Add FatLTO support for COFF #165529

thrimbor · 2025-10-29T09:04:45Z

This adds support for FatLTO to COFF targets in clang and lld.

The changes are adapted from 610fc5c and 14e3bec but much smaller because it just needed the COFF-specific parts wired in, and I tried my best to adapt the pre-existing ELF tests for the COFF version.

My main goal is to be able to use this for shipping pre-built https://github.com/XboxDev/nxdk container images someday, which uses the i386-pc-win32 target.

llvmbot · 2025-10-29T09:05:17Z

@llvm/pr-subscribers-llvm-transforms
@llvm/pr-subscribers-lld
@llvm/pr-subscribers-platform-windows

@llvm/pr-subscribers-clang-driver

Author: Stefan Schmidt (thrimbor)

Changes

This adds support for FatLTO to COFF targets in clang and lld.

The changes are adapted from 610fc5c and 14e3bec but much smaller because it just needed the COFF-specific parts wired in, and I tried my best to adapt the pre-existing ELF tests for the COFF version.

My main goal is to be able to use this for shipping pre-built https://github.com/XboxDev/nxdk container images someday, which uses the i386-pc-win32 target.

Full diff: https://github.com/llvm/llvm-project/pull/165529.diff

10 Files Affected:

(modified) clang/lib/Driver/ToolChains/Clang.cpp (+1-1)
(modified) clang/test/CodeGen/fat-lto-objects.c (+14)
(modified) clang/test/Driver/fat-lto-objects.c (+8)
(modified) lld/COFF/Config.h (+3)
(modified) lld/COFF/Driver.cpp (+33-2)
(modified) lld/COFF/Driver.h (+2)
(modified) lld/COFF/Options.td (+3)
(added) lld/test/COFF/fatlto/fatlto-invalid.s (+8)
(added) lld/test/COFF/fatlto/fatlto.test (+111)
(modified) llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp (+1-1)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 79edc561c551f..28cc91c620b0c 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7789,7 +7789,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                                options::OPT_fno_fat_lto_objects)) {
     if (IsUsingLTO && A->getOption().matches(options::OPT_ffat_lto_objects)) {
       assert(LTOMode == LTOK_Full || LTOMode == LTOK_Thin);
-      if (!Triple.isOSBinFormatELF()) {
+      if (!Triple.isOSBinFormatELF() && !Triple.isOSBinFormatCOFF()) {
         D.Diag(diag::err_drv_unsupported_opt_for_target)
             << A->getAsString(Args) << TC.getTripleString();
       }
diff --git a/clang/test/CodeGen/fat-lto-objects.c b/clang/test/CodeGen/fat-lto-objects.c
index 36a73684e7bfe..64ccd8ae75a4d 100644
--- a/clang/test/CodeGen/fat-lto-objects.c
+++ b/clang/test/CodeGen/fat-lto-objects.c
@@ -35,6 +35,9 @@
 // RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -fsplit-lto-unit -S < %s -o - \
 // RUN: | FileCheck %s --check-prefixes=ASM
 
+// RUN: %clang -cc1 -triple i386-pc-win32 -flto=full -ffat-lto-objects -fsplit-lto-unit -S < %s -o - \
+// RUN: | FileCheck %s --check-prefixes=ASM-COFF
+
 /// Make sure that FatLTO generates .llvm.lto sections that are the same as the output from normal LTO compilations
 // RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=full -ffat-lto-objects -c %s -o %t.fatlto.full.o
 // RUN: llvm-objcopy --dump-section=.llvm.lto=%t.fatlto.full.bc %t.fatlto.full.o
@@ -43,6 +46,13 @@
 // RUN: llvm-dis < %t.nofat.full.bc -o %t.nofat.full.ll
 // RUN: diff %t.fatlto.full.ll %t.nofat.full.ll
 
