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From: Lewis_Evans@Playstation.sony.com
Newsgroups: scee.yaroze.programming.gnu_compiler
Subject: -O Optimising levels
Date: 12 Mar 1998 15:49:12 -0000
Organization: Sony Computer Entertainment Europe - 119.SS5
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From: Lewis_Evans@Playstation.sony.com
To: news@playstation.co.uk

Firstly, odd-sized arrays are saving very little memory,
so best to make them even-sized.

As regards this buffering business, I can't say because I haven't seen your
code
and don't know what you mean by 'single virtual screen without
double buffering' during game. (Do you write everything to the same
rectangular
offscreen area of VRAM each frame, and then MoveImage to
whichever buffer is being drawn/displayed??).

One thing that definitely needs looking at under optimisation:
function calling and local variables. Without optimisation, the
locals are accessed by the frame pointer, but with O, they're accessed
via the stack pointer. If you put the program stack onto the data-cache
for higher speed, but try to access locals, you get a nasty trouble
because the stack pointer's moved and locals will not be accessed
correctly, and you can expect all kinds of trouble.

Other than that, try isolating which part(s) of the code are going
astray under optimisation, and use -O2 because that's bestest
(highest level before you get to O3 which unrolls loops,
sounds great but on the PSX is very liable to give you
instruction cache misses and hence anti-optimisation).

     Lewis


Hey there people,
If you haven't already seen Gravitation I suggest you check it out
now -- a must for fans of the Thrust genre.
I've done a lot of optimising the code from my early demos
(such as removing the floating point numbers, and using sprites for
bullets) and it seems to run perfectly okay without a -O compiler
 option.  BUT when I use any optimising things go pear-shaped!
I use a mixture of single and double buffering (single buffer virtual
screen during play, and double buffering for intro/menus) and for
some reason this is the first to go nuts, although I've had the game
crash in mid play once.
Is it something to do with having your data structures odd sizes?
(Like using an array[7] instead of an even array[8]?)
Anything obvious I've missed?
Thanks,
Jim
--
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James Shaughnessy    james@manc.u-net.com
http://www.netyaroze-europe.com/~shaughnj
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