Message-Id: <m0xTmOS-000S1fC@inti.gov.ar>
Comments: Authenticated sender is <salvador AT natacha DOT inti DOT gov DOT ar>
From: "Salvador Eduardo Tropea (SET)" <salvador AT inti DOT edu DOT ar>
Organization: INTI
To: Peter Palotas <blizzar AT hem1 DOT passagen DOT se>
Date: Fri, 7 Nov 1997 14:27:59 +0000
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Subject: Re: Code optimisations
CC: djgpp AT delorie DOT com
Precedence: bulk

Peter Palotas <blizzar AT hem1 DOT passagen DOT se> wrote:
> >> Here are some tips on code optimizing that I saw somewhere:
> >> * First, profile your code and find what needs improving; it's often not
> >> what you think.
> >> * A good algorithm is likely to make the most difference in speed.
> >> * Tricks in rearranging syntax to get the compiler to make better code
> >> rarely help.
> >
> >I agree with Nate for 1 and 2. Not for 3 if you'll use GCC because for 
> >somethings is really stupid and needs some help (I don't know about a better 
> >compiler).
> 
> Could you give some examples of situations when GCC really needs help to
> provide optimized code!

I don't remmember what steps I did, but when I did my plasma library I was 
touching C code, then generating the ASM, etc ... all the time, I discovered 
that sometimes you MUST take some desissions, like choose a register for a 
variable (I don't mean the register keyword, but say: this local var MUST be 
EAX and not other). Of course the difference range from 1 to 6 clocks, but in 
the inner loop of the plasma that's executed 64000*70 times per second = 
4,480,000 times/sec! so 1 or 2 clocks makes a difference.
I even discovered some bugs in GCC optimizer, I mean cases where is impossible 
to teach GCC that he can avoid a reload, but I can easilly see the way. And I'm 
not talking about ultra-complex things, these routines are 10 or less lines.

SET
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