+// RUN: %clang -O2 --target=i386-pc-win32 -flto=full -ffat-lto-objects -c %s -o %t.fatlto.full.coff.o
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.fatlto.full.coff.bc %t.fatlto.full.coff.o
+// RUN: llvm-dis < %t.fatlto.full.coff.bc -o %t.fatlto.full.coff.ll
+// RUN: %clang -O2 --target=i386-pc-win32 -flto=full -c %s -o %t.nofat.full.coff.bc
+// RUN: llvm-dis < %t.nofat.full.coff.bc -o %t.nofat.full.coff.ll
+// RUN: diff %t.fatlto.full.coff.ll %t.nofat.full.coff.ll
+
 // RUN: %clang -O2 --target=x86_64-unknown-linux-gnu -fPIE -flto=thin -ffat-lto-objects -c %s -o %t.fatlto.thin.o
 // RUN: llvm-objcopy --dump-section=.llvm.lto=%t.fatlto.thin.bc %t.fatlto.thin.o
 // RUN: llvm-dis < %t.fatlto.thin.bc -o %t.fatlto.thin.ll
@@ -67,6 +77,10 @@
 // ASM-NEXT:        .asciz  "BC
 // ASM-NEXT: .size   .Lllvm.embedded.object
 
+//      ASM-COFF: .section .llvm.lto,"ynD"
+// ASM-COFF-NEXT: L_llvm.embedded.object:
+// ASM-COFF-NEXT:        .asciz  "BC
+
 const char* foo = "foo";
 
 int test(void) {
diff --git a/clang/test/Driver/fat-lto-objects.c b/clang/test/Driver/fat-lto-objects.c
index 7b87e2b468886..84dd945e208b9 100644
--- a/clang/test/Driver/fat-lto-objects.c
+++ b/clang/test/Driver/fat-lto-objects.c
@@ -1,10 +1,12 @@
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC
 // CHECK-CC: -cc1
 // CHECK-CC-SAME: -emit-obj
 // CHECK-CC-SAME: -ffat-lto-objects
 
 /// Without -flto -S will just emit normal ASM, so we don't expect -emit-{llvm,obj} or -ffat-lto-objects to be passed to cc1.
 // RUN: %clang --target=x86_64-unknown-linux-gnu -ffat-lto-objects -### %s -S 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S
+// RUN: %clang --target=i386-pc-win32 -ffat-lto-objects -### %s -S 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S
 // CHECK-CC-S: -cc1
 // CHECK-CC-S: -S
 // CHECK-CC-S-NOT: -emit-obj
@@ -13,35 +15,41 @@
 
 /// When fat LTO is enabled with -S, we expect asm output and -ffat-lto-objects to be passed to cc1.
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -### %s -S 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S-LTO
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -### %s -S 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S-LTO
 // CHECK-CC-S-LTO: -cc1
 // CHECK-CC-S-NOT: -emit-llvm
 // CHECK-CC-S-LTO-SAME: -ffat-lto-objects
 
 /// When fat LTO is enabled with -S and -emit-llvm, we expect IR output and -ffat-lto-objects to be passed to cc1.
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -### %s -S -emit-llvm 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S-EL-LTO
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -### %s -S -emit-llvm 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S-EL-LTO
 // CHECK-CC-S-EL-LTO: -cc1
 // CHECK-CC-S-EL-LTO-SAME: -emit-llvm
 // CHECK-CC-S-EL-LTO-SAME: -ffat-lto-objects
 
 /// When fat LTO is enabled without -S we expect native object output and -ffat-lto-object to be passed to cc1.
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-C-LTO
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-C-LTO
 // CHECK-CC-C-LTO: -cc1
 // CHECK-CC-C-LTO-SAME: -emit-obj
 // CHECK-CC-C-LTO-SAME: -ffat-lto-objects
 
 /// When fat LTO is enabled with -c and -emit-llvm we expect bitcode output and -ffat-lto-object to be passed to cc1.
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -### %s -c -emit-llvm 2>&1 | FileCheck %s -check-prefix=CHECK-CC-C-EL-LTO
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -### %s -c -emit-llvm 2>&1 | FileCheck %s -check-prefix=CHECK-CC-C-EL-LTO
 // CHECK-CC-C-EL-LTO: -cc1
 // CHECK-CC-C-EL-LTO-SAME: -emit-llvm-bc
 // CHECK-CC-C-EL-LTO-SAME: -ffat-lto-objects
 
