[patch][rfa] Don't Generate Code to Support Unused Write Stacks

[Dave Brolley]

Frank Ch. Eigler wrote:

>brolley wrote:
>
>  
>
>>[...]
>>When generating write stacks and the supporting code in order to
>>suppor the (delay ...) rtl construct for SID, cgen currently generates
>>a write stack for all registers and memory modes regardless of whether
>>they are used or not. [...]
>>For the internal port for which performance was an issue, this yielded
>>an improvement of 17%. [...]
>>    
>>
>
>Sounds good.  I'm surprised though that your change should cause such
>a noticeable improvement.  It may be that the sid-side code to handle
>the write queue testing/iteration is rather deficient.  (Try adding
>some UNLIKELY markers to the CPU::writeback() function's while()
>conditions.)
>  
>
I think that the difference was 39 vs 2 write stacks to manage for this 
port.

Dave