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From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108038] New: GCC generates poor code for select of
 consecutive constants
Date: Fri, 09 Dec 2022 21:17:34 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108038

            Bug ID: 108038
           Summary: GCC generates poor code for select of consecutive
                    constants
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at gcc dot gnu.org
                CC: rzinsly at ventanamicro dot com
  Target Milestone: ---
            Target: riscv-64

This testcase:


typedef signed long int __int64_t;

#define X(OP, OPN) __int64_t test##OPN(__int64_t x, __int64_t y) { __int64_t
cmp; cmp =3D (x OP y) ? 2 : 3; return cmp; }

X(>,GT)
X(<,LT)

typedef unsigned long int __uint64_t;

#define XU(OP, OPN) __uint64_t test##OPN(__uint64_t x, __uint64_t y) {
__uint64_t cmp; cmp =3D (x OP y) ? 2 : 3; return cmp; }

X(>,GTU)
X(<,LTU)






Compiled for rv64 with -O2 -march=3Drv64gc_zba gets this code for the first=
 test.
 The others are similar in having the extraneous xori.=20=20


testGT:
        sgt     a0,a0,a1        # 25    [c=3D4 l=3D4]  *sgt_didi
        xori    a0,a0,1 # 27    [c=3D4 l=3D4]  xordi3/1
        addi    a0,a0,2 # 16    [c=3D4 l=3D4]  adddi3/1
        ret             # 35    [c=3D0 l=3D4]  simple_return


But ISTM we could generate this instead:

testGT:
        sgt     a0,a1,a0        # 24    [c=3D4 l=3D4]  *riscv.md:2535
        addi    a0,a0,2 # 16    [c=3D4 l=3D4]  adddi3/1
        ret             # 32    [c=3D0 l=3D4]  simple_return



I think the key is this:

Trying 25 -> 27:
   25: r139:DI=3Dr140:DI>r141:DI
      REG_DEAD r141:DI
      REG_DEAD r140:DI
   27: r134:DI=3Dr139:DI^0x1
      REG_DEAD r139:DI
Failed to match this instruction:
(set (reg:DI 134 [ <retval> ])
    (le:DI (reg:DI 140)
        (reg:DI 141)))


We can invert the condition and swap the operands with something like this:

(define_insn ""
  [(set (match_operand:GPR           0 "register_operand" "=3Dr")
    (any_le:GPR (match_operand:X 1 "register_operand" " r")
            (match_operand:X 2 "register_operand" "r")))]
  ""
  "sgt<u>\t%0,%2,%1"
  [(set_attr "type" "slt")
   (set_attr "mode" "<X:MODE>")])

We probably need another pattern to handle the LT/LTU cases which want to
match:

rying 24 -> 25:=20
   24: r139:DI=3Dr140:DI<r141:DI
      REG_DEAD r141:DI
      REG_DEAD r140:DI
   25: r138:SI=3Dr139:DI=3D=3D0
      REG_DEAD r139:DI
Failed to match this instruction:
(set (reg:SI 138)
    (ge:SI (reg:DI 140)=20
        (reg:DI 141)))

Again, I think by inverting the condition and swapping the operands we shou=
ld
be good to go.=