[wwwdocs] Porting-to-14: Mention new pragma GCC Target behavior

[Martin Jambor <mjambor@suse.cz>]

Hello,

when looking at a package build issue with GCC 14, Michal Jireš noted a
different behavior of pragma GCC Target.  This snippet tries to describe
the gist of the problem.  I have left it in the C section even though it
is not really C specific, but could not think of a good name for a new
section for it.  Ideas (and any other suggestions for improvements)
welcome, of course.

Otherwise, would this be good to go to the wwwdocs?

Thanks,

Martin

diff --git a/htdocs/gcc-14/porting_to.html b/htdocs/gcc-14/porting_to.html
index c825a68e..ae9a3cde 100644
--- a/htdocs/gcc-14/porting_to.html
+++ b/htdocs/gcc-14/porting_to.html
@@ -490,6 +490,43 @@ in C23.
 GCC will probably continue to support old-style function definitions
 even once C23 is used as the default language dialect.
 
+<h4 id="gcc-targte-pragma">Pragma GCC Target now affects preprocessor symbols</h4>
+
+<p>
+The behavior of pragma GCC Target has changed in GCC 14.  For example,
+GCC 13 and below defines <code>__AVX2__</code> only when the target
+is specified on the command line.  This has been considered <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87299">a
+bug</a> and since it was fixed in GCC 14, <code>__AVX2__</code> is now also
+defined with <code>#pragma GCC target("avx2")</code>.
+
+<p>
+Therefore, if macros expand to something like the snippet below,
+functions will be (silently) compiled for an incorrect instruction
+set.
+
+<pre>
+  #if ! __AVX2__
+  #pragma GCC push_options
+  #pragma GCC target("avx2")
+  #endif
+
+  /* Code to be compiled for AVX2. */
+
+  /* With GCC 14, __AVX2__ here will always be defined and pop_options
+  never called. */
+  #if ! __AVX2__
+  #pragma GCC pop_options
+  #endif
+
+  /* With GCC 14, all following functions will be compiled for AVX2
+  which was not intended. */
+</pre>
+
+<p>
+The fix in this case would be to remember
+whether <code>pop_options</code> needs to be performed in a new
+user-defined macro.
+
 <h2 id="cxx">C++ language issues</h2>
 
 <h3 id="header-dep-changes">Header dependency changes</h3>