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8297852: Backout 8252584 for the time being
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Reviewed-by: kbarrett
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TheShermanTanker committed Dec 1, 2022
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49 changes: 1 addition & 48 deletions doc/hotspot-style.html
Original file line number Diff line number Diff line change
Expand Up @@ -72,7 +72,6 @@ <h1 class="title">HotSpot Coding Style</h1>
<li><a href="#expression-sfinae" id="toc-expression-sfinae">Expression
SFINAE</a></li>
<li><a href="#enum" id="toc-enum">enum</a></li>
<li><a href="#alignas" id="toc-alignas">alignas</a></li>
<li><a href="#thread_local" id="toc-thread_local">thread_local</a></li>
<li><a href="#nullptr" id="toc-nullptr">nullptr</a></li>
<li><a href="#atomic" id="toc-atomic">&lt;atomic&gt;</a></li>
Expand Down Expand Up @@ -599,7 +598,7 @@ <h3 id="c-standard-library">C++ Standard Library</h3>
<code>std::numeric_limits</code>.</li>
<li><code>#include &lt;type_traits&gt;</code>.</li>
<li><code>#include &lt;cstddef&gt;</code> to use
<code>std::nullptr_t</code> and <code>std::max_align_t</code>.</li>
<code>std::nullptr_t</code>.</li>
</ul>
<p>TODO: Rather than directly #including (permitted) Standard Library
headers, use a convention of #including wrapper headers (in some
Expand Down Expand Up @@ -671,52 +670,6 @@ <h3 id="enum">enum</h3>
constant members. Compilers having such bugs are no longer supported.
Except where an enum is semantically appropriate, new code should use
integral constants.</p>
<h3 id="alignas">alignas</h3>
<p><em>Alignment-specifiers</em> (<code>alignas</code> <a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf">n2341</a>)
are permitted, with restrictions.</p>
<p><em>Alignment-specifiers</em> are permitted when the requested
alignment is a <em>fundamental alignment</em> (not greater than
<code>alignof(std::max_align_t)</code> <a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf">C++14
3.11/2</a>).</p>
<p><em>Alignment-specifiers</em> with an <em>extended alignment</em>
(greater than <code>alignof(std::max_align_t)</code> <a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf">C++14
3.11/3</a>) may only be used to align variables with static or automatic
storage duration (<a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf">C++14
3.7.1, 3.7.3</a>). As a consequence, <em>over-aligned types</em> are
forbidden; this may change if HotSpot updates to using C++17 or later <a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html">p0035r4</a>.</p>
<p>Large <em>extended alignments</em> should be avoided, particularly
for stack allocated objects. What is a large value may depend on the
platform and configuration. There may also be hard limits for some
platforms.</p>
<p>An <em>alignment-specifier</em> must always be applied to a
definition (<a
href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf">C++14
10.6.2/6</a>). (C++ allows an <em>alignment-specifier</em> to optionally
also be applied to a declaration, so long as the definition has
equivalent alignment. There isn't any known benefit from duplicating the
alignment in a non-definition declaration, so such duplication should be
avoided in HotSpot code.)</p>
<p>Enumerations are forbidden from having <em>alignment-specifiers</em>.
Aligned enumerations were originally permitted but insufficiently
specified, and were later (C++20) removed <a
href="https://cplusplus.github.io/CWG/issues/2354.html">CWG 2354</a>.
Permitting such usage in HotSpot now would just cause problems in the
future.</p>
<p><em>Alignment-specifiers</em> are forbidden in <code>typedef</code>
and <em>alias-declarations</em>. This may work or may have worked in
some versions of some compilers, but was later (C++14) explicitly
disallowed <a
href="https://cplusplus.github.io/CWG/issues/1437.html">CWG
1437</a>.</p>
<p>The HotSpot macro <code>ATTRIBUTE_ALIGNED</code> provides similar
capabilities for platforms that define it. This macro predates the use
by HotSpot of C++ versions providing <code>alignas</code>. New code
should use <code>alignas</code>.</p>
<h3 id="thread_local">thread_local</h3>
<p>Avoid use of <code>thread_local</code> (<a
href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2659.htm">n2659</a>);
Expand Down
47 changes: 1 addition & 46 deletions doc/hotspot-style.md
Original file line number Diff line number Diff line change
Expand Up @@ -573,7 +573,7 @@ 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 <cstddef>` to use `std::nullptr_t` and `std::max_align_t`.
* `#include <cstddef>` to use `std::nullptr_t`.

TODO: Rather than directly \#including (permitted) Standard Library
headers, use a convention of \#including wrapper headers (in some
Expand Down Expand Up @@ -651,51 +651,6 @@ constant members. Compilers having such bugs are no longer supported.
Except where an enum is semantically appropriate, new code should use
integral constants.

### alignas

_Alignment-specifiers_ (`alignas`
[n2341](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf))
are permitted, with restrictions.

_Alignment-specifiers_ are permitted when the requested alignment is a
_fundamental alignment_ (not greater than `alignof(std::max_align_t)`
[C++14 3.11/2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf)).

_Alignment-specifiers_ with an _extended alignment_ (greater than
`alignof(std::max_align_t)`
[C++14 3.11/3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf))
may only be used to align variables with static or automatic storage duration
([C++14 3.7.1, 3.7.3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf)).
As a consequence, _over-aligned types_ are forbidden; this may change if
HotSpot updates to using C++17 or later
[p0035r4](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html).

Large _extended alignments_ should be avoided, particularly for stack
allocated objects. What is a large value may depend on the platform and
configuration. There may also be hard limits for some platforms.

An _alignment-specifier_ must always be applied to a definition
([C++14 10.6.2/6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf)).
(C++ allows an _alignment-specifier_ to optionally also be applied to a
declaration, so long as the definition has equivalent alignment. There isn't
any known benefit from duplicating the alignment in a non-definition
declaration, so such duplication should be avoided in HotSpot code.)

Enumerations are forbidden from having _alignment-specifiers_. Aligned
enumerations were originally permitted but insufficiently specified, and were
later (C++20) removed
[CWG 2354](https://cplusplus.github.io/CWG/issues/2354.html).
Permitting such usage in HotSpot now would just cause problems in the future.

_Alignment-specifiers_ are forbidden in `typedef` and _alias-declarations_.
This may work or may have worked in some versions of some compilers, but was
later (C++14) explicitly disallowed
[CWG 1437](https://cplusplus.github.io/CWG/issues/1437.html).

The HotSpot macro `ATTRIBUTE_ALIGNED` provides similar capabilities for
platforms that define it. This macro predates the use by HotSpot of C++
versions providing `alignas`. New code should use `alignas`.

### thread_local

Avoid use of `thread_local`
Expand Down

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