Ajit Agarwal <aagar...@linux.ibm.com> writes: > On 03/06/24 5:03 pm, Richard Sandiford wrote: >> Ajit Agarwal <aagar...@linux.ibm.com> writes: >>>> [...] >>>> If it is intentional, what distinguishes things like vperm and xxinsertw >>>> (and all other unspecs) from plain addition? >>>> >>>> [(set (match_operand:VSX_F 0 "vsx_register_operand" "=wa") >>>> (plus:VSX_F (match_operand:VSX_F 1 "vsx_register_operand" "wa") >>>> (match_operand:VSX_F 2 "vsx_register_operand" "wa")))] >>>> >>> >>> Plain addition are not supported currently. >>> We have not seen many cases with plain addition and this patch >>> will not accept plain addition. >>> >>> >>>> This is why the intention behind the patch is important. As it stands, >>>> it isn't clear what criteria the patch is using to distinguish "valid" >>>> fuse candidates from "invalid" ones. >>>> >>> >>> Intention behind this patch all variants of UNSPEC instructions are >>> supported and uses without UNSPEC are not supported in this patch. >> >> But why make the distinction this way though? UNSPEC is a very >> GCC-specific concept. Whether something is an UNSPEC or some other >> RTL code depends largely on historical accident. E.g. we have specific >> codes for VEC_SELECT, VEC_MERGE, and VEC_DUPLICATE, but don't have one >> for VEC_PERM (even for VEC_PERM_EXPR exists in gimple). >> >> It seems unlikely that GCC's choice about whether to represent something >> as an UNSPEC or as another RTL code lines up neatly with the kind of >> codegen decisions that a good assembly programmer would make. >> >> I suppose another way of asking is to turn this around and say: what >> kind of uses are you trying to exclude? Presumably things are worse >> if you remove this function override. But what makes them worse? >> What kind of uses cause the regression? >> > > Uses of fused load where load with low address uses are modified with load > with high address uses. > > Similarly load with high address uses are modified with load low address > uses.
It sounds like something is going wrong the subreg updates. Can you give an example of where this occurs? For instance... > This is the semantics of lxvp instructions which can occur through > UNSPEC uses otherwise it breaks the functionality and seen failure > in almost all vect regressions and SPEC benchmarks. ...could you take one of the simpler vect regressions, show the before and after RTL, and why the transformation is wrong? Thanks, Richard
