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:
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
