On Fri, May 27, 2016 at 07:45:18AM +0200, Christophe Leroy wrote:

quoted

quoted

Wouldn't it be better to add nops before the function entry in order to
get the hot loop aligned, instead of adding nops in the middle of the
function ?

Why would that be better?  The nops are executed once per function call
in either case, there are the same number of nops in either case, and
on most CPUs nops aren't actually executed anyway (they are decoded and
the thrown away).

The idea was to not execute them:

|.balign 16 nop nop _GLOBAL(strcpy) addi	r5,r3,-1 addi	r4,r4,-1 1: 
lbzu r0,1(r4) cmpwi	0,r0,0 stbu	r0,1(r5) bne	1b blr |

That performs _worse_ on most modern CPUs (the first decode will decode
less, so instructions are available for execution later).  That's why
functions are aligned in the first place!


Segher