 /// Make sure we don't have a warning for -ffat-lto-objects being unused
 // RUN: %clang --target=x86_64-unknown-linux-gnu -ffat-lto-objects -fdriver-only -Werror -v %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-NOLTO
+// RUN: %clang --target=i386-pc-win32 -ffat-lto-objects -fdriver-only -Werror -v %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-NOLTO
 // CHECK-CC-NOLTO: -cc1
 // CHECK-CC-NOLTO-SAME: -emit-obj
 // CHECK-CC-NOLTO-NOT: -ffat-lto-objects
 
 // RUN: %clang --target=x86_64-unknown-linux-gnu -flto -ffat-lto-objects -fno-fat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-NOLTO
+// RUN: %clang --target=i386-pc-win32 -flto -ffat-lto-objects -fno-fat-lto-objects -### %s -c 2>&1 | FileCheck %s -check-prefix=CHECK-CC-NOLTO
 
 /// We need to pass an additional flag (--fat-lto-objects) to lld when linking w/ -flto -ffat-lto-objects
 /// But it should not be there when LTO is disabled w/ -fno-lto
diff --git a/lld/COFF/Config.h b/lld/COFF/Config.h
index 16997e4de780e..c5c0338c99e57 100644
--- a/lld/COFF/Config.h
+++ b/lld/COFF/Config.h
@@ -207,6 +207,9 @@ struct Configuration {
   // Used for /thinlto-remote-compiler-arg:<arg>
   llvm::SmallVector<llvm::StringRef, 0> dtltoCompilerArgs;
 
+  // Used for /fat-lto-objects
+  bool fatLTOObjects = false;
+
   // Used for /opt:[no]ltodebugpassmanager
   bool ltoDebugPassManager = false;
 
diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
index 0e528de9c3652..ac5942a3ac2d8 100644
--- a/lld/COFF/Driver.cpp
+++ b/lld/COFF/Driver.cpp
@@ -28,6 +28,7 @@
 #include "llvm/Config/llvm-config.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Object/COFFImportFile.h"
+#include "llvm/Object/IRObjectFile.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
@@ -256,6 +257,23 @@ MemoryBufferRef LinkerDriver::takeBuffer(std::unique_ptr<MemoryBuffer> mb) {
   return mbref;
 }
 
+InputFile *LinkerDriver::tryCreateFatLTOFile(MemoryBufferRef mb,
+                                             StringRef archiveName,
+                                             uint64_t offsetInArchive,
+                                             bool lazy) {
+  if (!ctx.config.fatLTOObjects)
+    return nullptr;
+
+  Expected<MemoryBufferRef> fatLTOData =
+      IRObjectFile::findBitcodeInMemBuffer(mb);
+
+  if (errorToBool(fatLTOData.takeError()))
+    return nullptr;
+
+  return BitcodeFile::create(ctx, *fatLTOData, archiveName, offsetInArchive,
+                             lazy);
+}
+
 void LinkerDriver::addBuffer(std::unique_ptr<MemoryBuffer> mb,
                              bool wholeArchive, bool lazy) {
   StringRef filename = mb->getBufferIdentifier();
@@ -289,7 +307,14 @@ void LinkerDriver::addBuffer(std::unique_ptr<MemoryBuffer> mb,
   case file_magic::bitcode:
     addFile(BitcodeFile::create(ctx, mbref, "", 0, lazy));
     break;
-  case file_magic::coff_object:
+  case file_magic::coff_object: {
+    InputFile *obj = tryCreateFatLTOFile(mbref, "", 0, lazy);
+    if (obj)
+      addFile(obj);
+    else
+      addFile(ObjFile::create(ctx, mbref, lazy));
+    break;
+  }
   case file_magic::coff_import_library:
     addFile(ObjFile::create(ctx, mbref, lazy));
     break;
@@ -374,7 +399,9 @@ void LinkerDriver::addArchiveBuffer(MemoryBufferRef mb, StringRef symName,
 
