[clang] Add intrin0.h header to mimic intrin0.h used by MSVC STL …

…for clang-cl (llvm#75711)

Fixes llvm#53520.

#### Description ####

Provide `intrin0.h` to be the minimal set of intrinsics that the MSVC
STL requires.
The `intrin0.h` header matches the latest header provided by MSVC 1939
which does include some extra intrinsics that the MSVC STL does not use.

Inside `BuiltinHeaders.def` I kept the header description as `intrin.h`.
If you want me to change those to `intrin0.h` for the moved intrinsics
let me know.

This should now allow `immintrin.h` to be used with function targets for
runtime cpu detection of simd instruction sets without worrying about
the compile-time overhead from MSVC STL including `intrin.h` on clang.

I still need to figure out how to best update MSVC STL to detect for the
presence of `intrin0.h` from clang and to use this header over
`intrin.h`.

#### Testing ####

Built clang locally and ran the test suite. I still need to do a pass
over the existing unit tests for the ms intrinsics to make sure there
aren't any gaps. Wanted to get this PR up for discussion first.

Modified latest MSVC STL from github to point to `intrin0.h` for clang.

Wrote some test files that included MSVC STL headers that rely on
intrinsics such as `atomic`, `bit` and `vector`. Built the unit tests
against x86, arm, aarch64, and x64.

#### Benchmarks ####

The following include times are based on the x64 target with the
modified headers in this PR.
These timings were done by using `clang-cl.exe -ftime-trace` and taking
the wall time for parsing `intrin.h` and `intrin0.h`.

`intrin.h` takes ~897ms to parse.
`intrin0.h` takes ~1ms to parse.

If there is anything required or a different approach is preferred let
me know. I would very much like to move this over the finish line so we
can use function targets with clang-cl.

clang/docs/ReleaseNotes.rst

@@ -454,6 +454,26 @@ Android Support
 Windows Support
 ^^^^^^^^^^^^^^^
 
+- 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

@@ -254,8 +254,10 @@ set(x86_files
   )
 
 set(windows_only_files
+  intrin0.h
   intrin.h
   vadefs.h
+  yvals_core.h
 )
 
 set(utility_files

clang/lib/Headers/bmiintrin.h

@@ -161,8 +161,7 @@ _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")))
@@ -610,7 +609,6 @@ __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 */