8274400: HotSpot Style Guide should permit use of alignof

Reviewed-by: kbarrett, kvn, dholmes, dcubed

doc/hotspot-style.html

@@ -598,10 +598,17 @@ <h3 id="c-standard-library">C++ Standard Library</h3>
 <code>std::nothrow</code>, and <code>std::nothrow_t</code>.</li>
 <li><code>#include &lt;limits&gt;</code> to use
 <code>std::numeric_limits</code>.</li>
-<li><code>#include &lt;type_traits&gt;</code>.</li>
+<li><code>#include &lt;type_traits&gt;</code> with some restrictions,
+listed below.</li>
 <li><code>#include &lt;cstddef&gt;</code> to use
 <code>std::nullptr_t</code> and <code>std::max_align_t</code>.</li>
 </ul>
+<p>Certain restrictions apply to the declarations provided by
+<code>&lt;type_traits&gt;</code>.</p>
+<ul>
+<li>The <code>alignof</code> operator should be used rather than
+<code>std::alignment_of&lt;&gt;</code>.</li>
+</ul>
 <p>TODO: Rather than directly #including (permitted) Standard Library
 headers, use a convention of #including wrapper headers (in some
 location like hotspot/shared/stdcpp). This provides a single place for
@@ -1086,6 +1093,8 @@ <h3 id="attributes">Attributes</h3>
 <h3 id="additional-permitted-features">Additional Permitted
 Features</h3>
 <ul>
+<li><p><code>alignof</code> (<a
+href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf">n2341</a>)</p></li>
 <li><p><code>constexpr</code> (<a
 href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2235.pdf">n2235</a>)
 (<a

doc/hotspot-style.md

@@ -572,9 +572,13 @@ There are a few exceptions to this rule.
 
 * `#include <new>` to use placement `new`, `std::nothrow`, and `std::nothrow_t`.
 * `#include <limits>` to use `std::numeric_limits`.
-* `#include <type_traits>`.
+* `#include <type_traits>` with some restrictions, listed below.
 * `#include <cstddef>` to use `std::nullptr_t` and `std::max_align_t`.
 
+Certain restrictions apply to the declarations provided by `<type_traits>`.
+
+* The `alignof` operator should be used rather than `std::alignment_of<>`.
+
 TODO: Rather than directly \#including (permitted) Standard Library
 headers, use a convention of \#including wrapper headers (in some
 location like hotspot/shared/stdcpp).  This provides a single place
@@ -1082,6 +1086,9 @@ The following attributes are expressly forbidden:
 
 ### Additional Permitted Features
 
+* `alignof`
+([n2341](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf))
+
 * `constexpr`
 ([n2235](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2235.pdf))
 ([n3652](https://isocpp.org/files/papers/N3652.html))