Build Windows pybind in CI (#11018)

Author: jathu

.github/workflows/build-presets.yml

@@ -72,24 +72,17 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        preset: [pybind]
+        preset: [pybind, llm]
     with:
       job-name: build
       ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
       submodules: recursive
       timeout: 90
       script: |
         set -eux
-        conda init powershell
-        powershell -Command "& {
-          \$ErrorActionPreference = 'Stop'
-          Set-PSDebug -Trace 1
+        conda create --yes --quiet -n et python=3.12
+        conda activate et
 
-          conda create --yes --quiet -n et python=3.12
-          conda activate et
-
-          python install_requirements.py
-          cmake --preset ${{ matrix.preset }}
-          \$numCores = [System.Environment]::GetEnvironmentVariable('NUMBER_OF_PROCESSORS') - 1
-          cmake --build cmake-out -j \$numCores
-        }"
+        python install_requirements.py
+        cmake --preset ${{ matrix.preset }}
+        cmake --build cmake-out -j$(( $(nproc) - 1 ))

CMakeLists.txt

@@ -72,7 +72,16 @@ if(NOT PYTHON_EXECUTABLE)
 endif()
 announce_configured_options(PYTHON_EXECUTABLE)
 
+# Check if we are on Windows
+if(WIN32)
+  if(DEFINED CMAKE_CXX_COMPILER_ID AND NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    message(FATAL_ERROR "Windows builds require Clang compiler. Current compiler: ${CMAKE_CXX_COMPILER_ID}")
+  elseif(NOT DEFINED CMAKE_CXX_COMPILER_ID)
+    set(CMAKE_CXX_COMPILER_ID "ClangCL")
+  endif()
+endif()
 announce_configured_options(CMAKE_CXX_COMPILER_ID)
+
 announce_configured_options(CMAKE_TOOLCHAIN_FILE)
 announce_configured_options(BUCK2)
 

install_executorch.py

@@ -221,12 +221,6 @@ def main(args):
         # latest PT commit otherwise
         use_pytorch_nightly = False
 
-    # Use ClangCL on Windows.
-    # ClangCL is an alias to Clang that configures it to work in an MSVC-compatible
-    # mode. Using it on Windows to avoid compiler compatibility issues for MSVC.
-    if os.name == "nt":
-        cmake_args.append("-T ClangCL")
-
     #
     # Install executorch pip package. This also makes `flatc` available on the path.
     # The --extra-index-url may be necessary if pyproject.toml has a dependency on a