Constraint for inline assembly with one register used for both an input variable and an output variable
Georg-Johann Lay
avr@gjlay.de
Thu Jun 5 12:14:24 GMT 2025
Am 05.06.25 um 13:43 schrieb Xi Ruoyao via Gcc-help:
> Hi,
>
> When we write syscall wrappers with inline assembly, we often need to
> assign one register for both an input variable and an output variable.
> Per https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html,
> such an inline assembly can be written
>
> register int *p1 asm ("r0") = …;
> register int *p2 asm ("r1") = …;
> register int *result asm ("r0");
> asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2));
>
> Note the use of constraint "0" for p1 which shares the same register as
> result. But currently in the Linux kernel vDSO, they are actually
> written like
>
> asm ("sysint" : "=r" (result) : "r" (p1), "r" (p2));
>
> So the question here: is using "r" for p1 valid or not? Do we need to
> "fix" this everywhere for the vDSO?
That code is perfectly fine, since the register numbers for p1 and
result are the same "r0" at the point of the inline asm.
Johann
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