From: Al Viro <v...@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: 16 April 2021 20:44
> On Fri, Apr 16, 2021 at 12:24:13PM -0700, Eric Dumazet wrote:
> > From: Eric Dumazet <eduma...@google.com>
> >
> > We have to loop only to copy u64 values.
> > After this first loop, we copy at most one u32, one u16 and one byte.
>
> Does it actually yield a better code?
>
> FWIW, this
> void bar(unsigned);
> void foo(unsigned n)
> {
> while (n >= 8) {
> bar(n);
> n -= 8;
> }
> while (n >= 4) {
> bar(n);
> n -= 4;
> }
> while (n >= 2) {
> bar(n);
> n -= 2;
> }
> while (n >= 1) {
> bar(n);
> n -= 1;
> }
> }
This variant might be better:
void foo(unsigned n)
{
while (n >= 8) {
bar(8);
n -= 8;
}
if (likely(!n))
return;
if (n & 4)
bar(4);
if (n & 2)
bar(2);
if (n & 1)
bar(1);
}
I think Al's version might have optimised down to this,
but Eric's asm contains the n -= 4/2/1;
OTOH gcc can make a real pig's breakfast of code like this!
David
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