CheckIPOSupported not regarded CMAKE_LINKER_TYPE

Usage
1

At the present time, GCC LTO cannot link with LLVM LLD, but CheckIPOSupported may give different answer depending on how cmake command line invoked.

For

CC=gcc CXX=g++ LDFLAGS=-fuse-ld=lld cmake

CheckIPOSupported returns failure, then the project can safely set IPO to OFF.

For

cmake -D CMAKE_C_COMPILER=gcc -D CMAKE_CXX_COMPILER=g++ -D CMAKE_LINKER_TYPE=LLD

CheckIPOSupported returns success, then the project may set IPO to ON, then get link-time errors.

2

I failed to reproduce your problem. Can you provide a snippet reproducing the problem?
By the way, can you specify your environment (platform, compiler type and version, CMake version)?

3

Environment

Arch Linux
cmake version 3.31.1
ninja 1.12.1
gcc (GCC) 14.2.1 20240910
LLD 18.1.8 (compatible with GNU linkers)

Project to Reproduce

CMakeLists.txt:

cmake_minimum_required(VERSION 3.20)
project(test_proj)
include(CheckIPOSupported)
check_ipo_supported(RESULT C_COMPILER_HAS_IPO_SUPPORT LANGUAGES C)
message(STATUS "LTO: ${C_COMPILER_HAS_IPO_SUPPORT}")
add_executable(main main.c)
set_property(TARGET main PROPERTY INTERPROCEDURAL_OPTIMIZATION ${C_COMPILER_HAS_IPO_SUPPORT})

main.c:

#include <stdio.h>
int main() {
  printf("Hello world!\n");
  return 0;
}

Command Line Outputs

$ cmake -B build -G Ninja -D CMAKE_C_COMPILER=gcc -D CMAKE_CXX_COMPILER=g++ -D CMAKE_LINKER_TYPE=LLD        
-- The C compiler identification is GNU 14.2.1
-- The CXX compiler identification is GNU 14.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/g++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- LTO: YES
-- Configuring done (1.0s)
-- Generating done (0.0s)
-- Build files have been written to: /tmp/tmp.J0m6gKPUMX/build
$ ninja -C build -v                                                                                 
ninja: Entering directory `build'
[1/2] /usr/bin/gcc   -flto=auto -fno-fat-lto-objects -MD -MT CMakeFiles/main.dir/main.c.o -MF CMakeFiles/main.dir/main.c.o.d -o CMakeFiles/main.dir/main.c.o -c /tmp/tmp.J0m6gKPUMX/main.c
[2/2] : && /usr/bin/gcc -flto=auto -fno-fat-lto-objects -fuse-ld=lld -Wl,--dependency-file,CMakeFiles/main.dir/link.d CMakeFiles/main.dir/main.c.o -o main   && :
FAILED: main 
: && /usr/bin/gcc -flto=auto -fno-fat-lto-objects -fuse-ld=lld -Wl,--dependency-file,CMakeFiles/main.dir/link.d CMakeFiles/main.dir/main.c.o -o main   && :
ld.lld: error: undefined symbol: main
>>> referenced by /usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/../../../../lib/Scrt1.o:(_start)
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

CMakeConfigureLog.yaml


---
-
    (Redacted)
-
    kind: "try_compile-v1"
    backtrace:
      - "/usr/share/cmake/Modules/CheckIPOSupported.cmake:138 (try_compile)"
      - "/usr/share/cmake/Modules/CheckIPOSupported.cmake:261 (_ipo_run_language_check)"
      - "CMakeLists.txt:4 (check_ipo_supported)"
    directories:
      source: "/tmp/tmp.J0m6gKPUMX/build/CMakeFiles/_CMakeLTOTest-C/src"
      binary: "/tmp/tmp.J0m6gKPUMX/build/CMakeFiles/_CMakeLTOTest-C/bin"
    buildResult:
      variable: "_IPO_LANGUAGE_CHECK_RESULT"
      cached: true
      stdout: |
        Change Dir: '/tmp/tmp.J0m6gKPUMX/build/CMakeFiles/_CMakeLTOTest-C/bin'
        
        Run Build Command(s): /usr/bin/ninja -v
        [1/4] /usr/bin/gcc   -flto=auto -fno-fat-lto-objects -o CMakeFiles/boo.dir/main.c.o -c /tmp/tmp.J0m6gKPUMX/build/CMakeFiles/_CMakeLTOTest-C/src/main.c
        [2/4] /usr/bin/gcc   -flto=auto -fno-fat-lto-objects -o CMakeFiles/foo.dir/foo.c.o -c /tmp/tmp.J0m6gKPUMX/build/CMakeFiles/_CMakeLTOTest-C/src/foo.c
        [3/4] : && /usr/bin/cmake -E rm -f libfoo.a && "/usr/bin/gcc-ar" qc libfoo.a  CMakeFiles/foo.dir/foo.c.o && "/usr/bin/gcc-ranlib" libfoo.a && :
        [4/4] : && /usr/bin/gcc -flto=auto -fno-fat-lto-objects  CMakeFiles/boo.dir/main.c.o -o boo  libfoo.a && :
        
      exitCode: 0
...
4

The problem comes from your CMake version specified through cmake_minumum_required() command. The policy CMP0138 is not defined, so flags are propagated to the IPO check.

Update cmake_minumum_required() to version to 3.24 or set explicitly CMP0138 to NEW with cmake_policy() command.