[lld] Preliminary fat-lto-object support

This patch adds support to lld for --fat-lto-objects. We add a new
--fat-lto-objects flag to LLD, and slightly change how it chooses input
files in the driver when the flag is set.

Fat LTO objects contain both LTO compatible IR, as well as generated object
code. This allows users to defer the choice of whether to use LTO or not to
link-time. This is a feature available in GCC for some time, and makes the
existing -ffat-lto-objects flag functional in the same way as GCC's.

If the --fat-lto-objects option is passed to LLD and the input files are fat
object files, then the linker will chose the LTO compatible bitcode sections
embedded within the fat object and link them together using LTO. Otherwise,
standard object file linking is done using the assembly section in the object
files.

Original RFC: https://discourse.llvm.org/t/rfc-ffat-lto-objects-support/63977

Depends on D146777

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D146778

lld/ELF/Config.h

@@ -125,7 +125,8 @@ class LinkerDriver {
   void inferMachineType();
   void link(llvm::opt::InputArgList &args);
   template <class ELFT> void compileBitcodeFiles(bool skipLinkedOutput);
-
+  bool tryAddFatLTOFile(MemoryBufferRef mb, StringRef archiveName,
+                        uint64_t offsetInArchive, bool lazy);
   // True if we are in --whole-archive and --no-whole-archive.
   bool inWholeArchive = false;
 
@@ -205,6 +206,7 @@ struct Config {
       callGraphProfile;
   bool cmseImplib = false;
   bool allowMultipleDefinition;
+  bool fatLTOObjects;
   bool androidPackDynRelocs = false;
   bool armHasBlx = false;
   bool armHasMovtMovw = false;

lld/ELF/Driver.cpp

@@ -52,6 +52,7 @@
 #include "llvm/Config/llvm-config.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Object/Archive.h"
+#include "llvm/Object/IRObjectFile.h"
 #include "llvm/Remarks/HotnessThresholdParser.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compression.h"
@@ -237,6 +238,19 @@ static bool isBitcode(MemoryBufferRef mb) {
   return identify_magic(mb.getBuffer()) == llvm::file_magic::bitcode;
 }
 
+bool LinkerDriver::tryAddFatLTOFile(MemoryBufferRef mb, StringRef archiveName,
+                                    uint64_t offsetInArchive, bool lazy) {
+  if (!config->fatLTOObjects)
+    return false;
+  Expected<MemoryBufferRef> fatLTOData =
+      IRObjectFile::findBitcodeInMemBuffer(mb);
+  if (errorToBool(fatLTOData.takeError()))
+    return false;
+  files.push_back(
+      make<BitcodeFile>(*fatLTOData, archiveName, offsetInArchive, lazy));
+  return true;
+}
+
 // Opens a file and create a file object. Path has to be resolved already.
 void LinkerDriver::addFile(StringRef path, bool withLOption) {
   using namespace sys::fs;
@@ -261,7 +275,7 @@ void LinkerDriver::addFile(StringRef path, bool withLOption) {
       for (const std::pair<MemoryBufferRef, uint64_t> &p : members) {
         if (isBitcode(p.first))
           files.push_back(make<BitcodeFile>(p.first, path, p.second, false));
-        else
+        else if (!tryAddFatLTOFile(p.first, path, p.second, false))
           files.push_back(createObjFile(p.first, path));
       }
       return;
@@ -285,9 +299,10 @@ void LinkerDriver::addFile(StringRef path, bool withLOption) {
     InputFile::isInGroup = true;
     for (const std::pair<MemoryBufferRef, uint64_t> &p : members) {
       auto magic = identify_magic(p.first.getBuffer());
-      if (magic == file_magic::elf_relocatable)
-        files.push_back(createObjFile(p.first, path, true));
-      else if (magic == file_magic::bitcode)
+      if (magic == file_magic::elf_relocatable) {
+        if (!tryAddFatLTOFile(p.first, path, p.second, true))
+          files.push_back(createObjFile(p.first, path, true));
+      } else if (magic == file_magic::bitcode)
         files.push_back(make<BitcodeFile>(p.first, path, p.second, true));
       else
         warn(path + ": archive member '" + p.first.getBufferIdentifier() +
@@ -319,7 +334,8 @@ void LinkerDriver::addFile(StringRef path, bool withLOption) {
     files.push_back(make<BitcodeFile>(mbref, "", 0, inLib));
     break;
   case file_magic::elf_relocatable:
-    files.push_back(createObjFile(mbref, "", inLib));
+    if (!tryAddFatLTOFile(mbref, "", 0, inLib))
+      files.push_back(createObjFile(mbref, "", inLib));
     break;
   default:
     error(path + ": unknown file type");
@@ -1138,6 +1154,8 @@ static void readConfigs(opt::InputArgList &args) {
       args.hasFlag(OPT_android_memtag_heap, OPT_no_android_memtag_heap, false);
   config->androidMemtagStack = args.hasFlag(OPT_android_memtag_stack,
                                             OPT_no_android_memtag_stack, false);
+  config->fatLTOObjects =
+      args.hasFlag(OPT_fat_lto_objects, OPT_no_fat_lto_objects, false);
   config->androidMemtagMode = getMemtagMode(args);
   config->auxiliaryList = args::getStrings(args, OPT_auxiliary);
   config->armBe8 = args.hasArg(OPT_be8);

lld/ELF/Options.td

@@ -647,6 +647,10 @@ def thinlto_prefix_replace_eq: JJ<"thinlto-prefix-replace=">;
 def thinlto_single_module_eq: JJ<"thinlto-single-module=">,
   HelpText<"Specify a single module to compile in ThinLTO mode, for debugging only">;
 
