[clang] [NFC][Docs] Documenting __builtin_cpu_supports. (PR #84098)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Pavel Iliin (ilinpv)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/84098.diff


1 Files Affected:

- (modified) clang/docs/LanguageExtensions.rst (+28) 


``````````diff
diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index bcd69198eafdbe..f939d1c765e91a 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -2799,6 +2799,34 @@ counter's true frequency will need to be provided by the user.
 
 Query for this feature with ``__has_builtin(__builtin_readsteadycounter)``.
 
+``__builtin_cpu_supports``
+--------------------------
+
+**Syntax**:
+
+.. code-block:: c++
+
+  int __builtin_cpu_supports(const char *features);
+
+**Example of Use:**:
+
+.. code-block:: c++
+
+  if (__builtin_cpu_supports("sve"))
+    sve_code();
+
+**Description**:
+
+The ``__builtin_cpu_supports`` function detects at runtime if target CPU
+supports features specified in string argument. It returns positive integer
+if all features are supported and 0 otherwise. Names of features and format is
+target specific. For example on AArch64 features are combined using ``+`` like
+this ``__builtin_cpu_supports("flagm+sha3+lse+rcpc2+fcma+memtag+bti+sme2")``.
+If feature name is not supported or format is wrong, compiler will issue a
+warning and replace builtin by constant 0.
+
+Query for this feature with ``__has_builtin(__builtin_cpu_supports)``.
+
 ``__builtin_dump_struct``
 -------------------------
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/84098