On Mon, Apr 6, 2020 at 1:20 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > "Richard Earnshaw (lists)" <richard.earns...@arm.com> writes: > > On 03/04/2020 16:03, Richard Sandiford wrote: > >> "Richard Earnshaw (lists)" <richard.earns...@arm.com> writes: > >>> On 03/04/2020 13:27, Richard Sandiford wrote: > >>>> "Richard Earnshaw (lists)" <richard.earns...@arm.com> writes: > >>>>> On 02/04/2020 19:53, Richard Henderson via Gcc-patches wrote: > >>>>>> This is attacking case 3 of PR 94174. > >>>>>> > >>>>>> In v2, I unify the various subtract-with-borrow and add-with-carry > >>>>>> patterns that also output flags with unspecs. As suggested by > >>>>>> Richard Sandiford during review of v1. It does seem cleaner. > >>>>>> > >>>>> > >>>>> Really? I didn't need to use any unspecs for the Arm version of this. > >>>>> > >>>>> R. > >>>> > >>>> See https://gcc.gnu.org/pipermail/gcc-patches/2020-April/543063.html > >>>> (including quoted context) for how we got here. > >>>> > >>>> The same problem affects the existing aarch64 patterns like > >>>> *usub<GPI:mode>3_carryinC. Although that pattern avoids unspecs, > >>>> the compare:CC doesn't seem to be correct. > >>>> > >>>> Richard > >>>> > >>> > >>> But I don't think you can use ANY_EXTEND in these comparisons. It > >>> doesn't describe what the instruction does, since the instruction does > >>> not really extend the values first. > >> > >> Yeah, that was the starting point in the thread above too. And using > >> zero_extend in the existing *usub<GPI:mode>3_carryinC pattern: > >> > >> (define_insn "*usub<GPI:mode>3_carryinC" > >> [(set (reg:CC CC_REGNUM) > >> (compare:CC > >> (zero_extend:<DWI> > >> (match_operand:GPI 1 "register_operand" "r")) > >> (plus:<DWI> > >> (zero_extend:<DWI> > >> (match_operand:GPI 2 "register_operand" "r")) > >> (match_operand:<DWI> 3 "aarch64_borrow_operation" "")))) > >> (set (match_operand:GPI 0 "register_operand" "=r") > >> (minus:GPI > >> (minus:GPI (match_dup 1) (match_dup 2)) > >> (match_operand:GPI 4 "aarch64_borrow_operation" "")))] > >> "" > >> "sbcs\\t%<w>0, %<w>1, %<w>2" > >> [(set_attr "type" "adc_reg")] > >> ) > >> > >> looks wrong for the same reason. But the main problem IMO isn't how the > >> inputs to the compare:CC are represented, but that we're using compare:CC > >> at all. Using compare doesn't accurately model the effect of SBCS on NZCV > >> for all inputs, so if we're going to use a compare here, it can't be :CC. > >> > >>> I would really expect this patch series to be pretty much a dual of this > >>> series that I posted last year for Arm. > >>> > >>> https://gcc.gnu.org/pipermail/gcc-patches/2019-October/532180.html > >> > >> That series uses compares with modes like CC_V and CC_B, so I think > >> you're saying that given the choice in the earlier thread between adding > >> a new CC mode or using unspecs, you would have preferred a new CC mode, > >> is that right? > >> > > > > Yes. It surprised me, when doing the aarch32 version, just how often > > the mid-end parts of the compiler were able to reason about parts of the > > parallel insn and optimize things accordingly (eg propagating the truth > > of the comparison). If you use an unspec that can never happen. > > That could be changed though. E.g. we could add something like a > simplify_unspec target hook if this becomes a problem (either here > or for other unspecs). A fancy implementation could even use > match.pd-style rules in the .md file. > > The reason I'm not keen on using special modes for this case is that > they'd describe one way in which the result can be used rather than > describing what the instruction actually does. The instruction really > does set all four flags to useful values. The "problem" is that they're > not the values associated with a compare between two values, so representing > them that way will always lose information.
Can't you recover the pieces by using a parallel with multiple set:CC_X that tie together the pieces in the "correct" way? Richard. > Thanks, > Richard