   InputFile *obj;
   if (magic == file_magic::coff_object) {
-    obj = ObjFile::create(ctx, mb);
+    obj = tryCreateFatLTOFile(mb, parentName, offsetInArchive, /*lazy=*/false);
+    if (!obj)
+      obj = ObjFile::create(ctx, mb);
   } else if (magic == file_magic::bitcode) {
     obj = BitcodeFile::create(ctx, mb, parentName, offsetInArchive,
                               /*lazy=*/false);
@@ -2121,6 +2148,10 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   config->dtltoCompilerArgs =
       args::getStrings(args, OPT_thinlto_remote_compiler_arg);
 
+  // Handle /fat-lto-objects
+  config->fatLTOObjects =
+      args.hasFlag(OPT_fat_lto_objects, OPT_fat_lto_objects_no, false);
+
   // Handle /dwodir
   config->dwoDir = args.getLastArgValue(OPT_dwodir);
 
diff --git a/lld/COFF/Driver.h b/lld/COFF/Driver.h
index 14710d5853bcf..8b28bd9f3c617 100644
--- a/lld/COFF/Driver.h
+++ b/lld/COFF/Driver.h
@@ -169,6 +169,8 @@ class LinkerDriver {
 
   std::set<std::string> visitedLibs;
 
+  InputFile *tryCreateFatLTOFile(MemoryBufferRef mb, StringRef archiveName,
+                                 uint64_t offsetInArchive, bool lazy);
   void addBuffer(std::unique_ptr<MemoryBuffer> mb, bool wholeArchive,
                  bool lazy);
   void addArchiveBuffer(MemoryBufferRef mbref, StringRef symName,
diff --git a/lld/COFF/Options.td b/lld/COFF/Options.td
index d77478fc9c987..911c5c55f249b 100644
--- a/lld/COFF/Options.td
+++ b/lld/COFF/Options.td
@@ -298,6 +298,9 @@ def thinlto_remote_compiler_prepend_arg : P<"thinlto-remote-compiler-prepend-arg
 def thinlto_remote_compiler_arg : P<"thinlto-remote-compiler-arg",
   "Compiler arguments for the ThinLTO distributor to pass for ThinLTO backend "
   "compilations">;
+defm fat_lto_objects: B<"fat-lto-objects",
+    "Use the .llvm.lto section, which contains LLVM bitcode, in fat LTO object files to perform LTO.",
+    "Ignore the .llvm.lto section in relocatable object files (default).">;
 def lto_obj_path : P<
     "lto-obj-path",
     "output native object for merged LTO unit to this path">;
diff --git a/lld/test/COFF/fatlto/fatlto-invalid.s b/lld/test/COFF/fatlto/fatlto-invalid.s
new file mode 100644
index 0000000000000..5e60633ef5f11
--- /dev/null
+++ b/lld/test/COFF/fatlto/fatlto-invalid.s
@@ -0,0 +1,8 @@
+# RUN: llvm-mc -filetype=obj -triple=i386-pc-win32 %s -o %t
+# RUN: env LLD_IN_TEST=1 not lld-link %t /out:/dev/null /fat-lto-objects 2>&1 | FileCheck %s
+
+# CHECK: error:{{.*}} Invalid bitcode signature
+
+.section        .llvm.lto,"ynD"
+L_llvm.embedded.object:
+        .asciz  "BC\300\3365\000"
diff --git a/lld/test/COFF/fatlto/fatlto.test b/lld/test/COFF/fatlto/fatlto.test
new file mode 100644
index 0000000000000..19b62a94ea21a
--- /dev/null
+++ b/lld/test/COFF/fatlto/fatlto.test
@@ -0,0 +1,111 @@
+;; Basic FatLTO tests.
+; REQUIRES: x86
+
+; RUN: rm -rf %t && split-file %s %t && cd %t
+
+;; Ensure that input files contain .llvm.lto section.
+; RUN: llc a-LTO.ll --filetype=obj -o a-fatLTO.o --relocation-model=pic
+; RUN: opt < a-LTO.ll --module-summary -o a-fatLTO.bc
+; RUN: llvm-objcopy --add-section=.llvm.lto=a-fatLTO.bc --set-section-flags=.llvm.lto=exclude a-fatLTO.o
+
+; RUN: llc main-LTO.ll --filetype=obj -o main-fatLTO.o --relocation-model=pic
+; RUN: opt < main-LTO.ll --module-summary -o main-fatLTO.bc
+; RUN: llvm-objcopy --add-section=.llvm.lto=main-fatLTO.bc --set-section-flags=.llvm.lto=exclude main-fatLTO.o
+
+; RUN: llvm-readobj --sections a-fatLTO.o | FileCheck --check-prefix=HAS_LLVM_LTO %s
+; RUN: llvm-readobj --sections main-fatLTO.o | FileCheck --check-prefix=HAS_LLVM_LTO %s
+
+;; Make sure that the section flags are set correctly
+; HAS_LLVM_LTO:          Name: .llvm.lto (2F 34 00 00 00 00 00 00)
+; HAS_LLVM_LTO:          Characteristics [
+; HAS_LLVM_LTO-SAME:     (0xC0000800)
+; HAS_LLVM_LTO-NEXT:       IMAGE_SCN_LNK_REMOVE (0x800)
+; HAS_LLVM_LTO-NEXT:       IMAGE_SCN_MEM_READ (0x40000000)
