[Bug rtl-optimization/56766] Fails to combine (vec_select (vec_concat ...)) to (vec_merge ...)

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56766

--- Comment #18 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(in reply to c#9)

Yes, this is a generic problem.  recog will not recognise patterns
where regs are swapped in some places but not others.  This can of
course be worked around in combine, but that will be expensive (and
not solve the real problem).