Le 10/03/2017 à 09:41, Michael Ellerman a écrit :
Christophe Leroy <christophe.le...@c-s.fr> 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
