Re: [PATCH] speed: reuse char instead of recreation in loop
- From
Daniel Barkalow <barkalow@iabervon.org>
- Date
- May 25, 2009, 22:08 UTC
- Message-ID
- <alpine.LNX.2.00.0905251733240.2147@iabervon.org>
- In-Reply-To
- <pan.2009.05.25.20.40.20@fedoraproject.org>
On Mon, 25 May 2009, Thomas Spura wrote:
Show 31 quoted lines
> Am Mon, 25 May 2009 22:16:02 +0200 schrieb Björn Steinbrink:
>
> > On 2009.05.25 19:44:10 +0000, Thomas Spura wrote:
> >> Move a char and a char * outside of a for loop for speed improvements
> >>
> >> Signed-off-by: Thomas Spura <tomspur@fedoraproject.org> ---
> >> Comments?
> >>
> >> transport.c | 7 +++----
> >> 1 files changed, 3 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/transport.c b/transport.c index 17891d5..e350937 100644
> >> --- a/transport.c
> >> +++ b/transport.c
> >> @@ -263,11 +263,10 @@ static int write_refs_to_temp_dir(struct strbuf
> >> *temp_dir,
> >> int refspec_nr, const char **refspec)
> >> {
> >> int i;
> >> + unsigned char sha1[20];
> >> + char *ref;
> >>
> >> for (i = 0; i < refspec_nr; i++) {
> >> - unsigned char sha1[20];
> >> - char *ref;
> >> -
> >
> > I doubt that this makes any difference at all.
>
> With ints, the loop costs about 40% of speed. Without recreation, it
> should be always faster.Actually, having the variables go out of scope should be at least as fast. The compiler doesn't actually do anything to make the old variable inaccessible and get a new variable; with the variable uninitialized, it's legitimate for the compiler to simply reuse the same storage for all iterations. Futhermore, with the variables declared inside the loop, the compiler is allowed to make optimizations that would fail to preserve those variables between iterations. There are probably no such optimizations in this code for it to make, but, in general, letting variables in loops go out of scope (in C) only improves optimization possibilities.
-Daniel *This .sig left intentionally blank*