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[CMake] Make precompiled headers opt-in for ccache builds #141927

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[CMake] Make precompiled headers opt-in for ccache builds #141927

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31b413d
[CMake] Make precompiled headers opt-in for ccache builds
mrkajetanp May 29, 2025
f75292a
Elaborate the comment, include flag in docs
mrkajetanp Jun 4, 2025
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8 changes: 8 additions & 0 deletions llvm/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -289,6 +289,14 @@ if(LLVM_CCACHE_BUILD)
set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
CACHE STRING "Parameters to pass through to ccache")

# FIXME: This is a workaround for an unknown underlying issue where the header file
# changes but then the pch does not get rebuilt before its dependencies.
# See: https://github.com/llvm/llvm-project/issues/142449
set(LLVM_CCACHE_PCH OFF CACHE BOOL "Use precompiled headers in ccache builds")
if (NOT LLVM_CCACHE_PCH)
set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
endif()

if(NOT CMAKE_HOST_WIN32)
set(CCACHE_PROGRAM "${LLVM_CCACHE_PARAMS} ${CCACHE_PROGRAM}")
if (LLVM_CCACHE_MAXSIZE)
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4 changes: 3 additions & 1 deletion llvm/docs/CMake.rst
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Expand Up @@ -412,7 +412,9 @@ enabled sub-projects. Nearly all of these variable names begin with
Defaults to OFF. The size and location of the cache maintained
by ``ccache`` can be adjusted via the LLVM_CCACHE_MAXSIZE and LLVM_CCACHE_DIR
options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment
variables, respectively.
variables, respectively. By default, enabling the ccache build will disable
using precompiled headers as they have been known to cause intermittent CI
failures. This can be adjusted via the LLVM_CCACHE_PCH option.

**LLVM_CODE_COVERAGE_TARGETS**:STRING
If set to a semicolon separated list of targets, those targets will be used
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