* asm() / goto() concern
[not found] <1538337561.2146473.1653382920615.ref@mail.yahoo.com>
@ 2022-05-24 9:02 ` Ian Worthington
2022-05-24 20:56 ` Segher Boessenkool
0 siblings, 1 reply; 2+ messages in thread
From: Ian Worthington @ 2022-05-24 9:02 UTC (permalink / raw)
To: gcc-help
I'm trying to write some inline assembler but I see that asm() does not support branching to labels, and that asm goto() does not support output (at least prior to gcc 11).
If I tie them together by declaring an output of the asm() as an input to asm goto() I assume that this will prevent the optimiser from reordering them, but how can I ensure that they are run without any intervening instructions if the goto() has a dependency on the cc generated by the asm(), eg:
asm ( "algr %[rs],%[rb] \n\t"
: [rs] "+r" (sum) // output
: [rb] "r" (u64b) // input
);
asm goto ( "bc 3,%l[done]" // branch if carry bit set (cc 2 or 3)
: // output
: [rs] "r" (sum) // input: ensure this comes after the algr
: // clobbers
: done
);
Best wishes / Mejores deseos / Meilleurs vœux
Ian ...
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: asm() / goto() concern
2022-05-24 9:02 ` asm() / goto() concern Ian Worthington
@ 2022-05-24 20:56 ` Segher Boessenkool
0 siblings, 0 replies; 2+ messages in thread
From: Segher Boessenkool @ 2022-05-24 20:56 UTC (permalink / raw)
To: Ian Worthington; +Cc: gcc-help
Hi!
On Tue, May 24, 2022 at 09:02:00AM +0000, Ian Worthington via Gcc-help wrote:
> I'm trying to write some inline assembler but I see that asm() does not support branching to labels, and that asm goto() does not support output (at least prior to gcc 11).
>
> If I tie them together by declaring an output of the asm() as an input to asm goto() I assume that this will prevent the optimiser from reordering them, but how can I ensure that they are run without any intervening instructions if the goto() has a dependency on the cc generated by the asm(), eg:
You cannot. Sorry.
> asm ( "algr %[rs],%[rb] \n\t"
> : [rs] "+r" (sum) // output
> : [rb] "r" (u64b) // input
> );
>
> asm goto ( "bc 3,%l[done]" // branch if carry bit set (cc 2 or 3)
> : // output
> : [rs] "r" (sum) // input: ensure this comes after the algr
> : // clobbers
> : done
> );
You have to write to something that is a C variable in that first asm,
and read from that in the asm goto. You don't get any advantage over
plain C that way, of course, for your example at least.
Alternatively you can use a newer compiler. Or you can write the code
you want as a real assembler function (in a .s file, not inline asm).
gl;hf,
Segher
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2022-05-24 9:02 ` asm() / goto() concern Ian Worthington
2022-05-24 20:56 ` Segher Boessenkool
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