Re: Are you optimising properly?

[mike at penbex dot com dot tw (mike)]

I'd like to give some my ideas.
Seeing is always believing, but you can see more if you add flag '-v'
to see what gcc done to your code. Once you got a conclusion, it
may works on one platform.
In the cross platform developing, I will choose 'currect' code first.
BTW, if your are interested on x86 optimising, a group has done
a lot of effect. (http://goof.com/pcg/)

----- Original Message -----
From: "Robert de Bath" <robert$@home-box.demon.co.uk>
To: <crossgcc@sources.redhat.com>
Sent: Sunday, January 14, 2001 7:47 PM
Subject: Are you optimising properly?


> 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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