On Wed, 2 Apr 2025, Jakub Jelinek wrote:
> On Wed, Apr 02, 2025 at 10:32:20AM +0200, Richard Biener wrote:
> > I wonder if we can amend the documentation to suggest to end lifetime
> > of variables explicitly by proper scoping?
>
> In the -Wmaybe-musttail-local-addr attribute description I've already
> tried to show that in the example, but if you think something like
> the following would make it clearer:
OK.
Thanks,
Richard.
> 2025-04-02 Jakub Jelinek <ja...@redhat.com>
>
> * doc/extend.texi (musttail statement attribute): Hint how
> to avoid -Wmaybe-musttail-local-addr warnings.
>
> --- gcc/doc/extend.texi.jj 2025-04-02 10:46:40.447281167 +0200
> +++ gcc/doc/extend.texi 2025-04-02 11:24:25.042027221 +0200
> @@ -9368,6 +9368,31 @@ baz (int *x)
> @}
> @}
> @end smallexample
> +
> +To avoid the @option{-Wmaybe-musttail-local-addr} warning in the
> +above @code{*x == 2} case and similar code, consider defining the
> +maybe escaped variables in a separate scope which will end before the
> +return statement if possible to make it clear that the variable is not
> +live during the call. So
> +
> +@smallexample
> + else if (*x == 2)
> + @{
> + @{
> + int a = 42;
> + bar (&a);
> + @}
> + /* The call will be tail called (would not be without the
> + attribute), if bar stores the pointer anywhere, dereferencing
> + it in foo will be undefined behavior and there will be a warning
> + emitted for this with @option{-Wextra}, which implies
> + @option{-Wmaybe-musttail-local-addr}. */
> + [[gnu::musttail]] return foo (nullptr);
> + @}
> +@end smallexample
> +
> +in this case. That is not possible if it is function argument which
> +is address taken because those are in scope for the whole function.
> @end table
>
> @node Attribute Syntax
>
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
