LLVM ignores CMAKE_CXX_STANDARD when used without CACHE STRING FORCE

[mccakit]

When building llvm unless I pass the c++ standard as a forced cache variable, it defaults to c++17. No matter what I use

[tstellar]

@mccakit Can you give an example of the cmake invocation that isn't working?

[mccakit]

Sure

Toolchain

set(CMAKE_C_COMPILER /home/mccakit/dev/llvm/bin/clang)
set(CMAKE_CXX_COMPILER /home/mccakit/dev/llvm/bin/clang++)
set(CMAKE_ASM_COMPILER /home/mccakit/dev/llvm/bin/clang)
set(CMAKE_RC_COMPILER /home/mccakit/dev/llvm/bin/llvm-rc)
set(CMAKE_AR /home/mccakit/dev/llvm/bin/llvm-ar)
set(CMAKE_RANLIB /home/mccakit/dev/llvm/bin/llvm-ranlib)
set(CMAKE_MT /home/mccakit/dev/llvm/bin/llvm-mt)


set(CMAKE_POSITION_INDEPENDENT_CODE ON)
add_compile_options(
    -flto=thin
    -nostdinc++
    -nostdlib++
    -isystem /home/mccakit/dev/libcxx/native/include/c++/v1
)
add_link_options(
    -flto=thin
    -fuse-ld=lld
    -rtlib=compiler-rt
    -nostdlib++
    /home/mccakit/dev/libcxx/native/lib/libunwind.a
    /home/mccakit/dev/libcxx/native/lib/libc++abi.a
    /home/mccakit/dev/libcxx/native/lib/libc++.a
)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_C_STANDARD 23 CACHE STRING "" FORCE)
set(CMAKE_C_EXTENSIONS ON CACHE BOOL "" FORCE)

set(PKG_CONFIG_EXECUTABLE "/home/mccakit/dev/pkgconf/bin/pkgconf.py" CACHE FILEPATH "" FORCE)
set(CMAKE_BUILD_TYPE RELEASE)
set(BUILD_SHARED_LIBS OFF)

command
cmake -S llvm -B build -G Ninja -DLLVM_TARGETS_TO_BUILD=all -DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=all -DLLVM_ENABLE_PROJECTS="clang;lld;clang-tools-extra;lldb" -DLLVM_ENABLE_RUNTIMES="libc;libcxx;libcxxabi;libunwind;compiler-rt;libsycl" -DLLVM_BINUTILS_INCDIR=/usr/include -DCMAKE_TOOLCHAIN_FILE=$HOME/dev/toolchains/static/native.cmake

build log

Run Build Command(s): /home/mccakit/dev/depot_tools/ninja -v
[1/7896] /home/mccakit/dev/llvm/bin/clang++ -DGTEST_HAS_RTTI=0 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/mccakit/dev/llvm-project/build/lib/Support -I/home/mccakit/dev/llvm-project/llvm/lib/Support -I/home/mccakit/dev/llvm-project/build/include -I/home/mccakit/dev/llvm-project/llvm/include -I/home/mccakit/dev/llvm-project/llvm/../third-party/siphash/include -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections  -O3 -DNDEBUG -std=c++17 -fPIC -flto=thin -nostdinc++ -nostdlib++ -isystem /home/mccakit/dev/libcxx/native/include/c++/v1  -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/ABIBreak.cpp.o -MF lib/Support/CMakeFiles/LLVMSupport.dir/ABIBreak.cpp.o.d -o lib/Support/CMakeFiles/LLVMSupport.dir/ABIBreak.cpp.o -c /home/mccakit/dev/llvm-project/llvm/lib/Support/ABIBreak.cpp

[tstellar]

Does it work if you use CACHE without FORCE ?

[mccakit]

No, further llvm only builds on cxx20, it fails to build on 23 and 26