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List:       gcc-bugs
Subject:    rather odd bug with arrays - possibly compiler?
From:       J Grant <jg () jguk ! org>
Date:       2001-12-13 8:41:13
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Hello

I have a 3D array ulong v[32][768[1024], x (32) is access packed 32
bits.

i traverse the array in all three axis ( 3 sections, with the each axis
in the centre of its own loop)
e.g.

loop z
 loop y
   loop x
    .........
   loop z end
 loop y end
loop x end


loop z
 loop x
   loop y
    .........
   loop end
 loop end
loop end



loop y
 loop x
   loop z
    .........
   loop end
 loop end
loop end


the X is slightly different as it is packed, but they all use the same C
code, just cut n pasted into different loop order to traverse along
different axis.

However I was amazed to find the centre loop above took 10 seconds when
X took 1.1 and Z took 1.2

this is rather odd, all I could think of during testing was that it is
to do with the way a flat array is converted to 3D so it is actually
reading along the Z axis for that one, but it does not make much sense
to myself.

maybe it is missing processor Cache, due to bad prediction and having to
wait. I dont know.

If anyone has an idea why this is please let me know. I compiled with
-O3 and -O2 and the others but no difference to the Y axis over the
others

if you need more info just let me know!

JG
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