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.
Maciej
