Re: [PATCH 1/2] x86_64/memset: avoid multiply insn if possible

[Rich Felker <dalias@libc.org>]

On Thu, Feb 12, 2015 at 09:36:26PM +0100, Denys Vlasenko wrote:
> On Thu, Feb 12, 2015 at 8:26 PM, Denys Vlasenko
> <vda.linux@googlemail.com> wrote:
> >> I'd actually like to extend the "short" range up to at least 32 bytes
> >> using two 8-byte writes for the middle, unless the savings from using
> >> 32-bit imul instead of 64-bit are sufficient to justify 4 4-byte
> >> writes for the middle. On the cpu I tested on, the difference is 11
> >> cycles vs 32 cycles for non-rep path versus rep path at size 32.
> >
> > The short path causes mixed feelings in me.
> >
> > On one hand, it's elegant in a contrived way.
> >
> > On the other hand, multiple
> > overlaying stores must be causing hell in store unit.
> > I'm thinking, maybe there's a faster way to do that.

In practice it performs quite well. x86's are good at this. The
generic C code in memset.c does not do any overlapping writes of
different sizes for the short buffer code path -- all writes there are
single-byte, and multiple-write only happens for some of the inner
bytes depending on the value of n.

> For example, like in the attached implementation.
> 
> This one will not perform eight stores to memory
> to fill 15 byte area... only two.

I could try comparing its performance, but I expect branches to cost a
lot more than redundant stores to cached memory. My approach in the C
code seems to be the absolute minimum possible number of branches for
short memsets, and it pays off -- it's even faster than the current
asm for these small sizes.

Rich