Am 05.06.25 um 15:18 schrieb Segher Boessenkool:
Hi!
On Thu, Jun 05, 2025 at 02:14:24PM +0200, Georg-Johann Lay via Gcc-help wrote:
Am 05.06.25 um 13:43 schrieb Xi Ruoyao via Gcc-help:
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.
Yup. Various compiler passes will not realise "result" is the same reg
as "p1", but that probably won't limit optimisation here.
Btw, you can write the code as
register int *p1 asm ("r0") = …;
register int *p2 asm ("r1") = …;
int *result;
asm ("sysint" : "+r" (p1) : "0" (p1), "r" (p2));
result = p1;
With "+r" (p1), the input operand "0" (p1) is not needed any more. Just:
asm ("sysint" : "+r" (p1) : "r" (p2));
Johann
(and by choosing better variable names, "r0" and "r1" perhaps, you don't
need to write a copy).
Segher
