Hi!
This is my humble attempt to document the C++ ABI changes.
Or do you have something better?
Not sure e.g. how C++ calls the TREE_ADDRESSABLE types which are
passed/returned by invisible reference, perhaps it would be nice
to make it clear that those aren't affected.
As the release tarballs contain a copy of changes.html, I'm afraid
we need to resolve this before I can roll the tarballs.
--- htdocs/gcc-10/changes.html 2020-05-07 10:44:17.391465227 +0200
+++ htdocs/gcc-10/changes.html 2020-05-07 11:02:50.580691123 +0200
@@ -31,6 +31,27 @@ a work-in-progress.</p>
<h2>Caveats</h2>
<ul>
<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 AArch64, ARM, PowerPC ELFv2,
+ S/390 and Itanium. In <code>-std=c++17</code> and <code>-std=c++20</code>
+ modes for classes with empty bases which otherwise contain only a single
+ element or are handled as homogeneous aggregates in
+ <code>-std=c++14</code> and earlier modes the existence of the empty
+ bases resulted in those classes not to be considered as having a single
+ element or be homogeneous aggregate and so could be passed differently,
+ making <code>-std=c++17</code> and <code>-std=c++14</code> compiled code
+ ABI incompatible. This has been corrected and the empty bases are
+ ignored in those ABI decisions, so <code>-std=c++14</code> and
+ <code>-std=c++17</code> compiled codes are now again ABI compatible.
+ Example: <code>struct empty {}; struct S : public empty { float f;
}</code>.
+ Similarly, in classes containing non-static data members with empty
+ class types and with 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>
+ <li>
The deprecated Profile Mode and <code>array_allocator</code> extensions
have been removed from libstdc++.
</li>
Jakub
