From 3c87ab39a36b2811e7a562ede5c9e758247a3e52 Mon Sep 17 00:00:00 2001
From: Leonard Chan <leonardchan@google.com>
Date: Thu, 18 Jun 2026 18:28:53 +0000
Subject: [PATCH] rustc_llvm: Emit module summaries when using -Clto=fat

Currently, module summaries are only emitted with thin lto. If we would
link full/fat lto'd rust code against lto'd c++ code built with CFI (or
WPD), those passes would fail during the link step because the
participating rust modules are missing module summaries. Rust code does
not know at compile-time if it would be participating in some special
link which may require module summaries, so this PR ensures module
summaries are unconditionally emitted for full/fat lto, just like with
thin lto.

The WriteBitcodeToFile function just invokes the normal
BitcodeWriterPass under the hood, but doesn't provide a way to set the
argument for emitting module summaries. So this patch just adds the pass
directly and sets that argument.
---
 .../rustc_llvm/llvm-wrapper/PassWrapper.cpp   | 45 +++++++++++++------
 tests/run-make/fat-lto-module-summary/foo.rs  |  1 +
 .../run-make/fat-lto-module-summary/rmake.rs  | 10 +++++
 3 files changed, 42 insertions(+), 14 deletions(-)
 create mode 100644 tests/run-make/fat-lto-module-summary/foo.rs
 create mode 100644 tests/run-make/fat-lto-module-summary/rmake.rs

diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 5dcaa5f6f84b8..d6e9c72125597 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -924,7 +924,15 @@ extern "C" LLVMRustResult LLVMRustOptimize(
       MPM.addPass(ThinLTOBitcodeWriterPass(
           ThinLTODataOS, ThinLTOSummaryBufferRef ? &ThinLinkDataOS : nullptr));
     } else {
-      MPM.addPass(BitcodeWriterPass(ThinLTODataOS));
+      // Force Full LTO summary for embedded bitcode under Fat LTO.
+      //
+      // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
+      // metadata is defined and explicitly set to zero. Otherwise, the thin LTO block
+      // ID will be emitted.
+      auto *Zero = ConstantInt::get(Type::getInt32Ty(TheModule->getContext()), 0);
+      TheModule->addModuleFlag(Module::Error, "ThinLTO", Zero);
+      MPM.addPass(BitcodeWriterPass(ThinLTODataOS, /*ShouldPreserveUseListOrder=*/false,
+                                    /*EmitSummaryIndex=*/true));
     }
     *ThinLTOBufferRef = ThinLTOBuffer.release();
     if (ThinLTOSummaryBufferRef) {
@@ -1442,23 +1450,32 @@ extern "C" LLVMRustBuffer *LLVMRustModuleSerialize(LLVMModuleRef M,
   {
     auto OS = raw_string_ostream(Ret->data);
     {
+      PassBuilder PB;
+      LoopAnalysisManager LAM;
+      FunctionAnalysisManager FAM;
+      CGSCCAnalysisManager CGAM;
+      ModuleAnalysisManager MAM;
+      PB.registerModuleAnalyses(MAM);
+      PB.registerCGSCCAnalyses(CGAM);
+      PB.registerFunctionAnalyses(FAM);
+      PB.registerLoopAnalyses(LAM);
+      PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
+
+      ModulePassManager MPM;
       if (is_thin) {
-        PassBuilder PB;
-        LoopAnalysisManager LAM;
-        FunctionAnalysisManager FAM;
-        CGSCCAnalysisManager CGAM;
-        ModuleAnalysisManager MAM;
-        PB.registerModuleAnalyses(MAM);
-        PB.registerCGSCCAnalyses(CGAM);
-        PB.registerFunctionAnalyses(FAM);
-        PB.registerLoopAnalyses(LAM);
-        PB.crossRegisterProxies(LAM, FAM, CGAM, MAM);
-        ModulePassManager MPM;
         MPM.addPass(ThinLTOBitcodeWriterPass(OS, nullptr));
-        MPM.run(*unwrap(M), MAM);
       } else {
-        WriteBitcodeToFile(*unwrap(M), OS);
+        // Force Full LTO summary for embedded bitcode under Fat LTO.
+        //
+        // Note the bitcode writer will only emit the full LTO block ID if the "ThinLTO"
+        // metadata is defined and explicitly set to zero. Otherwise, the thin LTO block
+        // ID will be emitted.
+        auto *Zero = ConstantInt::get(Type::getInt32Ty(unwrap(M)->getContext()), 0);
+        unwrap(M)->addModuleFlag(Module::Error, "ThinLTO", Zero);
+        MPM.addPass(BitcodeWriterPass(OS, /*ShouldPreserveUseListOrder=*/false,
+                                      /*EmitSummaryIndex=*/true));
       }
+      MPM.run(*unwrap(M), MAM);
     }
   }
   return Ret.release();
diff --git a/tests/run-make/fat-lto-module-summary/foo.rs b/tests/run-make/fat-lto-module-summary/foo.rs
new file mode 100644
index 0000000000000..b76b4321d62aa
--- /dev/null
+++ b/tests/run-make/fat-lto-module-summary/foo.rs
@@ -0,0 +1 @@
+pub fn foo() {}
diff --git a/tests/run-make/fat-lto-module-summary/rmake.rs b/tests/run-make/fat-lto-module-summary/rmake.rs
new file mode 100644
index 0000000000000..f04416c68fd96
--- /dev/null
+++ b/tests/run-make/fat-lto-module-summary/rmake.rs
@@ -0,0 +1,10 @@
+use run_make_support::{llvm_bcanalyzer, rustc};
+
+fn main() {
+    rustc().input("foo.rs").crate_type("lib").arg("-Clto=fat").arg("--emit=llvm-bc").run();
+
+    llvm_bcanalyzer()
+        .input("foo.bc")
+        .run()
+        .assert_stdout_contains("FULL_LTO_GLOBALVAL_SUMMARY_BLOCK");
+}