[clang] Preliminary fat-lto-object support

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.

This patch adds support for that flag in the driver, as well as setting the
necessary codegen options for the backend. Largely, this means we select
the newly added pass pipeline for generating fat objects.

Users are expected to pass -ffat-lto-objects to clang in addition to one
of the -flto variants. Without the -flto flag, -ffat-lto-objects has no
effect.

// Compile and link. Use the object code from the fat object w/o LTO.
clang -fno-lto -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Select full LTO at link time.
clang -flto -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Select ThinLTO at link time.
clang -flto=thin -ffat-lto-objects -fuse-ld=lld foo.c

// Compile and link. Use ThinLTO  with the UnifiedLTO pipeline.
clang -flto=thin -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c

// Compile and link. Use full LTO  with the UnifiedLTO pipeline.
clang -flto -ffat-lto-objects -funified-lto -fuse-ld=lld foo.c

// Link separately, using ThinLTO.
clang -c -flto=thin -ffat-lto-objects foo.c
clang -flto=thin -fuse-ld=lld foo.o -ffat-lto-objects  # pass --lto=thin --fat-lto-objects to ld.lld

// Link separately, using full LTO.
clang -c -flto -ffat-lto-objects foo.c
clang -flto -fuse-ld=lld foo.o  # pass --lto=full --fat-lto-objects to ld.lld

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

Depends on D146776

Reviewed By: tejohnson, MaskRay

Differential Revision: https://reviews.llvm.org/D146777

clang/docs/ReleaseNotes.rst

@@ -267,6 +267,10 @@ Modified Compiler Flags
   directory (``/tmp`` on \*NIX systems, if none of the environment variables
   TMPDIR, TMP, and TEMP are specified).
 
+- ``-ffat-lto-objects`` can now be used to emit object files with both object
+  code and LLVM bitcode. Previously this flag was ignored for GCC compatibility.
+  (`See patch <https://reviews.llvm.org/D146777>`_).
+
 Removed Compiler Flags
 -------------------------
 - The deprecated flag `-fmodules-ts` is removed. Please use ``-std=c++20``

clang/include/clang/Basic/CodeGenOptions.def

@@ -164,6 +164,7 @@ CODEGENOPT(PrepareForThinLTO , 1, 0) ///< Set when -flto=thin is enabled on the
                                      ///< compile step.
 CODEGENOPT(LTOUnit, 1, 0) ///< Emit IR to support LTO unit features (CFI, whole
                           ///< program vtable opt).
+CODEGENOPT(FatLTO, 1, 0) ///< Set when -ffat-lto-objects is enabled.
 CODEGENOPT(EnableSplitLTOUnit, 1, 0) ///< Enable LTO unit splitting to support
 				     /// CFI and traditional whole program
 				     /// devirtualization that require whole

clang/include/clang/Driver/Options.td

@@ -2370,6 +2370,11 @@ def fthin_link_bitcode_EQ : Joined<["-"], "fthin-link-bitcode=">,
   Flags<[CoreOption, CC1Option]>, Group<f_Group>,
   HelpText<"Write minimized bitcode to <file> for the ThinLTO thin link only">,
   MarshallingInfoString<CodeGenOpts<"ThinLinkBitcodeFile">>;
+defm fat_lto_objects : BoolFOption<"fat-lto-objects",
+  CodeGenOpts<"FatLTO">, DefaultFalse,
+  PosFlag<SetTrue, [CC1Option], "Enable">,
+  NegFlag<SetFalse, [CC1Option], "Disable">,
+  BothFlags<[CC1Option], " fat LTO object support">>;
 def fmacro_backtrace_limit_EQ : Joined<["-"], "fmacro-backtrace-limit=">,
   Group<f_Group>, Flags<[NoXarchOption, CC1Option, CoreOption]>,
   HelpText<"Set the maximum number of entries to print in a macro expansion backtrace (0 = no limit)">,
@@ -5140,7 +5145,6 @@ defm caller_saves : BooleanFFlag<"caller-saves">, Group<clang_ignored_gcc_optimi
 defm reorder_blocks : BooleanFFlag<"reorder-blocks">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm branch_count_reg : BooleanFFlag<"branch-count-reg">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm default_inline : BooleanFFlag<"default-inline">, Group<clang_ignored_gcc_optimization_f_Group>;
-defm fat_lto_objects : BooleanFFlag<"fat-lto-objects">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm float_store : BooleanFFlag<"float-store">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm friend_injection : BooleanFFlag<"friend-injection">, Group<clang_ignored_f_Group>;
 defm function_attribute_list : BooleanFFlag<"function-attribute-list">, Group<clang_ignored_f_Group>;

clang/lib/CodeGen/BackendUtil.cpp

@@ -55,6 +55,7 @@
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/TargetParser/SubtargetFeature.h"
 #include "llvm/TargetParser/Triple.h"
+#include "llvm/Transforms/IPO/EmbedBitcodePass.h"
 #include "llvm/Transforms/IPO/LowerTypeTests.h"
 #include "llvm/Transforms/IPO/ThinLTOBitcodeWriter.h"
 #include "llvm/Transforms/InstCombine/InstCombine.h"
@@ -1015,7 +1016,12 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
           });
     }
 
