[compiler-rt] CMake 4.0.x breaks build on macOS

[1cebp]

CMake 4.0.x breaks compiler-rt during the second stage configure step.

It looks like the new version doesn't properly pass -isysroot=... to configuration targets on macOS, causing the build to fail. (Clarification: CMake intentionally does not specify a macOS SDK path as of 4.0.x, see @bradking's reply)

[build] -- Got ld supported ARCHES: armv6 armv7 armv7s arm64 arm64e arm64_32 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em
[build] -- Toolchain supported arches: armv6;armv7;armv7s;arm64;arm64e;arm64_32;i386;x86_64;x86_64h;armv6m;armv7k;armv7m;armv7em
[build] -- Using cached valid architectures for osx.
[build] -- OSX supported arches: arm64;arm64e;x86_64;x86_64h
[build] -- Compiler-RT supported architectures: arm64;arm64e;x86_64;x86_64h
[build] -- Host linker version: 1167.4.1
[build] CMake Error at /Users/icebp/llvm/compiler-rt/CMakeLists.txt:500 (message):
[build]   -g is not supported by host compiler
[build] 
[build] 
[build] -- Configuring incomplete, errors occurred!
[build] FAILED: runtimes/runtimes-stamps/runtimes-configure /Users/icebp/llvm/build/runtimes/runtimes-stamps/runtimes-configure 

Downstream users have mitigated by adding -isysroot manually.
microsoft/vcpkg#44712

[izuzak]

I've run into this as well and had to apply the same medicine (manually set isysroot). There seems to be a related issue in the CMake project: https://gitlab.kitware.com/cmake/cmake/-/issues/26863

[ADKaster]

fwiw the linked vcpkg change does not pass -isysroot manually in the problematic cases. It avoids a "missing/empty variable" error by not passing -isysroot when CMake has not automatically set CMAKE_OSX_SYSROOT. This is not a problem when using Apple Clang, or an Xcode.app/CommandLineTools compiler. But it does become a problem when using homebrew clang, or clang from the llvm.org official GitHub releases.

[bradking]

This was an intentional change to CMake 4.0, and is called out in the release notes:

• Builds targeting macOS no longer choose any SDK or pass an -isysroot flag to the compiler by default. Instead, compilers are expected to choose a default macOS SDK on their own. In order to use a compiler that does not do this, users must now specify CMAKE_OSX_SYSROOT when configuring their build, e.g., -DCMAKE_OSX_SYSROOT=macosx.

In general CMake expects the toolchain to support $CC hello.c out of the box. The only reason CMake ever gained a default CMAKE_OSX_SYSROOT was because it was needed even with macOS system compilers /usr/bin/{cc,c++} for a brief period around the macOS 10.8 days. Then it was kept for compatibility until a major version bump, which has now arrived.

CMake Issue 26863 was opened to track LLVM/Clang's lack of default macOS SDK selection. I'd been meaning to come over here to open an issue to discuss it.

[1cebp]

This was an intentional change to CMake 4.0, and is called out in the release notes:

Thanks for the clarification. Edited.

[bradking]

I'd been meaning to come over here to open an issue to discuss it.

I've opened #137352 as a dedicated place to discuss the underlying problem.