On Wed, 18 Apr 2012 23:29:31 -0700, Kenneth Graunke <kenn...@whitecape.org>
wrote:
> Consider the following code:
> dp4(8) g8<1>.xF g1<4,4,1>F g5<4,4,1>F { align16 WE_normal 1Q };
> mov(8) m3<1>.xF g8<4,4,1>.xF { align16 WE_normal 1Q };
>
> Thanks to our existing compute-to-MRF code, this becomes:
> dp4(8) m3<1>.xF g1<4,4,1>F g5<4,4,1>F { align16 WE_normal 1Q };
>
> However:
> dp4(8) g8<1>.xF g1<4,4,1>F g5<4,4,1>F { align16 WE_normal 1Q };
> mov(8) m3<1>.yF g8<4,4,1>.xF { align16 WE_normal 1Q };
> does not get optimized since the MRF and temporary GRF use different
> components, and the code does not yet support rewriting swizzles in the
> general case. Scalars are an easy special case: since there's only one
> component, you can simply change the writemask to store it in the proper
> component for the MRF.
>
> Reduces a simple shader in Unigine Tropics from 12 instructions to 9
> by eliminating superfluous MOVs for 3 of the 4 vector components.This looks to me like it would also apply itself to a series of, say, MULs to individual channels. But it wouldn't reswizzle the source channels as necessary, so you'd get wrong results.
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