matmul, dotprod, transpose performance patch proposal

[Tim Prince]

Tim Prince wrote:

> I have been examining performance improvements which can be obtained 
> by 1st step optimizations of these intrinsics.  I'm sure it's no 
> secret, that a performance increase can be obtained by usual methods:
>
> matmul_r[48]:  unroll and jam the stride 1 loops, combining 2 outer 
> loop iterations in one inner loop.  For Pentium-M, but not for Xeon, 
> Opteron, or Itanium, a much bigger boost is obtained by using 2 
> parallel dot products.  Change the general stride case to dot product, 
> dictating the strength reduction which gfortran misses.  20% to 100% 
> gain in performance
>
> dotprod_r[48]:  unroll, performing addition in pairs first, before 
> adding to sum accumulation, effectively cutting the performance 
> limitation due to latency of addition in half.  Typical 30% 
> performance gain
>
> transpose:  swap inner and outer loops, so the small stride (normally 
> 1) occurs on the destination rather than the source side, taking 
> advantage of Write Combine buffering, where present.  This should be 
> faster, as long as the operand size is not large enough for cache 
> eviction to become important.
>
> Would such a patch be offered, under what circumstance?
>
As this did not appear within 20 hours, I am sending again.  My apology, 
if it is waiting for moderator approval.