https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69671
--- Comment #23 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 17 Feb 2016, jakub at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69671 > > --- Comment #22 from Jakub Jelinek <jakub at gcc dot gnu.org> --- > Created attachment 37722 > --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37722&action=edit > gcc6-pr69671.patch > > Actually, on a closer look, I believe the only problem are the patterns that > use a vector_move_operand "0C" inside of vec_select with only constants as the > parallel's operands. Because fwprop is able to propagate constants into > instructions (thus undo the CSE effect), but doesn't do anything on these, > because it also simplifies them, so instead of the expected say > (vec_select:V4QI (const_vector:V16QI [ > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > ]) > (parallel [ > (const_int 0 [0]) > (const_int 1 [0x1]) > (const_int 2 [0x2]) > (const_int 3 [0x3]) > ])) > we get in there simplified: > (const_vector:V4QI [ > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > (const_int 0 [0]) > ]) > So, by adding extra patterns for that simplification fwprop is able to do its > job even if CSE did a better job. Of course then I wonder why we didn't simplify this in the first place when generating RTL and need to wait for forwprop ... But yes, sounds like the easiest way to go forward.
