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CMake: Opt in for NEW CMP0156 on WebAssembly 

Otherwise duplicate symbol errors occur when -sMAIN_MODULE is passed
to the executable and a static library is linked multiple times into
the executable, which is common in Qt builds, due to how qt plugin
propagation causes link cycles.

Note that having CMP0156 set to NEW relies on CMake having proper
support for de-duplication of libraries when targeting Emscripten /
WebAssembly, which is only being considered to be added in CMake
4.2+, which as of now is not released yet.

For earlier CMake versions, for CMP0156 to apply properly, a project
can workaround it by setting

set(CMAKE_CXX_PLATFORM_LINKER_ID LLD)
set(CMAKE_CXX_LINK_LIBRARIES_PROCESSING ORDER=REVERSE DEDUPLICATION=ALL)

in the project manually, even though that is not supported by CMake
itself, and relies on CMake internals that might change between
versions.

Pick-to: 6.9 6.8
Task-number: QTBUG-140211
Change-Id: Iece0f6893bbd0528e83c5cdbeddd29a51ff35991
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from commit d6d832d0d2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Alexandru Croitor 2025-09-17 18:59:35 +02:00 committed by Qt Cherry-pick Bot
parent 8e08522adc
commit 531c49d307
1 changed files with 10 additions and 4 deletions
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14
cmake/QtPublicCMakeVersionHelpers.cmake
View File

@ -155,14 +155,20 @@ function(__qt_internal_set_cmp0156)
if(APPLE)
set(default_policy_value NEW)
set(unsupported_policy_value OLD)
set(platform_string "Apple")
elseif(EMSCRIPTEN OR WASM)
# It's a similar story for Emscripten WebAssembly. If a project passes -sMAIN_MODULE=1,
# this will cause duplicate symbol errors when qt's static libraries are mentioned
# more than once on the link line, unless we let CMake deduplicate them. This only works
# for CMake 4.2+, which lets CMake know that the emscripten linker can resolve all symbols
# even if the library is not repeated.
set(default_policy_value NEW)
set(unsupported_policy_value OLD)
else()
# For non-Apple linkers, we keep the previous behavior of not deduplicating libraries,
# because we haven't done the necessary testing to identify on which platforms
# it is safe to deduplicate.
set(default_policy_value OLD)
set(unsupported_policy_value NEW)
set(platform_string "non-Apple")
endif()
# Force set the default policy value for the given platform, even if the policy value is
@ -171,7 +177,7 @@ function(__qt_internal_set_cmp0156)
get_cmake_property(debug_message_shown _qt_internal_cmp0156_debug_message_shown)
if(NOT debug_message_shown)
message(DEBUG "Force setting the CMP0156 policy to '${default_policy_value}' "
"for ${platform_string} platforms.")
"for platform '${CMAKE_SYSTEM_NAME}'.")
set_property(GLOBAL PROPERTY _qt_internal_cmp0156_debug_message_shown TRUE)
endif()
@ -183,7 +189,7 @@ function(__qt_internal_set_cmp0156)
if("${policy_value}" STREQUAL "${unsupported_policy_value}" AND NOT QT_BUILDING_QT)
message(WARNING
"CMP0156 is set to '${policy_value}'. Qt forces the '${default_policy_value}'"
" behavior of this policy for ${platform_string} platforms by default."
" behavior of this policy for the '${CMAKE_SYSTEM_NAME}' platform by default."
" Set QT_FORCE_CMP0156_TO_VALUE=${unsupported_policy_value} to force"
" the '${unsupported_policy_value}' behavior for Qt commands that create"
" library or executable targets.")