[PATCH v2 3/2] RISC-V: sbi: remove sbi_ecall tracepoints

[David Laight]

On Wed, 25 Jun 2025 09:51:45 +0200
Radim Krčmář <rkrcmar at ventanamicro.com> wrote:

> 2025-06-24T15:09:09+02:00, Radim Krčmář <rkrcmar at ventanamicro.com>:
> > For another example, let's have the following function:
> >
> >   struct sbiret some_sbi_ecall(uintptr_t a0, uintptr_t a1)
> >   {
> >     return sbi_ecall(123, 456, a0, a1);
> >   }
> >
...
> 
> GCC 15.1 still leaves "mv" outside the branch, but at least seems to be
> on the right track (undesired overhead is marked with leading stars):
> 
>    0xffffffff800236e8 <+0>:	addi	sp,sp,-48
>    0xffffffff800236ea <+2>:	sd	s0,32(sp)
>    0xffffffff800236ec <+4>:	sd	ra,40(sp)
>    0xffffffff800236ee <+6>:	addi	s0,sp,48
> *  0xffffffff800236f0 <+8>:	mv	a4,a0
> *  0xffffffff800236f2 <+10>:	mv	a5,a1
>    0xffffffff800236f4 <+12>:	nop
> *  0xffffffff800236f8 <+16>:	mv	a0,a4
> *  0xffffffff800236fa <+18>:	mv	a1,a5
>    0xffffffff800236fc <+20>:	li	a7,123
>    0xffffffff80023700 <+24>:	li	a6,456
>    0xffffffff80023704 <+28>:	ecall
> *  0xffffffff80023708 <+32>:	mv	a5,a0
> *  0xffffffff8002370a <+34>:	mv	a2,a1
>    0xffffffff8002370c <+36>:	nop
>    0xffffffff80023710 <+40>:	ld	ra,40(sp)
>    0xffffffff80023712 <+42>:	ld	s0,32(sp)
> *  0xffffffff80023714 <+44>:	mv	a0,a5
> *  0xffffffff80023716 <+46>:	mv	a1,a2
>    0xffffffff80023718 <+48>:	addi	sp,sp,48
>    0xffffffff8002371a <+50>:	ret
>    [Tracing goes to +126]

How much do a few register moves/spills matter compared to the
cost of the called code?
There will but much worse things out there if you look.

	David