[libcxx] Run tests on Windows/arm64 too

This qualifies this configuration as a supported target platform.

MinGW on aarch64 doesn't need defining __USE_MINGW_ANSI_STDIO=1 for
having working printfs of long doubles, so skip it for this
target, to get some more corner case test coverage.

Author: mstorsjo

.github/workflows/libcxx-build-and-test.yaml

@@ -233,7 +233,6 @@ jobs:
             **/crash_diagnostics/*
 
   windows:
-    runs-on: windows-2022
     strategy:
       fail-fast: false
       matrix:
@@ -243,6 +242,8 @@ jobs:
         - { config: mingw-static, mingw: true }
         - { config: mingw-dll-i686, mingw: true }
         - { config: mingw-incomplete-sysroot, mingw: true }
+        - { config: mingw-static-aarch64, mingw: true, runner: windows-11-arm }
+    runs-on: ${{ matrix.runner != '' && matrix.runner || 'windows-latest' }}
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       - name: Install dependencies
@@ -256,7 +257,7 @@ jobs:
       - name: Install llvm-mingw
         if: ${{ matrix.mingw == true }}
         run: |
-          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20250709/llvm-mingw-20250709-ucrt-x86_64.zip
+          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20250709/llvm-mingw-20250709-ucrt-${{ matrix.runner == 'windows-11-arm' && 'aarch64' || 'x86_64' }}.zip
           powershell Expand-Archive llvm-mingw*.zip -DestinationPath .
           del llvm-mingw*.zip
           mv llvm-mingw* c:\llvm-mingw

libcxx/cmake/caches/MinGW.cmake

@@ -9,4 +9,9 @@ set(LIBUNWIND_USE_COMPILER_RT ON CACHE BOOL "")
 
 # Without this flag, 'long double' (which is 80 bit on x86 mingw, but
 # 64 bit in MSVC) isn't handled correctly in printf.
-set(LIBCXX_EXTRA_SITE_DEFINES "__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
+if (CMAKE_SYSTEM_PROCESSOR MATCHES "ARM.*" OR CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
+  # On aarch64, long doubles use the same ABI as in MSVC mode, so we don't need
+  # to enable the MinGW specific stdio routines.
+else()
+  set(LIBCXX_EXTRA_SITE_DEFINES "__USE_MINGW_ANSI_STDIO=1" CACHE STRING "")
+endif()

libcxx/docs/index.rst

@@ -147,7 +147,7 @@ macOS 10.13+          i386, x86_64, arm64
 FreeBSD 12+           i386, x86_64, arm
 Linux                 i386, x86_64, arm, arm64  Only glibc-2.24 and later and no other libc is officially supported
 Android 5.0+          i386, x86_64, arm, arm64
-Windows               i386, x86_64              Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
+Windows               i386, x86_64, arm64       Both MSVC and MinGW style environments, ABI in MSVC environments is :doc:`unstable <DesignDocs/ABIVersioning>`
 AIX 7.2TL5+           powerpc, powerpc64
 Embedded (picolibc)   arm
 ===================== ========================= ============================

libcxx/utils/ci/run-buildbot

@@ -706,7 +706,7 @@ mingw-dll)
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
     check-runtimes
 ;;
-mingw-static)
+mingw-static|mingw-static-aarch64)
     clean
     generate-cmake \
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake" \