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>
My understanding, and I've followed the development of the Intel family since 
the 8080, each generation since the 386 has introduced better and faster 
instructions. I.E.:
The 486 introduced I.E.E.E floating point instructions by incorporating an 
FPU on board. The first few generations were flaky, so Intel disabled the 
dodgy ones and sold then as 486SX, I.E. without the FPU. Later generations 
were better, this is why you only find slow 486SX's ;-) Therefore 486's, with 
working FPU's can calculate floats faster than 386's, but you must generate 
the proper codes to take advantage of this.

The Pentium's improved the FPU logic and introduced pipelining. The first 
generations of Pentiums had faulty FPu logic programmed into them, the 
Pentium Bug, but subsequent ones were fine. These added instructions are 
faster again then the 486 equivulents. Also, the pipelining needs careful 
instruction ordering to take full advantage of it's speed improvements, 
again, something the compiler must know about to utilise to full effect. 
According the the PGCC site GCC does this poorly, but that info seems to be 
dated, GCC3.1.x might be better. This is one of the areas I am researching 
closely to get an answer.

MMX added the ability to perform matrix calculations on int's with single 
instructions and using special DMA features within the Pentium to speed this 
up. Two problems with this:
1) int's are not very useful for most matrix calculations, floats would be 
betters.
2) this is not something that can be optimised well by a compiler. It need to 
be identified and provided for in the sources.
I.E. not much use to anyone, but makes great ad copy ;-)

The PentiumPro improved the pipeline enormously. Again, a properly written 
compiler should be able to optimise for this, once it can organise the code 
properly.

The PIII added MMX-type instructions for floats! Now this IS useful! 
Graphic-Intensive programs can take greate advantage of this, but it must be 
provided for in the source code. Compilers can not, usually optimise for this 
sort of thing. XFree86 and DRI are two prime examples that do provide for 
this, so the PIII can run XFree86 and DRI quite a bit faster, IF it's 
compiled for these SSE instructions!

Not sure what improvements the P4 introduce? I think it's mostly just speed 
improvements rather than any execution changes.

So, The difference between the 386 and the PentiumMMX 'should' yield a 
significant speed boost if optimised correctly. There are faster floating 
point instructions and pipelining that need optimising for. I'm not sure if 
GCC can optimise properly for the pipelining, at least the PGCC group found 
significant improvements to add to GCC2.95.3 to gain speed improvements of 
upto 30%. 30% of 48 hours is 14.4 hours. Of course none of this will have any 
effect on O/I bound processes but GUI's are mostly CPU bound. I am also 
finding out about object pre-linking optimisations which should give even 
better performance for QT and KDE.

Now if I had another PIII for my box, I could take advantage of those SSE 
instructions to optimise XFree86 as well!

> kevin

An, of course, I've loads of time on my hands now and I need something to 
keep me busy. At least I can say that I've sucessfully built a full Linux 
system, including KDE3 from scratch when I'm done ;-)
Cheers,

	John Gay

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