ThinLTO — Clang 20.0.0git documentation (2024)

Clang/LLVM¶

The 3.9 release of clang includes ThinLTO support. However, ThinLTOis under active development, and new features, improvements and bugfixesare being added for the next release. For the latest ThinLTO support,build a recent version of clang and LLVM.

Linkers¶

ThinLTO is currently supported for the following linkers:

• gold (via the gold-plugin):Similar to monolithic LTO, this requires usinga gold linker configured with plugins enabled.

• ld64:Starting with Xcode 8.

  See Also

  Comparing clang to other open source compilersToolChain/LTO - Ubuntu Wiki4.3 Optimizing gcc CompilationGCC Vs. G++: What's the Difference? | Testbook

• lld:Starting with r284050 for ELF, r298942 for COFF.

Basic¶

To utilize ThinLTO, simply add the -flto=thin option to compile and link. E.g.

% clang -flto=thin -O2 file1.c file2.c -c% clang -flto=thin -O2 file1.o file2.o -o a.out

When using lld-link, the -flto option need only be added to the compile step:

% clang-cl -flto=thin -O2 -c file1.c file2.c% lld-link /out:a.exe file1.obj file2.obj

As mentioned earlier, by default the linkers will launch the ThinLTO backendthreads in parallel, passing the resulting native object files back to thelinker for the final native link. As such, the usage model is the same asnon-LTO.

With gold, if you see an error during the link of the form:

/usr/bin/ld: error: /path/to/clang/bin/../lib/LLVMgold.so: could not load plugin library: /path/to/clang/bin/../lib/LLVMgold.so: cannot open shared object file: No such file or directory

Then either gold was not configured with plugins enabled, or clangwas not built with -DLLVM_BINUTILS_INCDIR set properly. Seethe instructions for theLLVM gold plugin.

Controlling Backend Parallelism¶

By default, the ThinLTO link step will launch as manythreads in parallel as there are cores. If the number ofcores can’t be computed for the architecture, then it will launchstd::thread::hardware_concurrency number of threads in parallel.For machines with hyper-threading, this is the total number ofvirtual cores. For some applications and machine configurations thismay be too aggressive, in which case the amount of parallelism canbe reduced to N via:

• gold:-Wl,-plugin-opt,jobs=N

• ld64:-Wl,-mllvm,-threads=N

• ld.lld, ld64.lld:-Wl,--thinlto-jobs=N

• lld-link:/opt:lldltojobs=N

Other possible values for N are:

• 0:Use one thread per physical core (default)

  See Also

  Design and Implementation — LLVM 20.0.0git documentation

• 1:Use a single thread only (disable multi-threading)

• all:Use one thread per logical core (uses all hyper-threads)

Incremental¶

ThinLTO supports fast incremental builds through the use of a cache,which currently must be enabled through a linker option.

• gold (as of LLVM 4.0):-Wl,-plugin-opt,cache-dir=/path/to/cache

• ld64 (supported since clang 3.9 and Xcode 8) and Mach-O ld64.lld (as of LLVM15.0):-Wl,-cache_path_lto,/path/to/cache

• ELF ld.lld (as of LLVM 5.0):-Wl,--thinlto-cache-dir=/path/to/cache

• COFF lld-link (as of LLVM 6.0):/lldltocache:/path/to/cache

Cache Pruning¶

To help keep the size of the cache under control, ThinLTO supports cachepruning. Cache pruning is supported with gold, ld64, and lld, but currently onlygold and lld allow you to control the policy with a policy string. The cachepolicy must be specified with a linker option.

• gold (as of LLVM 6.0):-Wl,-plugin-opt,cache-policy=POLICY

• ELF ld.lld (as of LLVM 5.0), Mach-O ld64.lld (as of LLVM 15.0):-Wl,--thinlto-cache-policy=POLICY

• COFF lld-link (as of LLVM 6.0):/lldltocachepolicy:POLICY

A policy string is a series of key-value pairs separated by : characters.Possible key-value pairs are:

• cache_size=X%: The maximum size for the cache directory is X percentof the available space on the disk. Set to 100 to indicate no limit,50 to indicate that the cache size will not be left over half the availabledisk space. A value over 100 is invalid. A value of 0 disables the percentagesize-based pruning. The default is 75%.

• cache_size_bytes=X, cache_size_bytes=Xk, cache_size_bytes=Xm,cache_size_bytes=Xg:Sets the maximum size for the cache directory to X bytes (or KB, MB,GB respectively). A value over the amount of available space on the diskwill be reduced to the amount of available space. A value of 0 disablesthe byte size-based pruning. The default is no byte size-based pruning.

  Note that ThinLTO will apply both size-based pruning policies simultaneously,and changing one does not affect the other. For example, a policy ofcache_size_bytes=1g on its own will cause both the 1GB and default 75%policies to be applied unless the default cache_size is overridden.

• cache_size_files=X:Set the maximum number of files in the cache directory. Set to 0 to indicateno limit. The default is 1000000 files.

• prune_after=Xs, prune_after=Xm, prune_after=Xh: Sets theexpiration time for cache files to X seconds (or minutes, hoursrespectively). When a file hasn’t been accessed for prune_after seconds,it is removed from the cache. A value of 0 disables the expiration-basedpruning. The default is 1 week.

• prune_interval=Xs, prune_interval=Xm, prune_interval=Xh:Sets the pruning interval to X seconds (or minutes, hoursrespectively). This is intended to be used to avoid scanning the directorytoo often. It does not impact the decision of which files to prune. Avalue of 0 forces the scan to occur. The default is every 20 minutes.

Clang Bootstrap¶

To bootstrap clang/LLVMwith ThinLTO, follow these steps:

1. The host compiler must be a version of clang that supports ThinLTO.

2. The host linker must support ThinLTO (and in the case of gold, must beconfigured with plugins enabled).

3. Use the following additional CMake variableswhen configuring the bootstrap compiler build:

• -DLLVM_ENABLE_LTO=Thin

• -DCMAKE_C_COMPILER=/path/to/host/clang

• -DCMAKE_CXX_COMPILER=/path/to/host/clang++

• -DCMAKE_RANLIB=/path/to/host/llvm-ranlib

• -DCMAKE_AR=/path/to/host/llvm-ar

Or, on Windows:

• -DLLVM_ENABLE_LTO=Thin

• -DCMAKE_C_COMPILER=/path/to/host/clang-cl.exe

• -DCMAKE_CXX_COMPILER=/path/to/host/clang-cl.exe

• -DCMAKE_LINKER=/path/to/host/lld-link.exe

• -DCMAKE_RANLIB=/path/to/host/llvm-ranlib.exe

• -DCMAKE_AR=/path/to/host/llvm-ar.exe

1. To use additional linker arguments for controlling the backendparallelism or enabling incremental builds of the bootstrap compiler,after configuring the build, modify the resulting CMakeCache.txt file in thebuild directory. Specify any additional linker options afterCMAKE_EXE_LINKER_FLAGS:STRING=. Note the configure may fail iflinker plugin options are instead specified directly in the previous step.

The BOOTSTRAP_LLVM_ENABLE_LTO=Thin will enable ThinLTO for stage 2 andstage 3 in case the compiler used for stage 1 does not support the ThinLTOoption.