On Tue, Apr 20, 2021 at 04:50:22AM +0200, Maciej W. Rozycki wrote:
> Hi,
>
> As Huacai has recently discovered the MIPS backend for `do_div' has been
> broken and inadvertently disabled with commit c21004cd5b4c ("MIPS: Rewrite
> <asm/div64.h> to work with gcc 4.4.0."). As it is code I have originally
> written myself and Huacai had issues bringing it back to life leading to a
> request to discard it even I have decided to step in.
>
> In the end I have fixed the code and measured its performance to be ~100%
> better on average than our generic code. I have decided it would be worth
> having the test module I have prepared for correctness evaluation as well
> as benchmarking, so I have included it with the series, also so that I can
> refer to the results easily.
>
> In the end I have included four patches on this occasion: 1/4 is the test
> module, 2/4 is an inline documentation fix/clarification for the `do_div'
> wrapper, 3/4 enables the MIPS `__div64_32' backend and 4/4 adds a small
> performance improvement to it.
>
> I have investigated a fifth change as a potential improvement where I
> replaced the call to `do_div64_32' with a DIVU instruction for cases where
> the high part of the intermediate divident is zero, but it has turned out
> to regress performance a little, so I have discarded it.
>
> Also a follow-up change might be worth having to reduce the code size and
> place `__div64_32' out of line for CC_OPTIMIZE_FOR_SIZE configurations,
> but I have not fully prepared such a change at this time. I did use the
> WIP form I have for performance evaluation however; see the figures quoted
> with 4/4.
>
> These changes have been verified with a DECstation system with an R3400
> MIPS I processor @40MHz and a MTI Malta system with a 5Kc MIPS64 processor
> @160MHz.
>
> See individual change descriptions and any additional discussions for
> further details.
>
> Questions, comments or concerns? Otherwise please apply.
series applied to mips-next.
Thomas.
--
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea. [ RFC1925, 2.3 ]
