* Tell GCC functions do not modify struct
@ 2022-03-11 7:03 Yangfl
2022-03-14 21:43 ` Martin Sebor
0 siblings, 1 reply; 2+ messages in thread
From: Yangfl @ 2022-03-11 7:03 UTC (permalink / raw)
To: gcc-help
Hello Everyone,
I'm playing with GCC optimization and come up with a quite common scenario:
struct A { int a; };
void print_a(const struct A *);
int test() {
const struct A a = {3};
print_a(&a);
return a.a == 3;
}
GCC always products a load operation of a.a, that's reasonable. But my
question is, how to tell GCC that the function is guaranteed to not
modify the struct?
I've tried `access` attribute, but that does not help. Of course I can
use __builtin_unreachable around the function call to make a promise,
but that is not a struct-independent solution.
Thanks
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Tell GCC functions do not modify struct
2022-03-11 7:03 Tell GCC functions do not modify struct Yangfl
@ 2022-03-14 21:43 ` Martin Sebor
0 siblings, 0 replies; 2+ messages in thread
From: Martin Sebor @ 2022-03-14 21:43 UTC (permalink / raw)
To: Yangfl, gcc-help
On 3/11/22 00:03, Yangfl via Gcc-help wrote:
> Hello Everyone,
>
> I'm playing with GCC optimization and come up with a quite common scenario:
>
> struct A { int a; };
>
> void print_a(const struct A *);
>
> int test() {
> const struct A a = {3};
> print_a(&a);
> return a.a == 3;
> }
>
> GCC always products a load operation of a.a, that's reasonable. But my
> question is, how to tell GCC that the function is guaranteed to not
> modify the struct?
>
> I've tried `access` attribute, but that does not help. Of course I can
> use __builtin_unreachable around the function call to make a promise,
> but that is not a struct-independent solution.
GCC doesn't yet use the access attribute for optimization.
The solution in standard C is to declare the pointer argument restrict:
void print_a (const struct A * restrict);
Regrettably, GCC doesn't yet take advantage of this notation for
optimization either. A request for this enhancement is being
tracked in pr81009:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81009
Martin
>
> Thanks
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2022-03-14 21:43 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-03-11 7:03 Tell GCC functions do not modify struct Yangfl
2022-03-14 21:43 ` Martin Sebor
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).