Build Fails on Windows 10: CMake Cannot Locate `clang` / `clang++` Compiler

setup_env.py から始めて、Windows の OS_EXTRA_ARGS と CMake コンパイラ引数を確認し、その後 CMakeLists.txt を調べて、python setup_env.py -md models/BitNet-b1.58-2B-4T -q i2_s で再現します。CMake が Windows の C/C++ コンパイラを見つけ、利用できない clang または clang++ のパスを必要とせずに、ビルドが構成以降まで完了すれば完了です。

Environment

• OS: Windows 10
• Command: python setup_env.py -md models/BitNet-b1.58-2B-4T -q i2_s

Problem

When running the setup script, CMake fails to identify and locate the C/C++ compilers (clang / clang++). The build does not proceed beyond the configuration phase.

I also referenced #513 for potential solutions, but the suggestions there did not resolve the issue in my environment.

Error Output

-- The C compiler identification is unknown
-- The CXX compiler identification is unknown

CMake Error at CMakeLists.txt:2 (project):
  The CMAKE_C_COMPILER:
    clang
  is not a full path and was not found in the PATH.  Perhaps the extension is
  missing?
  Tell CMake where to find the compiler by setting either the environment
  variable "CC" or the CMake cache entry CMAKE_C_COMPILER to the full path to
  the compiler, or to the compiler name if it is in the PATH.

CMake Error at CMakeLists.txt:2 (project):
  The CMAKE_CXX_COMPILER:
    clang++
  is not a full path and was not found in the PATH.  Perhaps the extension is
  missing?
  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

-- Configuring incomplete, errors occurred!

Root Cause

The script attempts to use the ClangCL toolset on Windows with explicitly set clang / clang++ compilers. However, on some Windows environments:

1. ClangCL is not installed or not properly configured.
2. clang / clang++ are not present in the system PATH, causing CMake to fail.

---

Solution

I made two modifications in setup_env.py, after which the build completed successfully.

Change 1: Switch CMake Generator from ClangCL to MinGW Makefiles

Original:

OS_EXTRA_ARGS = {
    "Windows": ["-T", "ClangCL"],
}

Modified:

OS_EXTRA_ARGS = {
    "Windows": ["-G", "MinGW Makefiles"],
}

The -T ClangCL option requires a specific Clang installation that is compatible with MSVC. If this toolchain is not properly set up, CMake cannot locate the compiler.

Switching to MinGW Makefiles allows CMake to use the default system compiler (typically gcc/g++ from MinGW) without requiring ClangCL.

Change 2: Remove Explicit clang / clang++ Compiler Specification

Original:

run_command([
    "cmake", "-B", "build",
    _COMPILER_EXTRA_ARGS[arch],
    OS_EXTRA_ARGS.get(platform.system(), []),
    "-DCMAKE_C_COMPILER=clang",
    "-DCMAKE_CXX_COMPILER=clang++"
], log_step="generate_build_files")

Modified:

run_command([
    "cmake", "-B", "build",
    _COMPILER_EXTRA_ARGS[arch],
    OS_EXTRA_ARGS.get(platform.system(), [])
], log_step="generate_build_files")

The -DCMAKE_C_COMPILER=clang and -DCMAKE_CXX_COMPILER=clang++ flags forced CMake to look for Clang in PATH, which was not available.

Removing these flags allows CMake to auto-detect the default compiler provided by the MinGW Makefiles generator.