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[DTLTO][LLD][ELF] Add support for Integrated Distributed ThinLTO
This patch introduces support for Integrated Distributed ThinLTO (DTLTO) in ELF LLD. DTLTO enables the distribution of ThinLTO backend compilations via external distribution systems, such as Incredibuild, during the traditional link step: https://llvm.org/docs/DTLTO.html. It is expected that users will invoke DTLTO through the compiler driver (e.g., Clang) rather than calling LLD directly. A Clang-side interface for DTLTO will be added in a follow-up patch. Note: Bitcode members of archives (both thin and non-thin) are not currently supported. This will be addressed in a future change. Testing: - ELF LLD `lit` test coverage has been added, using a mock distributor to avoid requiring Clang. - Cross-project `lit` tests cover integration with Clang. For the design discussion of the DTLTO feature, see: llvm#126654
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‎cross-project-tests/CMakeLists.txt

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@@ -94,6 +94,13 @@ add_lit_testsuite(check-cross-amdgpu "Running AMDGPU cross-project tests"
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DEPENDS clang
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)
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# DTLTO tests.
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add_lit_testsuite(check-cross-dtlto "Running DTLTO cross-project tests"
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${CMAKE_CURRENT_BINARY_DIR}/dtlto
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EXCLUDE_FROM_CHECK_ALL
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DEPENDS ${CROSS_PROJECT_TEST_DEPS}
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)
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# Add check-cross-project-* targets.
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add_lit_testsuites(CROSS_PROJECT ${CMAKE_CURRENT_SOURCE_DIR}
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DEPENDS ${CROSS_PROJECT_TEST_DEPS}

‎cross-project-tests/dtlto/README.md

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Tests for DTLTO (Integrated Distributed ThinLTO) functionality.
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These are integration tests as DTLTO invokes `clang` for code-generation.

‎cross-project-tests/dtlto/dtlto.c

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// REQUIRES: x86-registered-target,ld.lld
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/// Simple test that DTLTO works with a single input bitcode file and that
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/// --save-temps can be applied to the remote compilation.
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// RUN: rm -rf %t && mkdir %t && cd %t
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// RUN: %clang --target=x86_64-linux-gnu -c -flto=thin %s
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// RUN: ld.lld dtlto.o \
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// RUN: --thinlto-distributor=%python \
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// RUN: --thinlto-distributor-arg=%llvm_src_root/utils/dtlto/local.py \
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// RUN: --thinlto-remote-compiler=%clang \
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// RUN: --thinlto-remote-compiler-arg=--save-temps
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/// Check that the required output files have been created.
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// RUN: ls | count 10
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// RUN: ls | FileCheck %s
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/// Produced by the bitcode compilation.
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// CHECK-DAG: {{^}}dtlto.o{{$}}
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/// Linked ELF.
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// CHECK-DAG: {{^}}a.out{{$}}
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/// --save-temps output for the backend compilation.
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// CHECK-DAG: {{^}}dtlto.s{{$}}
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// CHECK-DAG: {{^}}dtlto.s.0.preopt.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.1.promote.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.2.internalize.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.3.import.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.4.opt.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.5.precodegen.bc{{$}}
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// CHECK-DAG: {{^}}dtlto.s.resolution.txt{{$}}
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int _start() { return 0; }

‎cross-project-tests/dtlto/lit.local.cfg

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if "clang" not in config.available_features:
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config.unsupported = True

‎lld/ELF/Config.h

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@@ -249,6 +249,10 @@ struct Config {
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llvm::SmallVector<llvm::StringRef, 0> searchPaths;
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llvm::SmallVector<llvm::StringRef, 0> symbolOrderingFile;
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llvm::SmallVector<llvm::StringRef, 0> thinLTOModulesToCompile;
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llvm::StringRef dtltoDistributor;
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llvm::SmallVector<llvm::StringRef, 0> dtltoDistributorArgs;
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llvm::StringRef dtltoCompiler;
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llvm::SmallVector<llvm::StringRef, 0> dtltoCompilerArgs;
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llvm::SmallVector<llvm::StringRef, 0> undefined;
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llvm::SmallVector<SymbolVersion, 0> dynamicList;
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llvm::SmallVector<uint8_t, 0> buildIdVector;

‎lld/ELF/Driver.cpp

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@@ -1341,6 +1341,11 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
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args.hasFlag(OPT_dependent_libraries, OPT_no_dependent_libraries, true);
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ctx.arg.disableVerify = args.hasArg(OPT_disable_verify);
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ctx.arg.discard = getDiscard(args);
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ctx.arg.dtltoDistributor = args.getLastArgValue(OPT_thinlto_distributor_eq);
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ctx.arg.dtltoDistributorArgs =
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args::getStrings(args, OPT_thinlto_distributor_arg);
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ctx.arg.dtltoCompiler = args.getLastArgValue(OPT_thinlto_compiler_eq);
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ctx.arg.dtltoCompilerArgs = args::getStrings(args, OPT_thinlto_compiler_arg);
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ctx.arg.dwoDir = args.getLastArgValue(OPT_plugin_opt_dwo_dir_eq);
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ctx.arg.dynamicLinker = getDynamicLinker(ctx, args);
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ctx.arg.ehFrameHdr =

