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[flang][Driver] Enables lto-partitions and fat-lto-object. #158125

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Merged
merged 3 commits into from
Sep 18, 2025
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[flang][Driver] Enables lto-partitions and fat-lto-object. #158125

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[flang][Driver] Enables lto-partitions and fat-lto-object.
anchuraj Sep 2, 2025
9023172
R2: Addressing review comments
anchuraj Sep 16, 2025
05a1730
Removing white space
anchuraj Sep 17, 2025
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5 changes: 3 additions & 2 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -3171,10 +3171,11 @@ def fthin_link_bitcode_EQ : Joined<["-"], "fthin-link-bitcode=">,
MarshallingInfoString<CodeGenOpts<"ThinLinkBitcodeFile">>;
defm fat_lto_objects : BoolFOption<"fat-lto-objects",
CodeGenOpts<"FatLTO">, DefaultFalse,
PosFlag<SetTrue, [], [ClangOption, CC1Option], "Enable">,
NegFlag<SetFalse, [], [ClangOption, CC1Option], "Disable">,
PosFlag<SetTrue, [], [ClangOption, CC1Option, FlangOption, FC1Option], "Enable">,
NegFlag<SetFalse, [], [ClangOption, CC1Option, FlangOption, FC1Option], "Disable">,
BothFlags<[], [ClangOption, CC1Option], " fat LTO object support">>;
def flto_partitions_EQ : Joined<["-"], "flto-partitions=">, Group<f_Group>,
Visibility<[ClangOption, FlangOption]>,
HelpText<"Number of partitions to use for parallel full LTO codegen, ld.lld only.">;
def fmacro_backtrace_limit_EQ : Joined<["-"], "fmacro-backtrace-limit=">,
Group<f_Group>, Visibility<[ClangOption, CC1Option, CLOption]>,
Expand Down
36 changes: 24 additions & 12 deletions clang/lib/Driver/ToolChains/Flang.cpp
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Expand Up @@ -182,6 +182,29 @@ void Flang::addCodegenOptions(const ArgList &Args,
CmdArgs.push_back("-fcoarray");
}

void Flang::addLTOOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
const auto &TC = getToolChain();
const Driver &D = TC.getDriver();
DiagnosticsEngine &Diags = D.getDiags();
LTOKind LTOMode = D.getLTOMode();
// LTO mode is parsed by the Clang driver library.
assert(LTOMode != LTOK_Unknown && "Unknown LTO mode.");
if (LTOMode == LTOK_Full)
CmdArgs.push_back("-flto=full");
else if (LTOMode == LTOK_Thin) {
Diags.Report(
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@tarunprabhu tarunprabhu Sep 17, 2025

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I know that this isn't part of this PR, but do you know what flto=thin does in flang? Does it do anything at all or do we just accept this option, then ignore it? This warning is not very informative, and I wonder if it is worth providing something different.

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@anchuraj anchuraj Sep 17, 2025

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https://reviews.llvm.org/D142420 is the changes I can see. It adds the pipeline buildThinLTOPreLinkDefaultPipeline(level) in FrontendAction.cpp. It does not ignore the option but says further development is required.

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@anchuraj anchuraj Sep 17, 2025

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Thank you for bringing this up. There are TODOs mentioned in the above PR:

The TODOs would be:

Add support for -f[no]split-lto-unit
Emit LTO summary
Use ThinLTOBitcodeWriterPass for thin lto

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@tarunprabhu tarunprabhu Sep 17, 2025

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Thanks for looking into this. It looks like there were some concerns about that commit message even back then. Did you have plans to work on any of these?

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@anchuraj anchuraj Sep 18, 2025

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I plan to work on enabling module summary.

Diags.getCustomDiagID(DiagnosticsEngine::Warning,
"the option '-flto=thin' is a work in progress"));
CmdArgs.push_back("-flto=thin");
}
if (Args.hasArg(options::OPT_flto_partitions_EQ)) {
StringRef A = Args.getLastArgValue(options::OPT_flto_partitions_EQ, "8");
CmdArgs.push_back(Args.MakeArgString("-flto-partitions=" + A));
}
Args.addAllArgs(CmdArgs, {options::OPT_ffat_lto_objects,
options::OPT_fno_fat_lto_objects});
}

void Flang::addPicOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
// ParsePICArgs parses -fPIC/-fPIE and their variants and returns a tuple of
// (RelocationModel, PICLevel, IsPIE).
Expand Down Expand Up @@ -821,7 +844,6 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,

const Driver &D = TC.getDriver();
ArgStringList CmdArgs;
DiagnosticsEngine &Diags = D.getDiags();

// Invoke ourselves in -fc1 mode.
CmdArgs.push_back("-fc1");
Expand Down Expand Up @@ -884,17 +906,7 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,

handleColorDiagnosticsArgs(D, Args, CmdArgs);

// LTO mode is parsed by the Clang driver library.
LTOKind LTOMode = D.getLTOMode();
assert(LTOMode != LTOK_Unknown && "Unknown LTO mode.");
if (LTOMode == LTOK_Full)
CmdArgs.push_back("-flto=full");
else if (LTOMode == LTOK_Thin) {
Diags.Report(
Diags.getCustomDiagID(DiagnosticsEngine::Warning,
"the option '-flto=thin' is a work in progress"));
CmdArgs.push_back("-flto=thin");
}
addLTOOptions(Args, CmdArgs);

