[ydl-gen] Programming for speed on the PS3

[Jonathan Bartlett]

Warren Nagourney wrote:

>I found that the xlc compiler produces code that runs about 15%-20%  
>faster than gcc. This is on a program with a moderate amount of  
>single precision arithmetic. The interesting thing is that the same  
>(scalar) code runs about 30% faster on the spu as it does on the ppu  
>(with the same 15% advantage to xlc). Provided the code and data fit  
>in the local store of the spu, it seems better to use the spu  for  
>scalar code! Compared to a G4, the ppu is more or less equivalent to  
>a 1.2 GHz processor and the spu is equivalent to a 1.7-2 GHz G4.
>
>  
>
I'm not surprised that it is this fast with single-precision arithmetic, 
even as scalar code.  Do you have any data for double-precision? 

For single-precision arithmetic, the SPU can process a new value each 
clock tick (it's a 6-cycle calculation, but fully-pipeline-able).  For 
double-precision, not only does it take 13 cycles for the calculation, 
but it completely stalls the processor for the first 6 cycles!  It's 
almost as bad as a branch!

Jon
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