[llvm-branch-commits] [llvm] [llvm][EmbedBitcodePass] Prevent modifying the module with ThinLTO (PR #139999)

[Paul Kirth via llvm-branch-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/139999

Since ThinLTOBitcodeWriterPass handles many things for CFI and WPD, like
updating vtable linkage, we need to prevent those changes from
persisting in the non-LTO object code we will compile under FatLTO.

The only non-invasive way to do that is to clone the module when
serializing the module in ThinLTOBitcodeWriterPass. We may be able to
avoid cloning in the future with additional infrastructure to restore
the IR to its original state.

Fixes #139440

>From 0bc0cc2c64a7c7a347cf71888e9a57339537ce25 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Wed, 14 May 2025 20:47:26 -0700
Subject: [PATCH] [llvm][EmbedBitcodePass] Prevent modifying the module with
 ThinLTO

Since ThinLTOBitcodeWriterPass handles many things for CFI and WPD, like
updating vtable linkage, we need to prevent those changes from
persisting in the non-LTO object code we will compile under FatLTO.

The only non-invasive way to do that is to clone the module when
serializing the module in ThinLTOBitcodeWriterPass. We may be able to
avoid cloning in the future with additional infrastructure to restore
the IR to its original state.

Fixes #139440
---
 llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp   | 6 +++++-
 llvm/test/Transforms/EmbedBitcode/embed-wpd.ll | 7 ++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
index 73f567734a91b..7a378c61cb899 100644
--- a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
+++ b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Triple.h"
 #include "llvm/Transforms/IPO/ThinLTOBitcodeWriter.h"
+#include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/ModuleUtils.h"
 
 #include <string>
@@ -33,8 +34,11 @@ PreservedAnalyses EmbedBitcodePass::run(Module &M, ModuleAnalysisManager &AM) {
 
   std::string Data;
   raw_string_ostream OS(Data);
+  // Clone the module with with Thin LTO, since ThinLTOBitcodeWriterPass changes
+  // vtable linkage that would break the non-lto object code for FatLTO.
   if (IsThinLTO)
-    ThinLTOBitcodeWriterPass(OS, /*ThinLinkOS=*/nullptr).run(M, AM);
+    ThinLTOBitcodeWriterPass(OS, /*ThinLinkOS=*/nullptr)
+        .run(*llvm::CloneModule(M), AM);
   else
     BitcodeWriterPass(OS, /*ShouldPreserveUseListOrder=*/false, EmitLTOSummary)
         .run(M, AM);
diff --git a/llvm/test/Transforms/EmbedBitcode/embed-wpd.ll b/llvm/test/Transforms/EmbedBitcode/embed-wpd.ll
index f1f7712f54039..54931be42b4eb 100644
--- a/llvm/test/Transforms/EmbedBitcode/embed-wpd.ll
+++ b/llvm/test/Transforms/EmbedBitcode/embed-wpd.ll
@@ -1,12 +1,13 @@
 ; RUN: opt --mtriple x86_64-unknown-linux-gnu < %s -passes="embed-bitcode<thinlto>" -S | FileCheck %s
 
-; CHECK-NOT: $_ZTV3Foo = comdat any
+; CHECK: $_ZTV3Foo = comdat any
 $_ZTV3Foo = comdat any
 
 $_ZTI3Foo = comdat any
 
-; CHECK: @_ZTV3Foo = external hidden unnamed_addr constant { [5 x ptr] }, align 8
-; CHECK: @_ZTI3Foo = linkonce_odr hidden constant { ptr, ptr, ptr } { ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv120__si_class_type_infoE, i64 2), ptr @_ZTS3Foo, ptr @_ZTISt13runtime_error }, comdat, align 8
+;; ThinLTOBitcodeWriter will remove the vtable for Foo, and make it an external symbol
+; CHECK: @_ZTV3Foo = linkonce_odr hidden unnamed_addr constant { [5 x ptr] } { [5 x ptr] [ptr null, ptr @_ZTI3Foo, ptr @_ZN3FooD2Ev, ptr @_ZN3FooD0Ev, ptr @_ZNKSt13runtime_error4whatEv] }, comdat, align 8, !type !0, !type !1, !type !2, !type !3, !type !4, !type !5
+; CHECK-NOT: @foo = external unnamed_addr constant { [5 x ptr] }, align 8
 ; CHECK: @llvm.embedded.object = private constant {{.*}}, section ".llvm.lto", align 1
 ; CHECK: @llvm.compiler.used = appending global [1 x ptr] [ptr @llvm.embedded.object], section "llvm.metadata"
 @_ZTV3Foo = linkonce_odr hidden unnamed_addr constant { [5 x ptr] } { [5 x ptr] [ptr null, ptr @_ZTI3Foo, ptr @_ZN3FooD2Ev, ptr @_ZN3FooD0Ev, ptr @_ZNKSt13runtime_error4whatEv] }, comdat, align 8, !type !0, !type !1, !type !2, !type !3, !type !4, !type !5