🚀 go-pugleaf

mash@mips.COM (John Mashey) wrote:
> 2) OK, I give up.  There must be something unbelievably clever going on
> to use 128-bit loads for C-language string operations. I've looked
> at the i860 Programmer's Reference Manual a bunch, trying to figure
> out how to use either the FP unit or the graphics unit to do this.

Yeah... the Z-buffer check instructions could be used for this, but they're
only available in 16 and 32-bit versions, and you have to test the bits
from the psr, two cycles.  And even that would only be 64 bits at a time.

> The string copy on page 9-5 of the manual is the "natural" strcpy
> (which doesn't use anything but byte load/store, and takes about 5 cycles/
> byte).  I haven't been able to find anything like "branch on any byte zero",
> and the 860 doesn't have unaligned word operations.  For a fair test,
> you MUST use str* that only assume byte alignment of operands, and
> you can't inline the str*.  The only place I can think of using 128-bit
> loads is in the structure-copy, and it shouldn't be used there,
> unless structures whose largest entities are words are always aligned
> to 4-word boundaries, which seems unlikely.

Well, you can quickly cobble together some code using the
((x-0x01010101)&~x)&0x80808080 != 0 trick that works on words
at a time.  This would help in Dhrystone which, as has been observeed,
has unnaturally long strings.  If you get this going with a bit of
alternation  to allow for load latency, you can get strlen down to about
5 cycles/word.  Strcmp and strcpy would be slower, but would probably
be bandwidth-limited.

As for structure copies, what you want is for all structures 4 words
or larger in size to always be 4-word aligned.  Intel suggests the
stack is kept this way, for just the same reason.

(I admit this is starting to enter the realm of declining returns - you
can waste a lot of memory this way - but is still feasable.)

> Maybe somebody at Intel would care to post the str* routines
> and educate us?

I posted the instruction set - it's an exercise for the reader. :-)
-- 
	-Colin (uunet!microsoft!w-colinp)

"Don't listen to me.  I never do." - The Doctor