‎lld/ELF/InputFiles.cpp

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#include "llvm/ADT/CachedHashString.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/LTO/LTO.h"
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#include "llvm/Object/Archive.h"
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#include "llvm/Object/IRObjectFile.h"
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#include "llvm/Support/ARMAttributeParser.h"
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#include "llvm/Support/ARMBuildAttributes.h"

‎lld/ELF/LTO.cpp

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@@ -180,6 +180,13 @@ BitcodeCompiler::BitcodeCompiler(Ctx &ctx) : ctx(ctx) {
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std::string(ctx.arg.thinLTOPrefixReplaceNew),
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std::string(ctx.arg.thinLTOPrefixReplaceNativeObject),
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ctx.arg.thinLTOEmitImportsFiles, indexFile.get(), onIndexWrite);
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} else if (!ctx.arg.dtltoDistributor.empty()) {
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backend = lto::createOutOfProcessThinBackend(
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llvm::hardware_concurrency(ctx.arg.thinLTOJobs), onIndexWrite,
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ctx.arg.thinLTOEmitIndexFiles, ctx.arg.thinLTOEmitImportsFiles,
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ctx.arg.outputFile, ctx.arg.dtltoDistributor,
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ctx.arg.dtltoDistributorArgs, ctx.arg.dtltoCompiler,
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ctx.arg.dtltoCompilerArgs, !ctx.arg.saveTempsArgs.empty());
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} else {
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backend = lto::createInProcessThinBackend(
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llvm::heavyweight_hardware_concurrency(ctx.arg.thinLTOJobs),

‎lld/ELF/Options.td

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@@ -710,7 +710,17 @@ def thinlto_object_suffix_replace_eq: JJ<"thinlto-object-suffix-replace=">;
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def thinlto_prefix_replace_eq: JJ<"thinlto-prefix-replace=">;
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def thinlto_single_module_eq: JJ<"thinlto-single-module=">,
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HelpText<"Specify a single module to compile in ThinLTO mode, for debugging only">;
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def thinlto_distributor_eq: JJ<"thinlto-distributor=">,
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HelpText<"Distributor to use for ThinLTO backend compilations. If specified, "
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"ThinLTO backend compilations will be distributed.">;
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defm thinlto_distributor_arg: EEq<"thinlto-distributor-arg", "Arguments to "
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"pass to the ThinLTO distributor">;
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def thinlto_compiler_eq: JJ<"thinlto-remote-compiler=">,
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HelpText<"Compiler for the ThinLTO distributor to invoke for ThinLTO backend "
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"compilations">;
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defm thinlto_compiler_arg: EEq<"thinlto-remote-compiler-arg", "Compiler "
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"arguments for the ThinLTO distributor to pass for ThinLTO backend "
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"compilations">;
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defm fat_lto_objects: BB<"fat-lto-objects",
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"Use the .llvm.lto section, which contains LLVM bitcode, in fat LTO object files to perform LTO.",
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"Ignore the .llvm.lto section in relocatable object files (default).">;

‎lld/docs/DTLTO.rst

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Integrated Distributed ThinLTO (DTLTO)
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======================================
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Integrated Distributed ThinLTO (DTLTO) enables the distribution of backend
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ThinLTO compilations via external distribution systems, such as Incredibuild,
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during the traditional link step.
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The implementation is documented here: https://llvm.org/docs/DTLTO.html.
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Currently, DTLTO is only supported in ELF LLD. Support will be added to other
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LLD flavours in the future.
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ELF LLD
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-------
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The command-line interface is as follows:
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- ``--thinlto-distributor=<path>``
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Specifies the file to execute as the distributor process. If specified,
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ThinLTO backend compilations will be distributed.
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- ``--thinlto-remote-compiler=<path>``
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Specifies the path to the compiler that the distributor process will use for
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backend compilations. The compiler invoked must match the version of LLD.
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- ``--thinlto-distributor-arg=<arg>``
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Specifies ``<arg>`` on the command line when invoking the distributor.
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Can be specified multiple times.
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- ``--thinlto-remote-compiler-arg=<arg>``
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Appends ``<arg>`` to the remote compiler's command line.
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Can be specified multiple times.
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Options that introduce extra input/output files may cause miscompilation if
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the distribution system does not automatically handle pushing/fetching them to
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remote nodes. In such cases, configure the distributor - possibly using
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``--thinlto-distributor-arg=`` - to manage these dependencies. See the
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distributor documentation for details.
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Some LLD LTO options (e.g., ``--lto-sample-profile=<file>``) are supported.
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Currently, other options are silently accepted but do not have the intended
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effect. Support for such options will be expanded in the future.

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