-    if (IsThinLTO || (IsLTO && CodeGenOpts.UnifiedLTO)) {
+    bool IsThinOrUnifiedLTO = IsThinLTO || (IsLTO && CodeGenOpts.UnifiedLTO);
+    if (CodeGenOpts.FatLTO) {
+      MPM = PB.buildFatLTODefaultPipeline(Level, IsThinOrUnifiedLTO,
+                                          IsThinOrUnifiedLTO ||
+                                              shouldEmitRegularLTOSummary());
+    } else if (IsThinOrUnifiedLTO) {
       MPM = PB.buildThinLTOPreLinkDefaultPipeline(Level);
     } else if (IsLTO) {
       MPM = PB.buildLTOPreLinkDefaultPipeline(Level);
@@ -1071,6 +1077,21 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
                                     EmitLTOSummary));
     }
   }
+  if (CodeGenOpts.FatLTO) {
+    // Set module flags, like EnableSplitLTOUnit and UnifiedLTO, since FatLTO
+    // uses a different action than Backend_EmitBC or Backend_EmitLL.
+    bool IsThinOrUnifiedLTO =
+        CodeGenOpts.PrepareForThinLTO ||
+        (CodeGenOpts.PrepareForLTO && CodeGenOpts.UnifiedLTO);
+    if (!TheModule->getModuleFlag("ThinLTO"))
+      TheModule->addModuleFlag(Module::Error, "ThinLTO",
+                               uint32_t(IsThinOrUnifiedLTO));
+    if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
+      TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                               uint32_t(CodeGenOpts.EnableSplitLTOUnit));
+    if (CodeGenOpts.UnifiedLTO && !TheModule->getModuleFlag("UnifiedLTO"))
+      TheModule->addModuleFlag(Module::Error, "UnifiedLTO", uint32_t(1));
+  }
 
   // Now that we have all of the passes ready, run them.
   {

clang/lib/Driver/Driver.cpp

@@ -4732,8 +4732,13 @@ Action *Driver::ConstructPhaseAction(
   }
   case phases::Backend: {
     if (isUsingLTO() && TargetDeviceOffloadKind == Action::OFK_None) {
-      types::ID Output =
-          Args.hasArg(options::OPT_S) ? types::TY_LTO_IR : types::TY_LTO_BC;
+      types::ID Output;
+      if (Args.hasArg(options::OPT_S))
+        Output = types::TY_LTO_IR;
+      else if (Args.hasArg(options::OPT_ffat_lto_objects))
+        Output = types::TY_PP_Asm;
+      else
+        Output = types::TY_LTO_BC;
       return C.MakeAction<BackendJobAction>(Input, Output);
     }
     if (isUsingLTO(/* IsOffload */ true) &&

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7354,6 +7354,22 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (SplitLTOUnit)
     CmdArgs.push_back("-fsplit-lto-unit");
 
+  if (Arg *A = Args.getLastArg(options::OPT_ffat_lto_objects,
+                               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()) {
+        D.Diag(diag::err_drv_unsupported_opt_for_target)
+            << A->getAsString(Args) << TC.getTripleString();
+      }
+      CmdArgs.push_back(Args.MakeArgString(
+          Twine("-flto=") + (LTOMode == LTOK_Thin ? "thin" : "full")));
+      CmdArgs.push_back("-flto-unit");
+      CmdArgs.push_back("-ffat-lto-objects");
+      A->render(Args, CmdArgs);
+    }
+  }
+
   if (Arg *A = Args.getLastArg(options::OPT_fglobal_isel,
                                options::OPT_fno_global_isel)) {
     CmdArgs.push_back("-mllvm");

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -617,6 +617,11 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
                                 PluginName + Suffix,
                             Plugin);
     CmdArgs.push_back(Args.MakeArgString(Twine(PluginPrefix) + Plugin));
+  } else {
+    // Tell LLD to find and use .llvm.lto section in regular relocatable object
+    // files
+    if (Args.hasArg(options::OPT_ffat_lto_objects))
+      CmdArgs.push_back("--fat-lto-objects");
   }
 
   const char *PluginOptPrefix = IsOSAIX ? "-bplugin_opt:" : "-plugin-opt=";

