# Distributed under the MIT License.
# See LICENSE.txt for details.

# Use CCache if available to speed up builds
#
# CCache is particularly useful for speeding up builds on CI, where we can share
# the cache between runs. It is also very useful for general development.
#
# To support precompiled headers (PCH) we follow the recommendations in the
# ccache docs:
# https://ccache.dev/manual/latest.html#_precompiled_headers
# Our requirements for CCache with PCH are:
# - Works with GCC and Clang.
# - Deleting and recreating the build directory retains ~100% cache hit rate.
# - `make clean` retains ~100% cache hit rate.
# - Changing the PCH file in tools/SpectrePch.hpp invalidates the cache.
# - Sharing caches between runs on CI works.

option(USE_CCACHE "Use CCache if available to speed up builds" ON)

set(CCACHE_LAUNCHER_EXTRA_ENV_VARS "" CACHE STRING "Env vars for ccache")

if(NOT USE_CCACHE)
  return()
endif()

find_package(ccache)

if (NOT CCACHE_EXEC)
  return()
endif()

execute_process(COMMAND realpath ${CCACHE_EXEC}
                OUTPUT_STRIP_TRAILING_WHITESPACE
                OUTPUT_VARIABLE REAL_CCACHE_EXEC)

# Configure ccache with environment variables
set(_CCACHE_LAUNCHER_ENV_VARS
  ${CCACHE_LAUNCHER_EXTRA_ENV_VARS}
  "CCACHE_SLOPPINESS=pch_defines,time_macros,include_file_mtime,\
include_file_ctime")
# The `locale` sloppiness is only available in ccache 3.6 and later
if (CCACHE_VERSION VERSION_GREATER_EQUAL 3.6)
  string(APPEND _CCACHE_LAUNCHER_ENV_VARS ",locale")
endif()

# Invoke compiler through ccache
set(CMAKE_CXX_COMPILER_LAUNCHER
  ${_CCACHE_LAUNCHER_ENV_VARS} ${REAL_CCACHE_EXEC})
set(CMAKE_C_COMPILER_LAUNCHER ${CMAKE_CXX_COMPILER_LAUNCHER})
message(STATUS "Using ccache for compilation. It is invoked as: "
  "${CMAKE_CXX_COMPILER_LAUNCHER}")

# Add `-fno-pch-timestamp` flag to Clang to support precompiled headers
# (see https://ccache.dev/manual/4.8.2.html#_precompiled_headers)
# Note that `CMAKE_CXX_COMPILE_OPTIONS_CREATE_PCH` isn't part of the public API
# so this may break in the future. For a discussion see:
# https://discourse.cmake.org/t/ccache-clang-and-fno-pch-timestamp/7253/6
if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  list(APPEND CMAKE_CXX_COMPILE_OPTIONS_CREATE_PCH -Xclang -fno-pch-timestamp)
endif()