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CMake: Use -O2 instead of -O3 in CMAKE_BUILD_TYPE=Release.

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-O3 doesn't seem useful for speed but it makes the code bigger.
CMake makes is difficult for users to simply override the
optimization level: CFLAGS / CMAKE_C_FLAGS aren't helpful because
they go before CMAKE_C_FLAGS_RELEASE. Of course, users can override
CMAKE_C_FLAGS_RELEASE directly but then they have to remember to
add also -DNDEBUG to disable assertions.

This commit changes -O3 to -O2 in CMAKE_C_FLAGS_RELEASE if and only if
CMAKE_C_FLAGS_RELEASE cache variable doesn't already exist. So if
a custom value is passed on the command line (or reconfiguring an
already-configured build), the cache variable won't be modified.
This commit is contained in:
Lasse Collin 2024-02-17 21:27:48 +02:00
parent 025eb6d787
commit 4430e075f7
1 changed files with 19 additions and 0 deletions
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19
CMakeLists.txt
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@ -74,9 +74,28 @@ string(REGEX REPLACE
.*$" .*$"
"\\1.\\2.\\3" PACKAGE_VERSION "${PACKAGE_VERSION}") "\\1.\\2.\\3" PACKAGE_VERSION "${PACKAGE_VERSION}")
# With several compilers, CMAKE_BUILD_TYPE=Release uses -O3 optimization
# which results in bigger code without a clear difference in speed. If
# no user-defined CMAKE_C_FLAGS_RELEASE is present, override -O3 to -O2
# to make it possible to recommend CMAKE_BUILD_TYPE=Release.
if(NOT DEFINED CMAKE_C_FLAGS_RELEASE)
set(OVERRIDE_O3_IN_C_FLAGS_RELEASE ON)
endif()
# Among other things, this gives us variables xz_VERSION and xz_VERSION_MAJOR. # Among other things, this gives us variables xz_VERSION and xz_VERSION_MAJOR.
project(xz VERSION "${PACKAGE_VERSION}" LANGUAGES C) project(xz VERSION "${PACKAGE_VERSION}" LANGUAGES C)
if(OVERRIDE_O3_IN_C_FLAGS_RELEASE)
# Looking at CMake's source, there aren't any _FLAGS_RELEASE_INIT
# entries where "-O3" would appear as part of some other option,
# thus a simple search and replace should be fine.
string(REPLACE -O3 -O2 CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
# Update the cache value while keeping its docstring unchanged.
set_property(CACHE CMAKE_C_FLAGS_RELEASE
PROPERTY VALUE "${CMAKE_C_FLAGS_RELEASE}")
endif()
# We need a compiler that supports enough C99 or newer (variable-length arrays # We need a compiler that supports enough C99 or newer (variable-length arrays
# aren't needed, those are optional in C17). Setting CMAKE_C_STANDARD here # aren't needed, those are optional in C17). Setting CMAKE_C_STANDARD here
# makes it the default for all targets. It doesn't affect the INTERFACE so # makes it the default for all targets. It doesn't affect the INTERFACE so