https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117195
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The tree code represents the following obtab:
@cindex @code{widen_ssum@var{m}3} instruction pattern
@cindex @code{widen_usum@var{m}3} instruction pattern
@item @samp{widen_ssum@var{m}3}
@itemx @samp{widen_usum@var{m}3}
Operands 0 and 2 are of the same mode, which is wider than the mode of
operand 1. Add operand 1 to operand 2 and place the widened result in
operand 0. (This is used express accumulation of elements into an accumulator
of a wider mode.)
@var{m} is the mode of operand 1.
But it does not mention what happens if operand 1 has 2x the number of
elements.
