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

[Martin Jambor <mjambor@suse.cz>]

Hi,

On Wed, May 01 2024, Gerald Pfeifer wrote:
> On Tue, 30 Apr 2024, Martin Jambor wrote:
>> +<h3 id="gcc-targte-pragma">Pragma GCC Target now affects preprocessor symbols</h4>
>
> Note the id: should be "gcc-target-pragma", though I even suggest to 
> simplify and say "target-pragma".
>
>> +The behavior of pragma GCC Target and specifically how it affects ISA
>
> Seconding Jakub's
>  
>   "And here as well, perhaps even <code>#pragma GCC target</code>."
>
>> +macros has changed in GCC 14.  In GCC 13 and older, the <code>GCC
>> +target</code> pragma defined and undefined corresponding ISA macros in
>> +C when using integrated preprocessor during compilation but not when
>
> "...the integrated preprocessor..."
>
>> +preprocessor was invoked as a separate step or when using -save-temps.
>
> "...the preprocessor..."
>
> and <code>-save-temps</code>, or better "the <code>-save-temps</code> 
> option".
>
>> +This can lead to different behavior, especially in C++.  For example,
>> +functions the C++ snippet below will be (silently) compiled for an
>> +incorrect instruction set by GCC 14.
>
> "functions" above looks like it's extraneous and should be skipped?
>
>> +  /* With GCC 14, __AVX2__ here will always be defined and pop_options
>> +  never called. */
>> +  #if ! __AVX2__
>> +  #pragma GCC pop_options
>> +  #endif
>
> Maybe a bit subtle, I would not say a #pragma is called; how about invoked 
> or activated?
>
>> +<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.
>
> "The fix in this case is to remember" (or "...remembering...")
>

Thanks for your suggestions, this is what I am going to commit in a
moment.

Martin


diff --git a/htdocs/gcc-14/porting_to.html b/htdocs/gcc-14/porting_to.html
index c825a68e..a20d82c2 100644
--- a/htdocs/gcc-14/porting_to.html
+++ b/htdocs/gcc-14/porting_to.html
@@ -514,6 +514,48 @@ be included explicitly when compiling with GCC 14:
 </li>
 </ul>
 
+<h3 id="target-pragma">Pragma GCC target now affects preprocessor symbols</h4>
+
+<p>
+The behavior of pragma GCC target and specifically how it affects ISA
+macros has changed in GCC 14.  In GCC 13 and older, the <code>GCC
+target</code> pragma defined and undefined corresponding ISA macros in
+C when using the integrated preprocessor during compilation but not
+when the preprocessor was invoked as a separate step or when using
+the <code>-save-temps</code> option.  In C++ the ISA macro definitions
+were performed in a way which did not have any actual effect.
+
+In GCC 14 C++ behaves like C with integrated preprocessing in earlier
+versions. Moreover, in both languages ISA macros are defined and
+undefined as expected when preprocessing separately from compilation.
+
+<p>
+This can lead to different behavior, especially in C++.  For example,
+a part of the C++ snippet below will be (silently) compiled for an
+incorrect instruction set by GCC 14.
+
+<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 invoked. */
+  #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 is to remember whether <code>pop_options</code>
+needs to be performed in a new user-defined macro.
+
 <!-- <h2 id="fortran">Fortran language issues</h2> -->
 
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