Re: [PATCH] read-cache: avoid memcpy in expand_name_field in index v4

From: Duy Nguyen <pclouds@gmail.com>
Date: Mar 19, 2013, 01:29 UTC
Message-ID: <CACsJy8DhMpQsNY3f4U3FmE1Bd1PvrxDLTYWT6boycrJVtv8WVg@mail.gmail.com>
In-Reply-To: <7vboagoav9.fsf@alter.siamese.dyndns.org>

On Tue, Mar 19, 2013 at 12:50 AM, Junio C Hamano <gitster@pobox.com> wrote:

Show 13 quoted lines

> While it is true that strbuf_remove(&sb, sb.len - trim, trim) is
> equivalent to strbuf_setlen(&sb, sb.len - trim), I wonder why we see
> any memcpy() in the first place.
>
> strbuf_remove(&sb, sb.len - trim, trim) is turned into
> strbuf_splice(&sb, sb.len - trim, trim, NULL, 0) and then in turn it
> does these two:
>
>         memmove(sb.buf + (sb.len - trim) + 0,
>                 sb.buf + sb.len, 0);
>         memcpy(sb.buf + (sb.len - trim), NULL, 0);
>
> both of which should be a no-op, no?

Apparently my memcpy does not bail out early when the third arg is zero (glibc 2.11.2 on gentoo, x86). It cares more about memory alignment. This is the beginning of memcpy:

mov %edi,%eax mov 0x4(%esp),%edi mov %esi,%edx mov 0x8(%esp),%esi mov %edi,%ecx xor %esi,%ecx and $0x3,%ecx mov 0xc(%esp),%ecx cld jne 75946 <memcpy+0x56> cmp $0x3,%ecx jbe 75946 <memcpy+0x56>

Show 10 quoted lines

> There also is this call that has the same "trim at the right end":
>
>     pretty.c:       strbuf_remove(sb, sb->len - trimlen, trimlen);
>
> It almost makes me suggest that it may be a better solution to make
> strbuf_remove() more intelligent about such a call pattern _if_
> these empty memmove/memcpy are so expensive, perhaps like the
> attached.  It could be that strbuf_splice() should be the one that
> ought to be more intelligent, but I'll leave it up to you to
> benchmark to find out where the best place to optimize is.

memcpy is not expensive per-se, but this is (again) webkit, where expand_name_field (and memcpy) is called ~200k times. At that quantity I still prefer fixing in the "hot" call site expand_name_field(), and because strbuf_setlen is an inline function, we make no extra calls.

Making strbuf_remove/strbuf_splice more intelligent may be good (or may make it harder to read), I don't know. But I think it could be a separate topic.

-- 
Duy