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build: PCH + ccache launcher + fast-linker autodetect #6

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Merged
andylbrummer merged 2 commits into from
Jun 16, 2026
Merged

build: PCH + ccache launcher + fast-linker autodetect #6

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04e9945
build: precompiled headers + ccache launcher + fast-linker autodetect
andylbrummer Jun 16, 2026
a7da014
build: restrict fast-linker autodetect to non-Apple (mold is ELF-only)
andylbrummer Jun 16, 2026
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30 changes: 30 additions & 0 deletions CMakeLists.txt
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Original file line number Diff line number Diff line change
@@ -1,5 +1,18 @@
cmake_minimum_required(VERSION 3.25)

# Use ccache as the compiler launcher when it is installed — automatic for
# local dev builds, a no-op where absent. Must be set before project()/
# enable_language. `NOT DEFINED` respects an explicit -DCMAKE_CXX_COMPILER_LAUNCHER
# passed by CI, so this only fills the gap for developers.
if(NOT DEFINED CMAKE_CXX_COMPILER_LAUNCHER)
find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
set(CMAKE_C_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
set(CMAKE_CXX_COMPILER_LAUNCHER "${CCACHE_PROGRAM}")
message(STATUS "lci: using ccache compiler launcher (${CCACHE_PROGRAM})")
endif()
endif()

project(lci
VERSION 0.6.0
DESCRIPTION "Lightning Code Index - fast semantic code search"
Expand Down Expand Up @@ -67,6 +80,23 @@ if(LCI_WARNINGS_AS_ERRORS)
endif()
endif()

# Prefer a fast linker (mold > lld) when installed — the link of `lci` and the
# monolithic `lci_tests` (links all deps + ~1700 tests) is a real slice of
# build time, more so with debug info. Linux/ELF only: mold targets ELF (no
# Mach-O), and Apple's default ld64 is fine, so don't override the linker on
# macOS. No-op if neither is present.
if(NOT MSVC AND NOT APPLE)
find_program(MOLD_LINKER mold)
find_program(LLD_LINKER ld.lld)
if(MOLD_LINKER)
add_link_options(-fuse-ld=mold)
message(STATUS "lci: using mold linker (${MOLD_LINKER})")
elseif(LLD_LINKER)
add_link_options(-fuse-ld=lld)
message(STATUS "lci: using lld linker")
endif()
endif()

# Version header generation
configure_file(
"${CMAKE_CURRENT_SOURCE_DIR}/cmake/version.h.in"
Expand Down
16 changes: 16 additions & 0 deletions src/CMakeLists.txt
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Expand Up @@ -184,3 +184,19 @@ target_link_libraries(lci PRIVATE
# Project warning flags (target-scoped, do not leak into FetchContent libs).
target_compile_options(lci_lib PRIVATE ${LCI_WARNING_FLAGS})
target_compile_options(lci PRIVATE ${LCI_WARNING_FLAGS})

# Precompiled headers. The heaviest, most widely-included headers — nlohmann/json
# (~25k lines, in 26 TUs) and absl flat_hash_map (39 TUs) — are parsed once into
# the PCH instead of per translation unit. Largest clean-build compile-time win
# for the library. Both deps are linked PUBLIC above, so their include dirs are
# already on lci_lib's compile line; the std headers are ubiquitous (<string> in
# 134 TUs, <vector> in 100).
target_precompile_headers(lci_lib PRIVATE
<string>
<string_view>
<vector>
<memory>
<unordered_map>
<nlohmann/json.hpp>
<absl/container/flat_hash_map.h>
)
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