[clang] Add intrin0.h header to mimic intrin0.h used by MSVC STL for clang-cl

clang/docs/ReleaseNotes.rst

@@ -964,6 +964,26 @@ Windows Support
   linking may succeed but the resulting executables may expose issues at
   runtime.
 
+- Clang-cl now supports function targets with intrinsic headers. This allows
+  for runtime feature detection of intrinsics. Previously under clang-cl
+  ``immintrin.h`` and similar intrinsic headers would only include the intrinsics
+  if building with that feature enabled at compile time, e.g. ``avxintrin.h``
+  would only be included if AVX was enabled at compile time. This was done to work
+  around include times from MSVC STL including ``intrin.h`` under clang-cl.
+  Clang-cl now provides ``intrin0.h`` for MSVC STL and therefore all intrinsic
+  features without requiring enablement at compile time.
+  Fixes: (`#53520 <https://github.com/llvm/llvm-project/issues/53520>`_)
+
+- Improved compile times with MSVC STL. MSVC provides ``intrin0.h`` which is a
+  header that only includes intrinsics that are used by MSVC STL to avoid the
+  use of ``intrin.h``. MSVC STL when compiled under clang uses ``intrin.h``
+  instead. Clang-cl now provides ``intrin0.h`` for the same compiler throughput
+  purposes as MSVC. Clang-cl also provides ``yvals_core.h`` to redefine
+  ``_STL_INTRIN_HEADER`` to expand to ``intrin0.h`` instead of ``intrin.h``.
+  This also means that if all intrinsic features are enabled at compile time
+  including STL headers will no longer slow down compile times since ``intrin.h``
+  is not included from MSVC STL.
+
 LoongArch Support
 ^^^^^^^^^^^^^^^^^
 

clang/lib/Headers/CMakeLists.txt

@@ -253,8 +253,10 @@ set(x86_files
   )
 
 set(windows_only_files
+  intrin0.h
   intrin.h
   vadefs.h
+  yvals_core.h
 )
 
 set(utility_files

clang/lib/Headers/bmiintrin.h

@@ -115,8 +115,7 @@ _mm_tzcnt_64(unsigned long long __X)
 
 #undef __RELAXED_FN_ATTRS
 
-#if !(defined(_MSC_VER) || defined(__SCE__)) || __has_feature(modules) ||      \
-    defined(__BMI__)
+#if !defined(__SCE__) || __has_feature(modules) || defined(__BMI__)
 
 /* Define the default attributes for the functions in this file. */
 #define __DEFAULT_FN_ATTRS __attribute__((__always_inline__, __nodebug__, __target__("bmi")))
@@ -425,7 +424,6 @@ __blsr_u64(unsigned long long __X)
 
 #undef __DEFAULT_FN_ATTRS
 
-#endif /* !(defined(_MSC_VER) || defined(__SCE__)) || __has_feature(modules)   \
-          || defined(__BMI__) */
+#endif /* !defined(__SCE__) || __has_feature(modules) || defined(__BMI__) */
 
 #endif /* __BMIINTRIN_H */