+; HAS_LLVM_LTO-NEXT:       IMAGE_SCN_MEM_WRITE (0x80000000)
+; HAS_LLVM_LTO-NEXT:     ]
+
+;; Final executable should not have .llvm.lto section no matter what the target is.
+; RUN: lld-link /timestamp:0 /out:foo-fatLTO /entry:main a-fatLTO.o main-fatLTO.o -fat-lto-objects
+; RUN: llvm-readobj --sections foo-fatLTO | FileCheck --check-prefix=CHECK-LTO-TARGET %s
+
+;; Check that fat objects work w/ --start-lib.
+; RUN: lld-link /out:foo-fatLTO.start_lib /entry:main -start-lib a-fatLTO.o main-fatLTO.o -fat-lto-objects
+; RUN: llvm-readobj --sections foo-fatLTO.start_lib | FileCheck --check-prefix=CHECK-LTO-TARGET %s
+
+;; Check if .llvm.lto section gets aggregated in LTO target.
+; CHECK-LTO-TARGET-NOT: Name: .llvm.lto
+
+;; Final executable should not have .llvm.lto section no matter what the target is.
+; RUN: lld-link /timestamp:0 /out:foo-fatNoLTO /entry:main a-fatLTO.o %/t/main-fatLTO.o
+; RUN: llvm-readobj --sections foo-fatNoLTO | FileCheck --check-prefix=CHECK-NON-LTO-TARGET %s
+
+;; Check if .llvm.lto section gets aggregated in non-LTO target.
+; CHECK-NON-LTO-TARGET-NOT: Name: .llvm.lto
+
+;; Check if the LTO target executable produced from FatLTO object file is
+;; identical to the one produced from LTO modules.
+; RUN: opt < a-LTO.ll --module-summary -o a-LTO.bc
+; RUN: opt < main-LTO.ll --module-summary -o main-LTO.bc
+; RUN: lld-link /timestamp:0 /out:foo-LTO /entry:main a-LTO.bc main-LTO.bc
+; RUN: cmp foo-fatLTO foo-LTO
+
+;; Check if the no-LTO target executable produced from FatLTO object file is
+;; identical to the one produced from regular object files.
+; RUN: llc a-LTO.ll --filetype=obj -o a.o
+; RUN: llc main-LTO.ll --filetype=obj -o main.o
+; RUN: lld-link /timestamp:0 /out:foo-noLTO /entry:main a.o main.o
+; RUN: cmp foo-fatNoLTO foo-noLTO
+
+;; Check archive support.
+; RUN: llvm-ar rcs a.a a-fatLTO.o
+; RUN: lld-link /timestamp:0 /out:foo-fatLTO.archive /entry:main /wholearchive a.a main-LTO.bc -fat-lto-objects
+; RUN: cmp foo-fatLTO.archive foo-LTO
+
+;--- a-LTO.ll
+target datalayout = "e-m:x-p:32:32-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32-a:0:32-S32"
+target triple = "i386-pc-windows-msvc19.33.0"
+
+; Function Attrs: noinline nounwind uwtable
+define dso_local i32 @_start() #0 {
+entry:
+  ret i32 0
+}
+
+attributes #0 = { noinline nounwind uwtable }
+
+!llvm.module.flags = !{!0, !1, !2, !3, !4, !5, !6}
+
+!0 = !{i32 1, !"NumRegisterParameters", i32 0}
+!1 = !{i32 2, !"Debug Info Version", i32 3}
+!2 = !{i32 1, !"wchar_size", i32 2}
+!3 = !{i32 7, !"frame-pointer", i32 2}
+!4 = !{i32 1, !"MaxTLSAlign", i32 65536}
+!5 = !{i32 1, !"ThinLTO", i32 0}
+!6 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+
+;--- main-LTO.ll
+target datalayout = "e-m:x-p:32:32-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32-a:0:32-S32"
+target triple = "i386-pc-windows-msvc19.33.0"
+
+; Function Attrs: noinline nounwind uwtable
+define dso_local i32 @main() #0 {
+entry:
+  %retval = alloca i32, align 4
+  store i32 0, ptr %retval, align 4
+  %call = call i32 (...) @_start()
+  ret i32 %call
+}
+
+declare i32 @_start(...)
+
+attributes #0 = { noinline nounwind uwtable }
+
+!llvm.module.flags = !{!0, !1, !2, !3, !4, !5, !6}
+
+!0 = !{i32 1, !"NumRegisterParameters", i32 0}
+!1 = !{i32 2, !"Debug Info Version", i32 3}
+!2 = !{i32 1, !"wchar_size", i32 2}
+!3 = !{i32 7, !"frame-pointer", i32 2}
+!4 = !{i32 1, !"MaxTLSAlign", i32 65536}
+!5 = !{i32 1, !"ThinLTO", i32 0}
+!6 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
diff --git a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
index f02256f8a83b9..1c9183c03dab4 100644
--- a/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
+++ b/llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp
@@ -27,7 +27,7 @@ PreservedAnalyses EmbedBitcodePass::run(Module &M, ModuleAnalysisManager &AM) {
     reportFatalUsageError("Can only embed the module once");
 
