🚀 go-pugleaf

In article <242@bobkat.UUCP> m5d@bobkat.UUCP (Mr Mike McNally) writes:
>Various architectures involve a mechanism whereby the CPU is able to fetch
>code during unused/unusable bus cycles.  In the x86 series, the prefetch
>is dumb.  The prefetch queue is kept small because it was found that the
>point of diminishing returns was encountered at about 6 bytes.  The
>obvious problem is that branch instructions do not cooperate with the
>idea of prefetching.  Attempts to follow different paths beyond a branch
>seem like Koyaanisqatsi solutions to me.     

>A compiler with a minimal amount of smarts can detect runs of code which
>can be guaranteed to execute in sequence.  Since the compiler is
>generating the code, why not provide information on the size of the
>upcoming run of certainly sequential instructions?  ...  The advantages
>to such a system are even greater given a burst-mode bus cycle.   

>Mike McNally

The sequence approach still does not help in prefetching code past the 
branch point before actually finding out which way the branch will go.  
I suspect a variable length burst mode will be no more effective than a
fixed burst mode which is always applied on instruction fetch.  

One possible solution to this problem is to have the branch instructions
themselves indicate which branch is more likely.  The machine could then
prefetch the most likely sequence, and most often it will already have
the instructions it needs.  When the branch taken is the least common,
the prefetch queue must be flushed and re-filled.   

The RISC approach is to have the instructions after the branch executed
reguardless if the branch is taken or not.  For a small pipe or prefetch
queue, the compiler can usually fill this "delay slot" with a useful
instruction.  Though this represents a problem if the pipe or prefetch
queue gets to large.  
-- 
  Lawrence Crowl		716-275-5766	University of Rochester
			crowl@rochester.arpa	Computer Science Department
 ...!{allegra,decvax,seismo}!rochester!crowl	Rochester, New York,  14627