From 96f71a9a6bf7b52c50a1f52d4d401a48dc40480f Mon Sep 17 00:00:00 2001
From: Kim Barrett <kbarrett@openjdk.org>
Date: Fri, 20 Jun 2025 19:48:41 +0000
Subject: [PATCH] 8255082: HotSpot Style Guide should permit noexcept

Reviewed-by: kvn, dholmes, dcubed
---
 doc/hotspot-style.html | 53 ++++++++++++++++++++++++++++++++++++++++++
 doc/hotspot-style.md   | 51 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+)

diff --git a/doc/hotspot-style.html b/doc/hotspot-style.html
index 22fd7b8167b..dafd29d6f54 100644
--- a/doc/hotspot-style.html
+++ b/doc/hotspot-style.html
@@ -87,6 +87,7 @@ id="toc-local-function-objects">Local Function Objects</a></li>
 <li><a href="#inheriting-constructors"
 id="toc-inheriting-constructors">Inheriting constructors</a></li>
 <li><a href="#attributes" id="toc-attributes">Attributes</a></li>
+<li><a href="#noexcept" id="toc-noexcept">noexcept</a></li>
 <li><a href="#additional-permitted-features"
 id="toc-additional-permitted-features">Additional Permitted
 Features</a></li>
@@ -1140,6 +1141,58 @@ function name and the parameter list.</li>
 <code>memory_order_consume</code>.</li>
 <li><code>[[deprecated]]</code> - Not relevant in HotSpot code.</li>
 </ul>
+<h3 id="noexcept">noexcept</h3>
+<p>Use of <code>noexcept</code> exception specifications (<a
+href="http://wg21.link/n3050">n3050</a>) are permitted with restrictions
+described below.</p>
+<ul>
+<li>Only the argument-less form of <code>noexcept</code> exception
+specifications are permitted.</li>
+<li>Allocation functions that may return <code>nullptr</code> to
+indicate allocation failure must be declared <code>noexcept</code>.</li>
+<li>All other uses of <code>noexcept</code> exception specifications are
+forbidden.</li>
+<li><code>noexcept</code> expressions are forbidden.</li>
+<li>Dynamic exception specifications are forbidden.</li>
+</ul>
+<p>HotSpot is built with exceptions disabled, e.g. compile with
+<code>-fno-exceptions</code> (gcc, clang) or no <code>/EH</code> option
+(MSVC++). So why do we need to consider <code>noexcept</code> at all?
+It's because <code>noexcept</code> exception specifications serve two
+distinct purposes.</p>
+<p>The first is to allow the compiler to avoid generating code or data
+in support of exceptions being thrown by a function. But this is
+unnecessary, because exceptions are disabled.</p>
+<p>The second is to allow the compiler and library code to choose
+different algorithms, depending on whether some function may throw
+exceptions. This is only relevant to a certain set of functions.</p>
+<ul>
+<li><p>Some allocation functions (<code>operator new</code> and
+<code>operator new[]</code>) return <code>nullptr</code> to indicate
+allocation failure. If a <code>new</code> expression calls such an
+allocation function, it must check for and handle that possibility.
+Declaring such a function <code>noexcept</code> informs the compiler
+that <code>nullptr</code> is a possible result. If an allocation
+function is not declared <code>noexcept</code> then the compiler may
+elide that checking and handling for a <code>new</code> expression
+calling that function.</p></li>
+<li><p>Certain Standard Library facilities (notably containers) provide
+different guarantees for some operations (and may choose different
+algorithms to implement those operations), depending on whether certain
+functions (constructors, copy/move operations, swap) are nothrow or not.
+They detect this using type traits that test whether a function is
+declared <code>noexcept</code>. This can have a significant performance
+impact if, for example, copying is chosen over a potentially throwing
+move. But this isn't relevant, since HotSpot forbids the use of most
+Standard Library facilities.</p></li>
+</ul>
+<p>HotSpot code can assume no exceptions will ever be thrown, even from
+functions not declared <code>noexcept</code>. So HotSpot code doesn't
+ever need to check, either with conditional exception specifications or
+with <code>noexcept</code> expressions.</p>
+<p>Dynamic exception specifications were deprecated in C++11. C++17
+removed all but <code>throw()</code>, with that remaining a deprecated
+equivalent to <code>noexcept</code>.</p>
 <h3 id="additional-permitted-features">Additional Permitted
 Features</h3>
 <ul>
diff --git a/doc/hotspot-style.md b/doc/hotspot-style.md
index 92b756e10d3..0a0089ee454 100644
--- a/doc/hotspot-style.md
+++ b/doc/hotspot-style.md
@@ -1130,6 +1130,57 @@ The following attributes are expressly forbidden:
 * `[[carries_dependency]]` - Related to `memory_order_consume`.
 * `[[deprecated]]` - Not relevant in HotSpot code.
 
+### noexcept
+
+Use of `noexcept` exception specifications
+([n3050](http://wg21.link/n3050))
+are permitted with restrictions described below.
+
+* Only the argument-less form of `noexcept` exception specifications are
+permitted.
+* Allocation functions that may return `nullptr` to indicate allocation
+failure must be declared `noexcept`.
+* All other uses of `noexcept` exception specifications are forbidden.
+* `noexcept` expressions are forbidden.
+* Dynamic exception specifications are forbidden.
+
+HotSpot is built with exceptions disabled, e.g. compile with `-fno-exceptions`
+(gcc, clang) or no `/EH` option (MSVC++). So why do we need to consider
+`noexcept` at all? It's because `noexcept` exception specifications serve two
+distinct purposes.
+
+The first is to allow the compiler to avoid generating code or data in support
+of exceptions being thrown by a function. But this is unnecessary, because
+exceptions are disabled.
+
+The second is to allow the compiler and library code to choose different
+algorithms, depending on whether some function may throw exceptions. This is
+only relevant to a certain set of functions.
+
+* Some allocation functions (`operator new` and `operator new[]`) return
+`nullptr` to indicate allocation failure. If a `new` expression calls such an
+allocation function, it must check for and handle that possibility. Declaring
+such a function `noexcept` informs the compiler that `nullptr` is a possible
+result. If an allocation function is not declared `noexcept` then the compiler
+may elide that checking and handling for a `new` expression calling that
+function.
+
+* Certain Standard Library facilities (notably containers) provide different
+guarantees for some operations (and may choose different algorithms to
+implement those operations), depending on whether certain functions
+(constructors, copy/move operations, swap) are nothrow or not. They detect
+this using type traits that test whether a function is declared `noexcept`.
+This can have a significant performance impact if, for example, copying is
+chosen over a potentially throwing move. But this isn't relevant, since
+HotSpot forbids the use of most Standard Library facilities.
+
+HotSpot code can assume no exceptions will ever be thrown, even from functions
+not declared `noexcept`. So HotSpot code doesn't ever need to check, either
+with conditional exception specifications or with `noexcept` expressions.
+
+Dynamic exception specifications were deprecated in C++11. C++17 removed all
+but `throw()`, with that remaining a deprecated equivalent to `noexcept`.
+
 ### Additional Permitted Features
 
 * `alignof`