gate QBLAS subproject on disable_quadblas option

The new compiled QBLAS uses GCC-only flags (-march, -mavx2), POSIX-only
APIs (clock_gettime, sysconf, _SC_LEVEL2_CACHE_SIZE), and GCC built-ins
for CPUID detection - none of which build under MSVC. The legacy
header-only QBLAS happened to compile on Windows because the wrap was
five lines and produced no objects.

Add a disable_quadblas meson option (default false; auto-enabled on
Windows) that:

  - skips dependency('qblas', fallback: ...) entirely
  - declares an empty qblas_dep so the rest of the build proceeds
  - adds -DDISABLE_QUADBLAS to project args so quadblas_interface.cpp
    and umath/matmul.cpp take the naive-kernel branches that already
    exist behind that guard

This removes the CFLAGS=/DDISABLE_QUADBLAS hack from the Windows CI
since meson now sets the preprocessor flag itself (and translates -D
to /D for MSVC). Verified on Linux: -Ddisable_quadblas=true builds
without pulling the QBLAS subproject; default still builds it.

Author: SwayamInSync

.github/workflows/build_wheels.yml

@@ -114,7 +114,9 @@ jobs:
           path: ./wheelhouse/*.whl
           name: wheels-${{ matrix.os }}
 
-  # disabling QBLAS optimization for windows due to incompatibility with MSVC
+  # QBLAS is auto-disabled on Windows by meson.build (it uses GCC/POSIX-only
+  # constructs that MSVC does not support); the wheel falls back to the
+  # naive matmul kernel. No CFLAGS hack needed.
   build_wheels_windows:
     name: Build wheels on Windows
     runs-on: windows-latest
@@ -153,9 +155,6 @@ jobs:
           CIBW_BUILD_VERBOSITY: "3"
           DISTUTILS_USE_SDK: "1"
           MSSdk: "1"
-          CIBW_ENVIRONMENT: >
-            CFLAGS="/DDISABLE_QUADBLAS $CFLAGS"
-            CXXFLAGS="/DDISABLE_QUADBLAS $CXXFLAGS"
           CIBW_REPAIR_WHEEL_COMMAND: 'delvewheel repair -w {dest_dir} {wheel} --add-path C:\sleef\bin'
           CIBW_TEST_COMMAND_WINDOWS: pip install numpy && pip install --no-deps {wheel} && pip install pytest pytest-run-parallel && pytest -s {project}/tests
           CIBW_TEST_EXTRAS: test

meson.build

@@ -15,7 +15,22 @@ if is_windows
   add_project_arguments('-DWIN32', '-D_WINDOWS', language : ['c', 'cpp'])
 endif
 
-qblas_dep = dependency('qblas', fallback: ['qblas', 'qblas_dep'])
+# QBLAS does not build under MSVC (GCC-only flags, POSIX-only APIs, GCC
+# built-ins for CPUID); force-disable it on Windows. Users on other
+# platforms can opt out via -Ddisable_quadblas=true to fall back to the
+# naive matmul kernel.
+disable_quadblas = is_windows or get_option('disable_quadblas')
+if disable_quadblas
+  if is_windows
+    message('QBLAS disabled (Windows / MSVC) - using naive matmul kernel')
+  else
+    message('QBLAS disabled by user option - using naive matmul kernel')
+  endif
+  add_project_arguments('-DDISABLE_QUADBLAS', language: ['c', 'cpp'])
+  qblas_dep = declare_dependency()
+else
+  qblas_dep = dependency('qblas', fallback: ['qblas', 'qblas_dep'])
+endif
 
 # Try to find SLEEF system-wide first, fall back to subproject if not found
 # Required SLEEF version (must match sleef.wrap revision)

meson.options

@@ -1,3 +1,9 @@
 option('disable_fma', type: 'boolean', value: false,
        description: 'Disable FMA (Fused Multiply-Add) code paths' +
-                    'Set to true when building for older CPUs like Sandy Bridge that lack FMA support.')
+                    'Set to true when building for older CPUs like Sandy Bridge that lack FMA support.')
+
+option('disable_quadblas', type: 'boolean', value: false,
+       description: 'Skip the QBLAS subproject and fall back to naive ' +
+                    'matmul kernels. Auto-enabled on Windows because ' +
+                    'QBLAS uses GCC/POSIX-only constructs that do not ' +
+                    'build under MSVC.')