Rumour about IBM benchmarks

[jim frost]

wje at siia.mv.com (Bill Ezell) writes:
>In <1233 at torsqnt.UUCP> david at torsqnt.UUCP (David Haynes) writes:

>>I have heard a rumour that the benchmark results that IBM posted for
>>their RS6000 system were the results of hand-coded, hand-optimized...

>This would seem unlikely to me. According to IBM (a perhaps suspect source)
>their C compiler 'consistently produces code better than hand-coded assembler'.
>This isn't too surprizing when you consider that the compiler is tailored
>to generate instructions to take advantage of the pipelining inherent
>in the processor, a tedious process at best when done by hand.

Actually the compiler doesn't do as good a job as you might think,
although there's no doubt in my mind that it does a better job on a
large project than you could do when hand-coding.

One thing it doesn't seem to try very hard to do is instruction
scheduling -- moving cmp instructions away from branch instructions so
that the branches execute in zero cycles, for instance.  This is
surprising since zero-cycle branches are a nice feature of the system.

I haven't gone out of my way to see just which optimizations it does
well or poorly at, but it does seem like the compiler could do a bit
better.

Happy hacking,

jim frost
saber software
jimf at saber.com