https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106069
--- Comment #22 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 3 Aug 2022, linkw at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106069 > > --- Comment #21 from Kewen Lin <linkw at gcc dot gnu.org> --- > I didn't look into this in details, but something in the culprit commit caught > my eyes, take altivec_vmrghh as example: > > Before the patch, the pattern > > [(set (match_operand:V8HI 0 "register_operand" "=v") > (vec_select:V8HI > (vec_concat:V16HI > (match_operand:V8HI 1 "register_operand" "v") > (match_operand:V8HI 2 "register_operand" "v")) > (parallel [(const_int 0) (const_int 8) > (const_int 1) (const_int 9) > (const_int 2) (const_int 10) > (const_int 3) (const_int 11)])))] > > can match vmrghh on BE while vmrglh on LE. It indicates this pattern has > different semantic from underlying instruction perspectives. > > After the patch, this pattern only matches vmrghh. > > IMHO, this part has semantic change before and after the patch. The code > before > the patch looks more reasonable to me, since the pattern can have different > meanings on BE and LE (underlying behavior). Ideally we would avoid semantic difference of RTL depending on the target. If that's not avoidable there should be target macros/hooks that specify the desired semantics. I assume the semantic difference is in vec_concat behavior but that's just documented as @findex vec_concat @item (vec_concat:@var{m} @var{x1} @var{x2}) Describes a vector concat operation. The result is a concatenation of the vectors or scalars @var{x1} and @var{x2}; its length is the sum of the lengths of the two inputs. which is a bit unspecific. To me it implies that vec_select of a single lane N of the concat result can be distributed to the operands of the vec_concat in the obvious way (if N >= GET_MODE_NUNITS (x1) subtract GET_MODE_NUNITS and use x2)
