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From: "crazylht at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/104059] New: cprop_hardreg propgates hard registers for
 mov instructions between different REG_CLASS without considering cost
Date: Mon, 17 Jan 2022 03:36:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104059

            Bug ID: 104059
           Summary: cprop_hardreg propgates hard registers for mov
                    instructions between different REG_CLASS without
                    considering cost
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: crazylht at gmail dot com
  Target Milestone: ---

int test (uint8_t *p, uint32_t t[1][1], int n) {

  int sum =3D 0;
  uint32_t a0;
  for (int i =3D 0; i < 4; i++, p++)
    t[i][0] =3D p[0];

  for (int i =3D 0; i < 4; i++) {
    {
      int t0 =3D t[0][i] + t[0][i];
      a0 =3D t0;
    };
    sum +=3D a0;
  }
  return (((uint16_t)sum) + ((uint32_t)sum >> 16)) >> 1;
}

testcase is from PR104049, for x86 with -O3 -mavx2 ,before cprop_hardregs i=
t's

----before cprop_hardreg------
(insn 100 79 81 2 (set (reg:SI 1 dx [orig:90 stmp__9.14 ] [90])
        (reg:SI 20 xmm0 [114])) 81 {*movsi_internal}
     (expr_list:REG_DEAD (reg:SI 20 xmm0 [114])
        (nil)))
(debug_insn 81 100 96 2 (debug_marker) "/app/example.cpp":16:3 -1
     (nil))
(insn 96 81 82 2 (set (reg:SI 0 ax [116])
        (reg:SI 1 dx [orig:90 stmp__9.14 ] [90])) "/app/example.cpp":16:44 =
81
{*movsi_internal}
     (nil))
---end------------

------after cprop_hardreg--------
(insn 100 79 81 2 (set (reg:SI 1 dx [orig:90 stmp__9.14 ] [90])
        (reg:SI 20 xmm0 [114])) 81 {*movsi_internal}
     (nil))
(debug_insn 81 100 96 2 (debug_marker) "/app/example.cpp":16:3 -1
     (nil))
(insn 96 81 82 2 (set (reg:SI 0 ax [116])
        (reg:SI 20 xmm0 [orig:90 stmp__9.14 ] [90])) "/app/example.cpp":16:=
44
81 {*movsi_internal}
     (expr_list:REG_DEAD (reg:SI 20 xmm0 [orig:90 stmp__9.14 ] [90])
        (nil)))
------end--------------

it's
        vmovd   edx, xmm0
        movl   eax, edx

vs
        vmovd   edx, xmm0
        vmovd   eax, xmm0

vmovd is expensive for many x86 targets.=