutputThinLTOBitcode:
     MPM.addPass(ThinLTOBitcodeWriterPass(
-        Out->os(), ThinLTOLinkOut ? &ThinLTOLinkOut->os() : nullptr));
+        Out->os(), ThinLTOLinkOut ? &ThinLTOLinkOut->os() : nullptr,
+        ShouldPreserveBitcodeUseListOrder));
     break;
   }