clang/test/CodeGen/fat-lto-objects.c

@@ -0,0 +1,57 @@
+// REQUIRES: x86-registered-target
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -fsplit-lto-unit -emit-llvm < %s  | FileCheck %s --check-prefixes=FULL,SPLIT
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -emit-llvm < %s  | FileCheck %s --check-prefixes=FULL,SPLIT
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -fsplit-lto-unit -ffat-lto-objects -emit-llvm < %s  | FileCheck %s --check-prefixes=THIN,SPLIT
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -ffat-lto-objects -emit-llvm < %s  | FileCheck %s --check-prefixes=THIN,NOSPLIT
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -fsplit-lto-unit -emit-obj < %s -o %t.full.split.o
+// RUN: llvm-readelf -S %t.full.split.o | FileCheck %s --check-prefixes=ELF
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.split.bc %t.full.split.o
+// RUN: llvm-dis %t.full.split.bc -o - | FileCheck %s --check-prefixes=FULL,SPLIT,NOUNIFIED
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=full -ffat-lto-objects -emit-obj < %s -o %t.full.nosplit.o
+// RUN: llvm-readelf -S %t.full.nosplit.o | FileCheck %s --check-prefixes=ELF
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.full.nosplit.bc %t.full.nosplit.o
+// RUN: llvm-dis %t.full.nosplit.bc -o - | FileCheck %s --check-prefixes=FULL,NOSPLIT,NOUNIFIED
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -fsplit-lto-unit -ffat-lto-objects -emit-obj < %s -o %t.thin.split.o
+// RUN: llvm-readelf -S %t.thin.split.o | FileCheck %s --check-prefixes=ELF
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.thin.split.bc %t.thin.split.o
+// RUN: llvm-dis %t.thin.split.bc -o - | FileCheck %s --check-prefixes=THIN,SPLIT,NOUNIFIED
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -ffat-lto-objects -emit-obj < %s -o %t.thin.nosplit.o
+// RUN: llvm-readelf -S %t.thin.nosplit.o | FileCheck %s --check-prefixes=ELF
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.thin.nosplit.bc %t.thin.nosplit.o
+// RUN: llvm-dis %t.thin.nosplit.bc -o -  | FileCheck %s --check-prefixes=THIN,NOSPLIT,NOUNIFIED
+
+// RUN: %clang -cc1 -triple x86_64-unknown-linux-gnu -flto=thin -funified-lto -ffat-lto-objects -emit-obj < %s -o %t.unified.o
+// RUN: llvm-readelf -S %t.unified.o | FileCheck %s --check-prefixes=ELF
+// RUN: llvm-objcopy --dump-section=.llvm.lto=%t.unified.bc %t.unified.o
+// RUN: llvm-dis %t.unified.bc -o - | FileCheck %s --check-prefixes=THIN,NOSPLIT,UNIFIED
+
+// 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
+
+/// Check that the ThinLTO metadata is only set false for full LTO.
+// FULL: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
+// THIN-NOT: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
+
+/// Be sure we enable split LTO units correctly under -ffat-lto-objects.
+// SPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+// NOSPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 0}
+
+// UNIFIED: ![[#]] = !{i32 1, !"UnifiedLTO", i32 1}
+// NOUNIFIED-NOT: ![[#]] = !{i32 1, !"UnifiedLTO", i32 1}
+
+// ELF: .llvm.lto
+
+//      ASM: .section        .llvm.lto,"e",@progbits
+// ASM-NEXT: .Lllvm.embedded.object:
+// ASM-NEXT:        .asciz  "BC
+// ASM-NEXT: .size   .Lllvm.embedded.object
+
+int test(void) {
+  return 0xabcd;
+}

clang/test/Driver/clang_f_opts.c

@@ -424,7 +424,6 @@
 // CHECK-WARNING-DAG: optimization flag '-fwhole-program' is not supported
 // CHECK-WARNING-DAG: optimization flag '-fcaller-saves' is not supported
 // CHECK-WARNING-DAG: optimization flag '-freorder-blocks' is not supported
-// CHECK-WARNING-DAG: optimization flag '-ffat-lto-objects' is not supported
 // CHECK-WARNING-DAG: optimization flag '-fmerge-constants' is not supported
 // CHECK-WARNING-DAG: optimization flag '-finline-small-functions' is not supported
 // CHECK-WARNING-DAG: optimization flag '-ftree-dce' is not supported

clang/test/Driver/fat-lto-objects.c

@@ -0,0 +1,34 @@
+// RUN: %clang --target=x86_64-unknown-linux-gnu -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
+// CHECK-CC-S: -cc1
+// CHECK-CC-S: -S
+// CHECK-CC-S-NOT: -emit-obj
+// CHECK-CC-S-NOT: -emit-llvm
+// CHECK-CC-S-NOT: -ffat-lto-objects
+
+/// When LTO is enabled, we expect LLVM 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 2>&1 | FileCheck %s -check-prefix=CHECK-CC-S-LTO
+// 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-LTO
+// CHECK-CC-S-LTO: -cc1
+// CHECK-CC-S-LTO-SAME: -emit-llvm
+// CHECK-CC-S-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
+// CHECK-CC-NOLTO: -cc1
+// CHECK-CC-NOLTO-SAME: -emit-obj
+// CHECK-CC-NOLTO-NOT: -ffat-lto-objects
+
+/// 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
+// RUN: %clang --target=x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_cross_linux_tree %s \
+// RUN:   -fuse-ld=lld -flto -ffat-lto-objects -### 2>&1 | FileCheck --check-prefix=LTO %s
+// RUN: %clang --target=x86_64-unknown-linux-gnu --sysroot=%S/Inputs/basic_cross_linux_tree %s \
+// RUN:   -fuse-ld=lld -fno-lto -ffat-lto-objects -### 2>&1 | FileCheck --check-prefix=NOLTO %s
+// LTO: "--fat-lto-objects"
+// NOLTO-NOT: "--fat-lto-objects"