   Triple T(M.getTargetTriple());
-  if (T.getObjectFormat() != Triple::ELF)
+  if (T.getObjectFormat() != Triple::ELF && T.getObjectFormat() != Triple::COFF)
     reportFatalUsageError(
         "EmbedBitcode pass currently only supports ELF object format");

tru

Thanks for working on this. It needs a release note before it can be accepted.

tru · 2025-10-29T09:47:02Z

llvm/lib/Transforms/IPO/EmbedBitcodePass.cpp

-  if (T.getObjectFormat() != Triple::ELF)
+  if (T.getObjectFormat() != Triple::ELF && T.getObjectFormat() != Triple::COFF)
    reportFatalUsageError(
        "EmbedBitcode pass currently only supports ELF object format");


This string needs to be updated.

thrimbor · 2025-10-30T02:17:34Z

I rebased the PR and added a ReleaseNote entry for LLD based on the one that was added for ELF FatLTO. This is the first time I've done that, so if there's something not quite right with it please let me know.

tru

LGTM now, but wait for at least one more reviewer that looks at the code.

llvmbot added clang Clang issues not falling into any other category lld clang:driver 'clang' and 'clang++' user-facing binaries. Not 'clang-cl' lld:COFF platform:windows llvm:transforms labels Oct 29, 2025

tru requested changes Oct 29, 2025

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tru requested review from aganea, bd1976bris, mstorsjo and nikic October 29, 2025 09:48

nikic requested a review from MaskRay October 29, 2025 09:52

thrimbor force-pushed the fatlto-coff branch 2 times, most recently from 234945b to e40ae66 Compare October 30, 2025 01:26

thrimbor added 2 commits October 30, 2025 03:04

[clang][llvm] Enable fat-lto-object support for COFF targets

03f55c0

[LLD][COFF] Add fat-lto-object support

d5dcbc6

thrimbor force-pushed the fatlto-coff branch from e40ae66 to d5dcbc6 Compare October 30, 2025 02:15

tru approved these changes Oct 30, 2025

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Add FatLTO support for COFF #165529

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