Building LLVM from source fails on Debian 12 -- compiler can't invoke cc1 (file not found)

[blackeyeliner]

Additional Info

• I am using Windows Subsystem for Linux

Output of brew config

$ brew config
HOMEBREW_VERSION: 4.5.10-67-ge804860
ORIGIN: https://github.com/Homebrew/brew
HEAD: e8048606d1408678b1e375e8c2c70b797bb5a006
Last commit: 8 hours ago
Branch: main
Core tap JSON: 19 Jul 10:09 UTC
HOMEBREW_PREFIX: /home/linuxbrew/.linuxbrew
HOMEBREW_CASK_OPTS: []
HOMEBREW_MAKE_JOBS: 20
HOMEBREW_SORBET_RUNTIME: set
Homebrew Ruby: 3.4.5 => /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/3.4.5/bin/ruby
CPU: 20-core 64-bit alderlake
Clang: N/A
Git: 2.39.5 => /bin/git
Curl: 7.88.1 => /bin/curl
Kernel: Linux 6.1.0-35-amd64 x86_64 GNU/Linux
OS: Debian GNU/Linux 12 (bookworm) (bookworm)
Host glibc: 2.36
/usr/bin/gcc: 12.2.0
/usr/bin/ruby: N/A
glibc: N/A
gcc@11: N/A
gcc: 15.1.0
xorg: N/A

Output of brew doctor

$ brew doctor
Your system is ready to brew.

Description of issue

I am trying to build LLVM from source on Debian 12.

All dependencies do install correctly, but build persistently fails while looking for liblzma like so:

-- Looking for lzma_auto_decoder in /home/linuxbrew/.linuxbrew/lib/liblzma.so
-- Looking for lzma_auto_decoder in /home/linuxbrew/.linuxbrew/lib/liblzma.so - not found
-- Looking for lzma_easy_encoder in /home/linuxbrew/.linuxbrew/lib/liblzma.so
-- Looking for lzma_easy_encoder in /home/linuxbrew/.linuxbrew/lib/liblzma.so - not found
-- Looking for lzma_lzma_preset in /home/linuxbrew/.linuxbrew/lib/liblzma.so
-- Looking for lzma_lzma_preset in /home/linuxbrew/.linuxbrew/lib/liblzma.so - not found
CMake Error at /home/linuxbrew/.linuxbrew/opt/cmake/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:227 (message):
  Could NOT find LibLZMA (missing: LIBLZMA_HAS_AUTO_DECODER
  LIBLZMA_HAS_EASY_ENCODER LIBLZMA_HAS_LZMA_PRESET) (found version "5.8.1")

My (muti-hour) investigation narrowed it down to CheckSymbolExists.cmake behavior, and particularly to try_compile() function. It feels like when CMake is invoking compiler from this function, it's stripped of all environment variables (or many, at least), which makes it fallback to system gcc, which is 12.2 as opposed to gcc-15 that's installed by brew.

What DOES work is if I correct environment right before try_compile() in check_symbol_exists() in CheckSymbolExists.cmake

        set(ENV{HOMEBREW_CC} "gcc-15")
        set(ENV{HOMEBREW_CXX} "g++-15")-
        set(ENV{GCC_EXEC_PREFIX} "/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/libexec/gcc/x86_64-pc-linux-gnu/15/")
        set(ENV{PATH} "/home/linuxbrew/.linuxbrew/opt/binutils/bin:/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/libexec/gcc/x86_64-pc-linux-gnu/15:$ENV{PATH}")
        set(ENV{LIBRARY_PATH} "/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/lib/gcc/current/gcc/x86_64-pc-linux-gnu/15:/home/linuxbrew/.linuxbrew/lib:$ENV{LIBRARY_PATH}")
        try_compile( ... )

Then it compiles correctly → finds the library correctly → proceeds with compiling LLVM.

So it's definitely some sort of an environment issue, but I can't find a way to set these variables "the right way" for Homebrew so that CMake then sees them and respects them.

Any ideas?

[blackeyeliner]

And the bizarre behavior is - if I create a wrapper bash script that "just" executes the same gcc, and replace gcc executable with it in Homebrew folder, it starts to compile properly.

So somehow if CMake calls gcc directly, it can't find cc1, but if gcc is wrapped in a bash script, it can.

And another bizarre thing is, I can't reliably reproduce it - a fresh Debian 12 install with gcc + build-essential and Homebrew does install and build LLVM right out of the box.

[alebcay]

The Homebrew build environment does indeed unset environment variables. As far as forcing the build to occur with brewed GCC instead of system GCC, you can try the fails_with DSL as described here. If that doesn't work, what you have is probably the best reasonable way.

As you've pointed out, it would be nicer to understand the root cause; the fact that changing the compiler fixes it likely points to some other issue.

[blackeyeliner]

The Homebrew build environment does indeed unset environment variables. As far as forcing the build to occur with brewed GCC instead of system GCC, you can try the fails_with DSL as described here. If that doesn't work, what you have is probably the best reasonable way.

As you've pointed out, it would be nicer to understand the root cause; the fact that changing the compiler fixes it likely points to some other issue.

I just tried with fails_with - it worked in a way that gcc-15 is not used, but it didn't work in a way that the build still fails.

So basically I am getting "gcc-15: fatal error" instead of "gcc: fatal error" in CMakeConfigureLog.yaml:

        /home/linuxbrew/.linuxbrew/Homebrew/Library/Homebrew/shims/linux/super/gcc-15 -D_GNU_SOURCE  -fPIC -fno-semantic-interposition -Werror=date-time -fno-lifetime-dse -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wno-missing-field-initializers -pedantic -Wno-long-long -Wimplicit-fallthrough -Wno-comment -fdiagnostics-color -ffunction-sections -fdata-sections -ffile-prefix-map=/var/tmp/llvm-20250721-32566-e62xvb/llvm-project-20.1.8.src/llvm/build=../../ -ffile-prefix-map=/var/tmp/llvm-20250721-32566-e62xvb/llvm-project-20.1.8.src/=. -no-canonical-prefixes -DCHECK_FUNCTION_EXISTS=lzma_lzma_preset -O3 -DNDEBUG -fPIE -o CMakeFiles/cmTC_53ad0.dir/CheckFunctionExists.c.o -c /var/tmp/llvm-20250721-32566-e62xvb/llvm-project-20.1.8.src/llvm/build/CMakeFiles/CMakeScratch/TryCompile-TwP51t/CheckFunctionExists.c
        gcc-15: fatal error: cannot execute 'cc1': posix_spawnp: No such file or directory
        compilation terminated.
        ninja: build stopped: subcommand failed.

I checked both

$ gcc-15 -print-prog-name=cc1
/home/linuxbrew/.linuxbrew/Cellar/gcc/15.1.0/bin/../libexec/gcc/x86_64-pc-linux-gnu/15/cc1

gcc -print-prog-name=cc1
/usr/lib/gcc/x86_64-linux-gnu/12/cc1

Both exist, valid and working

[carlocab]

Why are you trying to build LLVM from source? You should be able to install it from a bottle.

Homebrew should use your system compiler by default, unless it is is too old (which it isn't, in your case).