RFA: pervasive SSE codegen inefficiency

[Dale Johannesen]

Consider the following SSE code
(-march=pentium4 -mtune=prescott -O2 -mfpmath=sse -msse2)
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The first inner loop compiles to

         paddq   %xmm0, %xmm1

Good.  The second compiles to

         movdqa  %xmm2, %xmm0
         paddw   %xmm1, %xmm0
         movdqa  %xmm0, %xmm1

when it could be using a single paddw.  The basic problem is that
our approach defines __m128i to be V2DI even though all the operations
on the object are V4SI, so there are a lot of subreg's that don't need
to generate code.  I'd like to fix this, but am not sure how to go 
about it.
The pattern-matching and RTL optimizers seem quite hostile to mismatched
mode operations.  If I were starting from scratch I'd define a single 
V128I mode
and distinguish paddw and paddq by operation codes, or possibly by using
subreg:SSEMODEI throughout the patterns.  Any less intrusive ideas?  
Thanks.

(ISTR some earlier discussion about this but can't find it; apologies if
I'm reopening something that shouldn't be:)