+defm fat_lto_objects: BB<"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: J<"plugin-opt=O">, Alias<lto_O>, HelpText<"Alias for --lto-O">;
 def: F<"plugin-opt=debug-pass-manager">,
   Alias<lto_debug_pass_manager>, HelpText<"Alias for --lto-debug-pass-manager">;

lld/docs/ReleaseNotes.rst

@@ -31,6 +31,10 @@ ELF Improvements
 * ``PT_RISCV_ATTRIBUTES`` is added to include the SHT_RISCV_ATTRIBUTES section.
   (`D152065 <https://reviews.llvm.org/D152065>`_)
 
+- ``--fat-lto-objects`` option is added to support LLVM FatLTO.
+  Without ``--fat-lto-objects``, LLD will link LLVM FatLTO objects using the
+  relocatable object file. (`D146778 <https://reviews.llvm.org/D146778>`_)
+
 Breaking changes
 ----------------
 

lld/docs/ld.lld.1

@@ -621,6 +621,10 @@ Number of threads.
 (default) means all of concurrent threads supported.
 .Cm 1
 disables multi-threading.
+.It Fl -fat-lto-objects
+Use the .llvm.lto section, which contains LLVM bitcode, in fat LTO object files to perform LTO.
+.It Fl -no-fat-lto-objects
+Ignore the .llvm.lto section in relocatable object files (default).
 .It Fl -time-trace
 Record time trace.
 .It Fl -time-trace-file Ns = Ns Ar file

lld/test/ELF/fatlto/fatlto.invalid.s

@@ -0,0 +1,9 @@
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: not ld.lld %t -o /dev/null --fat-lto-objects 2>&1 | FileCheck %s
+
+# CHECK: error:{{.*}} Invalid bitcode signature
+
+.section        .llvm.lto,"e",@progbits
+.Lllvm.embedded.object:
+        .asciz  "BC\300\3365\000"
+        .size   .Lllvm.embedded.object, 12

lld/test/ELF/fatlto/fatlto.test

@@ -0,0 +1,99 @@
+;; Basic FatLTO tests.
+; REQUIRES: x86
+
+; RUN: rm -rf %t && split-file %s %t
+
+;; Ensure that input files contain .llvm.lto section.
+; RUN: llc %t/a-LTO.ll --filetype=obj -o %t/a-fatLTO.o
+; RUN: opt < %t/a-LTO.ll --module-summary -o %t/a-fatLTO.bc
+; RUN: llvm-objcopy --add-section=.llvm.lto=%t/a-fatLTO.bc --set-section-flags=.llvm.lto=exclude %t/a-fatLTO.o
+
+; RUN: llc %t/main-LTO.ll --filetype=obj -o %t/main-fatLTO.o
+; RUN: opt < %t/main-LTO.ll --module-summary -o %t/main-fatLTO.bc
+; RUN: llvm-objcopy --add-section=.llvm.lto=%t/main-fatLTO.bc --set-section-flags=.llvm.lto=exclude %t/main-fatLTO.o
+
+;; Final executable should not have .llvm.lto section no matter what the target is.
+; RUN: ld.lld -o %t/foo-fatLTO %t/a-fatLTO.o %t/main-fatLTO.o --fat-lto-objects
+; RUN: llvm-readobj -S %t/foo-fatLTO | FileCheck --check-prefix=CHECK-LTO-TARGET %s
+
+;; Check that fat objects work w/ --start-lib.
+; RUN: ld.lld -o %t/foo-fatLTO.start_lib --start-lib %t/a-fatLTO.o %t/main-fatLTO.o --fat-lto-objects
+; RUN: llvm-readobj -S %t/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: ld.lld -o %t/foo-fatNoLTO %t/a-fatLTO.o %/t/main-fatLTO.o
+; RUN: llvm-readobj -S %t/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 < %t/a-LTO.ll --module-summary -o %t/a-LTO.bc
+; RUN: opt < %t/main-LTO.ll --module-summary -o %t/main-LTO.bc
+; RUN: ld.lld -o %t/foo-LTO %t/a-LTO.bc %t/main-LTO.bc
+; RUN: cmp %t/foo-fatLTO %t/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 %t/a-LTO.ll --filetype=obj -o %t/a.o
+; RUN: llc %t/main-LTO.ll --filetype=obj -o %t/main.o
+; RUN: ld.lld -o %t/foo-noLTO %t/a.o %t/main.o
+; RUN: cmp %t/foo-fatNoLTO %t/foo-noLTO
+
+;; Check archive support.
+; RUN: llvm-ar rcs %t/a.a %t/a-fatLTO.o 
+; RUN: ld.lld -o %t/foo-fatLTO.archive %t/a.a %t/main-LTO.bc --fat-lto-objects
+; RUN: cmp %t/foo-fatLTO.archive %t/foo-LTO
+
+;--- a-LTO.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; 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, !"wchar_size", i32 4}
+!1 = !{i32 7, !"PIC Level", i32 2}
+!2 = !{i32 7, !"PIE Level", i32 2}
+!3 = !{i32 7, !"uwtable", i32 2}
+!4 = !{i32 7, !"frame-pointer", i32 2}
+!5 = !{i32 1, !"ThinLTO", i32 0}
+!6 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+
+;--- main-LTO.ll
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; 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, !"wchar_size", i32 4}
+!1 = !{i32 7, !"PIC Level", i32 2}
+!2 = !{i32 7, !"PIE Level", i32 2}
+!3 = !{i32 7, !"uwtable", i32 2}
+!4 = !{i32 7, !"frame-pointer", i32 2}
+!5 = !{i32 1, !"ThinLTO", i32 0}
+!6 = !{i32 1, !"EnableSplitLTOUnit", i32 1}