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CMake: Improve automatic compiler embedding into the Qt toolchain 

Embedding the initial CMAKE_CXX_COMPILER into qt.toolchain.cmake
breaks Boot2Qt builds, because the CXX environment variable is not
used anymore when building qtsvg or other projects.

Disable automatic embedding when cross-compiling, while keeping it
enabled for non-cross-compiling cases (to keep Windows and
and ICC configurations working).

Allow opting in or out of the embedding in case if the default is
wrong, via QT_EMBED_TOOLCHAIN_COMPILER.

Task-number: QTBUG-85067
Change-Id: I1d8f9f580bc379b77c34eefb5728bb49f93cc81a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
This commit is contained in:
Alexandru Croitor 2020-06-18 17:06:22 +02:00
parent 28ef8d283d
commit e2b2cd9397
1 changed files with 42 additions and 4 deletions
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46
cmake/QtBaseGlobalTargets.cmake
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@ -108,10 +108,48 @@ if(VCPKG_TARGET_TRIPLET)
list(APPEND init_vcpkg "set(VCPKG_TARGET_TRIPLET \"${VCPKG_TARGET_TRIPLET}\" CACHE STRING \"\")")
endif()
# On Windows compilers aren't easily mixed. Avoid that qtbase is built using cl.exe for example and then for another
# build gcc is picked up from %PATH%. The same goes when using a custom compiler on other platforms, such as ICC.
list(APPEND init_platform "set(CMAKE_CXX_COMPILER \"${CMAKE_CXX_COMPILER}\" CACHE STRING \"\")")
list(APPEND init_platform "set(CMAKE_C_COMPILER \"${CMAKE_C_COMPILER}\" CACHE STRING \"\")")
# By default we don't want to allow mixing compilers for building different repositories, so we
# embed the initially chosen compilers into the toolchain.
# This is because on Windows compilers aren't easily mixed.
# We want to avoid that qtbase is built using cl.exe for example, and then for another repo
# gcc is picked up from %PATH%.
# The same goes when using a custom compiler on other platforms, such as ICC.
#
# There are a few exceptions though.
#
# When crosscompiling using Boot2Qt, the environment setup shell script sets up the CXX env var,
# which is used by CMake to determine the initial compiler that should be used.
# Unfortunately, the CXX env var contains not only the compiler name, but also a few required
# arch-specific compiler flags. This means that when building qtsvg, if the Qt created toolchain
# file sets the CMAKE_CXX_COMPILER variable, the CXX env var is ignored and thus the extra
# arch specific compiler flags are not picked up anymore, leading to a configuration failure.
#
# To avoid this issue, disable automatic embedding of the compilers into the qt toolchain when
# cross compiling. This is merely a heuristic, becacuse we don't have enough data to decide
# when to do it or not.
# For example on Linux one might want to allow mixing of clang and gcc (maybe).
#
# To allow such use cases when the default is wrong, one can provide a flag to explicitly opt-in
# or opt-out of the compiler embedding into the Qt toolchain.
#
# Passing -DQT_EMBED_TOOLCHAIN_COMPILER=ON will force embedding of the compilers.
# Passing -DQT_EMBED_TOOLCHAIN_COMPILER=OFF will disable embedding of the compilers.
set(__qt_embed_toolchain_compilers TRUE)
if(CMAKE_CROSSCOMPILING)
set(__qt_embed_toolchain_compilers FALSE)
endif()
if(DEFINED QT_EMBED_TOOLCHAIN_COMPILER)
if(QT_EMBED_TOOLCHAIN_COMPILER)
set(__qt_embed_toolchain_compilers TRUE)
else()
set(__qt_embed_toolchain_compilers FALSE)
endif()
endif()
if(__qt_embed_toolchain_compilers)
list(APPEND init_platform "set(CMAKE_CXX_COMPILER \"${CMAKE_CXX_COMPILER}\" CACHE STRING \"\")")
list(APPEND init_platform "set(CMAKE_C_COMPILER \"${CMAKE_C_COMPILER}\" CACHE STRING \"\")")
endif()
unset(__qt_embed_toolchain_compilers)
if(APPLE)
# For simulator_and_device build, we should not explicitly set the sysroot.