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From "Bill Stoddard" <b...@wstoddard.com>
Subject RE: [PATCH] example BUSEC patch for benchmarking only
Date Thu, 11 Jul 2002 02:28:26 GMT

> Bill Stoddard wrote:
>
> >>Humm... looking at this macro which is used all over the place, I see a
> >>division.
> >>
> >> #define apr_time_sec(time) ((apr_int64_t)((time) / APR_USEC_PER_SEC))
> >>
> >>Since APR_USEC_PER_SEC is now a binary representation, I assume
> >>the compiler
> >>will do the proper optimization.
> >>
> >>
> >
> >ie, turn the division into a shift, which is much less expensive.
> >
>
> It's definitely a valid optimization.  I just checked gcc on
> Sparc and it generates a shift rather than a division.  But
> if the busec code becomes part of APR, I'd rather define the
> macro as a shift, just to make sure that people get the benefit
> of the speedup even if their compilers don't do the optimization.
> (My fear is that some 32-bit compilers might immediately generate
> a call to a library function if they see anything that looks like
> a 64-bit division.)
>
> --Brian
>

I've not looked at the generated code, but profiling indicates that an
additional division is happening, adding an extra 231 instructions.
(xlc_r -O2)

Bill


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