[Bug rtl-optimization/53533] [4.7/4.8 regression] vectorization causes loop unrolling test slowdown as measured by Adobe's C++Benchmark

[rth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53533

--- Comment #12 from Richard Henderson <rth at gcc dot gnu.org> 2012-06-12 18:54:24 UTC ---
(In reply to comment #10)
> But maybe allowing const_vector in (some of) the define_insn_and_split would
> be the way to go ...

Maybe.  It certainly would ease some of the simplifications.
At the moment I don't think we can go from

  mem -> const -> simplify -> const ->newmem

On the other hand, for this particular test case, where all
of the vector_cst elements are the same, and a reasonably
small number of bits set, it would be great to be able to
leverage synth_mult.

The main complexity for sse2_mulv4si3 is due to the fact that
we have to decompose the operation into V8HImode multiplies.
Whereas if we decompose the multiply, we have the shifts and
adds in V4SImode.