// -fPIC and related options.
addPicOptions(Args, CmdArgs);
Expand Down
8 changes: 8 additions & 0 deletions clang/lib/Driver/ToolChains/Flang.h
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Expand Up @@ -40,6 +40,14 @@ class LLVM_LIBRARY_VISIBILITY Flang : public Tool {
void addPreprocessingOptions(const llvm::opt::ArgList &Args,
llvm::opt::ArgStringList &CmdArgs) const;

/// Extract LTO options from the driver arguments and add them to
/// the command arguments.
///
/// \param [in] Args The list of input driver arguments
/// \param [out] CmdArgs The list of output command arguments
void addLTOOptions(const llvm::opt::ArgList &Args,
llvm::opt::ArgStringList &CmdArgs) const;

/// Extract PIC options from the driver arguments and add them to
/// the command arguments.
///
Expand Down
1 change: 1 addition & 0 deletions flang/include/flang/Frontend/CodeGenOptions.def
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Expand Up @@ -35,6 +35,7 @@ CODEGENOPT(InstrumentFunctions, 1, 0) ///< Set when -finstrument_functions is

CODEGENOPT(IsPIE, 1, 0) ///< PIE level is the same as PIC Level.
CODEGENOPT(PICLevel, 2, 0) ///< PIC level of the LLVM module.
CODEGENOPT(PrepareForFatLTO , 1, 0) ///< Set when -ffat-lto-objects is enabled.
CODEGENOPT(PrepareForFullLTO , 1, 0) ///< Set when -flto is enabled on the
///< compile step.
CODEGENOPT(PrepareForThinLTO , 1, 0) ///< Set when -flto=thin is enabled on the
Expand Down
4 changes: 4 additions & 0 deletions flang/lib/Frontend/CompilerInvocation.cpp
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Expand Up @@ -325,6 +325,10 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
if (args.hasArg(clang::driver::options::OPT_finstrument_functions))
opts.InstrumentFunctions = 1;

opts.PrepareForFatLTO =
args.hasFlag(clang::driver::options::OPT_ffat_lto_objects,
clang::driver::options::OPT_fno_fat_lto_objects, false);

// -flto=full/thin option.
if (const llvm::opt::Arg *a =
args.getLastArg(clang::driver::options::OPT_flto_EQ)) {
Expand Down
4 changes: 3 additions & 1 deletion flang/lib/Frontend/FrontendActions.cpp
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Expand Up @@ -995,7 +995,9 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {

// Create the pass manager.
llvm::ModulePassManager mpm;
if (opts.PrepareForFullLTO)
if (opts.PrepareForFatLTO)
mpm = pb.buildFatLTODefaultPipeline(level, opts.PrepareForThinLTO, true);
else if (opts.PrepareForFullLTO)
mpm = pb.buildLTOPreLinkDefaultPipeline(level);
else if (opts.PrepareForThinLTO)
mpm = pb.buildThinLTOPreLinkDefaultPipeline(level);
Expand Down
20 changes: 20 additions & 0 deletions flang/test/Driver/lto-fatlto.f90
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@@ -0,0 +1,20 @@
! REQUIRES: x86-registered-target
! checks fatlto objects: that valid bitcode is included in the object file generated.

! RUN: %flang -fc1 -triple x86_64-unknown-linux-gnu -flto -ffat-lto-objects -emit-obj %s -o %t.o
! RUN: llvm-readelf -S %t.o | FileCheck %s --check-prefixes=ELF
! RUN: llvm-objcopy --dump-section=.llvm.lto=%t.bc %t.o
! RUN: llvm-dis %t.bc -o - | FileCheck %s --check-prefixes=DIS

! ELF: .llvm.lto
! DIS: define void @_QQmain()
! DIS-NEXT: ret void
! DIS-NEXT: }

! RUN: %flang -fc1 -triple x86_64-unknown-linux-gnu -flto -ffat-lto-objects -S %s -o - | FileCheck %s --check-prefixes=ASM

! ASM: .section .llvm.lto,"e",@llvm_lto
! ASM-NEXT: .Lllvm.embedded.object:
! ASM-NEXT: .asciz "BC
! ASM-NEXT: .size .Lllvm.embedded.object
end program
19 changes: 19 additions & 0 deletions flang/test/Driver/lto-lld-flags.f90
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@@ -0,0 +1,19 @@
! UNSUPPORTED: system-windows
! check flto-partitions is passed to lld, fc1
! RUN: %flang -### -fuse-ld=lld -flto=full -flto-partitions=16 %s 2>&1 | FileCheck %s --check-prefixes=LLD-PART,FC1-PART

! FC1-PART: "-fc1"
! FC1-PART-SAME: "-flto=full"
! FC1-PART-SAME: "-flto-partitions=16"
! LLD-PART: ld.lld
! LLD-PART-SAME: "--lto-partitions=16"

! check fat-lto-objects is passed to lld, fc1
! RUN: %flang -### -fuse-ld=lld -flto -ffat-lto-objects %s 2>&1 | FileCheck %s --check-prefixes=LLD-FAT,FC1-FAT

! FC1-FAT: "-fc1"
! FC1-FAT-SAME: "-flto=full"
! FC1-FAT-SAME: "-ffat-lto-objects"
! LLD-FAT: ld.lld
! LLD-FAT-SAME: "--fat-lto-objects"
end program
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@abidh abidh Sep 17, 2025

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No new line at the end of the file.

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@anchuraj anchuraj Sep 17, 2025

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Updated.

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