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* gcc-wwwdocs branch master updated. f1d2be6c9fcc52d676266e7ede123953d150aaf3
@ 2020-05-07 10:24 Jonathan Wakely
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From: Jonathan Wakely @ 2020-05-07 10:24 UTC (permalink / raw)
To: gcc-cvs-wwwdocs
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- Log -----------------------------------------------------------------
commit f1d2be6c9fcc52d676266e7ede123953d150aaf3
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu May 7 11:24:04 2020 +0100
Document C++17 ABI changes in GCC 10
diff --git a/htdocs/gcc-10/changes.html b/htdocs/gcc-10/changes.html
index f5b70eb4..52fd6e6a 100644
--- a/htdocs/gcc-10/changes.html
+++ b/htdocs/gcc-10/changes.html
@@ -30,6 +30,12 @@ a work-in-progress.</p>
<!-- .................................................................. -->
<h2>Caveats</h2>
<ul>
+ <li>
+ An ABI incompatibility between C++14 and C++17 has been fixed. On some
+ targets a class with a zero-sized subobject would be passed incorrectly
+ when compiled as C++17 or C++20.
+ See the <a href="#empty_base">C++ notes below</a> for more details.
+ </li>
<li>
The deprecated Profile Mode and <code>array_allocator</code> extensions
have been removed from libstdc++.
@@ -409,6 +415,39 @@ int get_naïve_pi() {
The attribute <code>deprecated</code> can now be used on
<code>namespace</code>s too.
</li>
+ <li>
+ <a name="empty_base">The ABI</a>
+ of passing and returning certain C++ classes by value changed
+ on several targets in GCC 10, including
+ <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94383">AArch64</a>,
+ <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94711">ARM</a>,
+ <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94383">PowerPC ELFv2</a>,
+ <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94704">S/390</a>
+ and
+ <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94706">Itanium</a>.
+ These changes affect classes with a zero-sized
+ subobject (an empty base class, or data member with the
+ <code>[[no_unique_address]]</code> attribute) where all other non-static
+ data members have the same type (this is called a "homogeneous aggregate"
+ in some ABI specifications, or if there is only one such member,
+ a "single element").
+ In <code>-std=c++17</code> and <code>-std=c++20</code> modes, classes with
+ an empty base class were not considered to have a single element or
+ to be a homogeneous aggregate, and so could be passed differently
+ (in the wrong registers or at the wrong stack address). This could make
+ code compiled with <code>-std=c++17</code> and <code>-std=c++14</code>
+ ABI incompatible. This has been corrected and the empty bases are
+ ignored in those ABI decisions, so functions compiled with
+ <code>-std=c++14</code> and <code>-std=c++17</code> are now ABI compatible
+ again.
+ Example:
+ <code>struct empty {}; struct S : empty { float f; }; void f(S);</code>.
+ Similarly, in classes containing non-static data members with empty
+ class types using the C++20 <code>[[no_unique_address]]</code> attribute,
+ those members weren't ignored in the ABI argument passing decisions
+ as they should be. Both of these ABI changes are now diagnosed with
+ <code>-Wpsabi</code>.
+ </li>
</ul>
<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
-----------------------------------------------------------------------
Summary of changes:
htdocs/gcc-10/changes.html | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)
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2020-05-07 10:24 gcc-wwwdocs branch master updated. f1d2be6c9fcc52d676266e7ede123953d150aaf3 Jonathan Wakely
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