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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/87555] There is no need for UNSPEC_FMADDSUB
Date: Thu, 17 Jun 2021 12:07:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D87555

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-06-17

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The complication is that the define_insn looks like

(define_insn "*fma_fmaddsub_<mode>"
  [(set (match_operand:VF_128_256 0 "register_operand" "=3Dv,v,v,x,x")
        (unspec:VF_128_256
          [(match_operand:VF_128_256 1 "nonimmediate_operand" "%0,0,v,x,x")
           (match_operand:VF_128_256 2 "nonimmediate_operand" "vm,v,vm,x,m")
           (match_operand:VF_128_256 3 "nonimmediate_operand" "v,vm,0,xm,x"=
)]
          UNSPEC_FMADDSUB))]
  "TARGET_FMA || TARGET_FMA4"
  "@
   vfmaddsub132<ssemodesuffix>\t{%2, %3, %0|%0, %3, %2}
   vfmaddsub213<ssemodesuffix>\t{%3, %2, %0|%0, %2, %3}
   vfmaddsub231<ssemodesuffix>\t{%2, %1, %0|%0, %1, %2}
   vfmaddsub<ssemodesuffix>\t{%3, %2, %1, %0|%0, %1, %2, %3}
   vfmaddsub<ssemodesuffix>\t{%3, %2, %1, %0|%0, %1, %2, %3}"

so it can handle the {132,213,231} variants without much duplication from a
single pattern.  But it seems this cannot prevail when we open-code the
operation.  Likewise handling multiple modes with the VF_128_256 iterator
becomes difficult since the addsub requires either a vec_merge (as in the
addsub patterns) with a constant selector specific to the mode (unless we
can provide a large one that is implicitely truncated) or a vector constant
when using the (vec_select (vec_concat ...)) form.  So the above would
split into 12 somewhat repetitive patterns.

Doable.

Repeat for the AVX512 ones.  Tedious ;)=