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[CMake] Add a cache file for building a highly-optimized LLVM toolchain #117802

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[CMake] Add a cache file for building a highly-optimized LLVM toolchain #117802

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3bddb3c
[CMake] Add a cache file for building a highly-optimized LLVM toolchain
tstellar Sep 23, 2024
d188a03
Fix documentation
tstellar Nov 26, 2024
7d1bf4d
Use 3stages
tstellar Nov 27, 2024
ed2ba87
Update clang/cmake/caches/llvm-toolchain/stage1.cmake
tstellar Jan 9, 2025
3c26cba
Stage 1
tstellar Jan 10, 2025
3425cc6
stage2
tstellar Jan 10, 2025
bf3e961
Stage2-instrumented
tstellar Jan 10, 2025
5845f09
Add Module Linker flags
tstellar Jan 10, 2025
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17 changes: 17 additions & 0 deletions clang/cmake/caches/llvm-toolchain/stage1.cmake
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# Stage 1
# * Build an LTO optimized libcxx, so we can staticially link it into stage 2
# clang.


set(CMAKE_BUILD_TYPE Release CACHE STRING "")
set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
set(LLVM_ENABLE_RUNTIMES compiler-rt libcxx libcxxabi libunwind CACHE STRING "")
set(LLVM_ENABLE_PROJECTS clang lld CACHE STRING "")

set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
set(CLANG_BOOTSTRAP_CMAKE_ARGS -C ${CMAKE_CURRENT_LIST_DIR}/stage2-instrumented.cmake CACHE BOOL "")
set(BOOTSTRAP_LLVM_BUILD_INSTRUMENTED IR CACHE BOOL "")
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Is it possible to set this in stage2-instrumented instead?

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No, it's not. This needs to be set in the stage1 config: https://github.com/llvm/llvm-project/blob/main/clang/CMakeLists.txt#L597

set(CLANG_BOOTSTRAP_TARGETS stage2-check-all stage2-distribution stage2-install-distribution stage2-clang stage2-clang-bolt CACHE BOOL "")
set(LIBCXX_STATICALLY_LINK_ABI_IN_STATIC_LIBRARY ON CACHE STRING "")
set(RUNTIMES_CMAKE_ARGS "-DCMAKE_INTERPROCEDURAL_OPTIMIZATION=ON" CACHE STRING "")
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Does this use FullLTO or ThinLTO? We're explicitly building the binaries with ThinLTO, so I'd assumed libc++ would also need to be ThinLTO to be able to be statically linked into them.

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It uses ThinLTO: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/Compiler/Clang.cmake?ref_type=heads#L62

set(LLVM_ENABLE_LLD ON CACHE STRING "")
11 changes: 11 additions & 0 deletions clang/cmake/caches/llvm-toolchain/stage2-instrumented.cmake
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# Stage 2 instrumented:
# * Build an instrumented clang, so we can generate profile data for stage 2.


set(CLANG_ENABLE_BOOTSTRAP ON CACHE BOOL "")
set(LLVM_BUILD_INSTRUMENTED IR CACHE STRING "")
set(CLANG_BOOTSTRAP_CMAKE_ARGS -C ${CMAKE_CURRENT_LIST_DIR}/stage2.cmake CACHE STRING "")
set(CLANG_BOOTSTRAP_TARGETS clang check-all distribution install-distribution clang-bolt CACHE STRING "")
set(CLANG_BOLT OFF CACHE STRING "")

include(${CMAKE_CURRENT_LIST_DIR}/stage2.cmake)
17 changes: 17 additions & 0 deletions clang/cmake/caches/llvm-toolchain/stage2.cmake
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# Stage 2:
# * This is the final stage.
# * The goal is to have a clang that is LTO, PGO, and bolt optimized and also
# statically linked to libcxx and compiler-rt.

set(LLVM_TARGETS_TO_BUILD Native CACHE STRING "")
set(LLVM_ENABLE_RUNTIMES compiler-rt libcxx libcxxabi libunwind CACHE STRING "" FORCE)
set(LLVM_ENABLE_PROJECTS clang lld bolt CACHE STRING "" FORCE)
set(LLVM_ENABLE_LLD ON CACHE BOOL "")
set(LLVM_ENABLE_LTO THIN CACHE STRING "")
set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
set(LLVM_STATIC_LINK_CXX_STDLIB ON CACHE BOOL "")
set(CLANG_BOLT "INSTRUMENT" CACHE STRING "")
set(CMAKE_EXE_LINKER_FLAGS "-Wl,--emit-relocs,-znow -rtlib=compiler-rt --unwindlib=libunwind -static-libgcc" CACHE STRING "")
set(CMAKE_SHARED_LINKER_FLAGS "-rtlib=compiler-rt --unwindlib=libunwind -static-libgcc" CACHE STRING "")
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I've had some issues in the past (that I can't remember the details of) if I didn't also set CMAKE_MODULE_LINKER_FLAGS.

set(CMAKE_MODULE_LINKER_FLAGS "-rtlib=compiler-rt --unwindlib=libunwind -static-libgcc" CACHE STRING "")
set(LLVM_DISTRIBUTION_COMPONENTS clang lld runtimes clang-resource-headers CACHE STRING "")
19 changes: 19 additions & 0 deletions clang/cmake/caches/llvm-toolchain/usage.rst
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# LLVM Toolchain Build

This directory contains cache files for building a complete LLVM-based toolchain.
The resulting clang build will be LTO, PGO, and BOLT optimized and statically
linked against libc++ and compiler-rt.

The build is done in 3 stages:

* Stage 1: Build an LTO optimized libc++ with Stage 1 clang/lld.
* Stage 2 Instrumented: Build clang with instrumentation in order to generate
profile data for PGO.
* Stage 2: Build clang with LTO, PGO, and BOLT optimizations and statically link
with stage2 libc++ and compiler-rt.

## Usage

::
$ cmake -S llvm -B build -C clang/cmake/caches/llvm-toolchain/stage1.cmake
$ ninja stage2-install-distribution
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