Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH] x86/uaccess: small optimization in unsafe_copy_to_user() | Date | Sat, 17 Apr 2021 13:59:32 +0000 |
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From: Al Viro <viro@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 <edumazet@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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