Re: [PATCH] powerpc: sysdev: cpm1: Optimise gpio bit calculation

[Christophe LEROY]

Le 10/03/2017 à 09:41, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@xxxxxx> writes:
>
> > Help a bit the compiler to provide better code:
> >
> > unsigned int f(int i)
> > {
> > 	return 1 << (31 - i);
> > }
> >
> > unsigned int g(int i)
> > {
> > 	return 0x80000000 >> i;
> > }
> >
> > Disassembly of section .text:
> >
> > 00000000 <f>:
> >    0:	20 63 00 1f 	subfic  r3,r3,31
> >    4:	39 20 00 01 	li      r9,1
> >    8:	7d 23 18 30 	slw     r3,r9,r3
> >    c:	4e 80 00 20 	blr
> >
> > 00000010 <g>:
> >   10:	3d 20 80 00 	lis     r9,-32768
> >   14:	7d 23 1c 30 	srw     r3,r9,r3
> >   18:	4e 80 00 20 	blr
>
> Well yeah, it saves one instruction, but is it worth it? Are these gpio
> routines in some hot path I don't know about?

It saves one instruction, and one register (see other exemple below 
where r3 is to be preserved)

gpio_get() and gpio_set() are used extensively by some GPIO based 
drivers like SPI, NAND, so it may be worth it as it doesn't impair 
readability (if anyone prefers, we could write  (1 << 31) >> i  instead 
of  0x80000000 >> i )

unsigned int f(int i, unsigned int *a)
{
	*a = 1 << (31 - i);

	return i;
}

unsigned int g(int i, unsigned int *a)
{
	*a = 0x80000000 >> i;

	return i;
}

toto.o:     file format elf32-powerpc


Disassembly of section .text:

00000000 <f>:
   0:	21 43 00 1f 	subfic  r10,r3,31
   4:	39 20 00 01 	li      r9,1
   8:	7d 29 50 30 	slw     r9,r9,r10
   c:	91 24 00 00 	stw     r9,0(r4)
  10:	4e 80 00 20 	blr

00000014 <g>:
  14:	3d 20 80 00 	lis     r9,-32768
  18:	7d 29 1c 30 	srw     r9,r9,r3
  1c:	91 24 00 00 	stw     r9,0(r4)
  20:	4e 80 00 20 	blr