https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69671
--- Comment #24 from Kirill Yukhin <kyukhin at gcc dot gnu.org> --- (In reply to rguent...@suse.de from comment #23) > 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 [details] > > --> 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. Agree.
