Are you optimising properly?

[Robert de Bath]

I has a thought today ... These modern processors, Athlons PIII's even
the now venerable K6 try very hard to optimise the x86 instructions they
collect into jobs for all their internal units.  It seemed to me that
compiler optimisation wouldn't make any difference 'cause the processor
does so much itself.

But it does ... in an unexpected way.

I dusted off a copy of the old dhrystone 1.1 benchmark and complied it up ...
The numbers were all in the supercomputer range of course :-)

Riddle me this ... why does it run about 10% faster when I optimise for
space than it does when I go all out for speed?

$ gcc -v
Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.2/specs
gcc version 2.95.2 20000220 (Debian GNU/Linux)

PIII - 750

$ cc dhrystone.c ; ./a.out
This machine benchmarks at 633218 dhrystones/second
$ cc -O2 dhrystone.c ; ./a.out
This machine benchmarks at 866454 dhrystones/second
$ cc -O6 dhrystone.c ; ./a.out
This machine benchmarks at 859864 dhrystones/second
$ cc -Os dhrystone.c ; ./a.out
This machine benchmarks at 982045 dhrystones/second
$

K6-II 450
$ cc dhrystone.c ; ./a.out
This machine benchmarks at 332348 dhrystones/second
$ cc -O2 dhrystone.c ; ./a.out
This machine benchmarks at 487460 dhrystones/second
$ cc -O6 dhrystone.c ; ./a.out
This machine benchmarks at 496377 dhrystones/second
$ cc -Os dhrystone.c ; ./a.out
This machine benchmarks at 539380 dhrystones/second
$ cc -mcpu=k6 -Os dhrystone.c ; ./a.out
This machine benchmarks at 541724 dhrystones/second
$ cc -march=k6 -Os dhrystone.c ; ./a.out
This machine benchmarks at 542061 dhrystones/second
$

It seems to apply to other programs too, not just synthetic benchmarks.
It makes me wonder if everybody else is wrong. :-)

The -mcpu= options have a small effect, -march has a tiny bit more.

The K6-II runs hotter too with -Os, implying that it _is_ doing more work.

-- 
Rob.                          (Robert de Bath < http://poboxes.com/rdebath >)
                    <rdebath @ poboxes.com> < http://www.cix.co.uk/~mayday >




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