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From: "juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107445] New: Redundant moves for subregs move
Date: Thu, 27 Oct 2022 22:50:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107445

            Bug ID: 107445
           Summary: Redundant moves for subregs move
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juzhe.zhong at rivai dot ai
  Target Milestone: ---

Here is the testcase:
https://godbolt.org/z/4bcfx5a86

coalesce:
        ld4d    {z24.d - z27.d}, p0/z, [x0]
        mov     z5.d, z24.d
        mov     z4.d, z25.d
        mov     z3.d, z26.d
        mov     z2.d, z27.d
        cmp     w1, 0
        ble     .L2
        mov     w0, 0
        ld1d    z1.d, p0/z, [x2]
        ld1d    z0.d, p0/z, [x3]
.L3:
        add     w0, w0, 1
        movprfx z5.d, p0/z, z5.d
        mad     z5.d, p0/m, z1.d, z0.d
        movprfx z4.d, p0/z, z4.d
        mad     z4.d, p0/m, z1.d, z0.d
        movprfx z3.d, p0/z, z3.d
        mad     z3.d, p0/m, z1.d, z0.d
        movprfx z2.d, p0/z, z2.d
        mad     z2.d, p0/m, z1.d, z0.d
        cmp     w1, w0
        bne     .L3

It's obvious that we should be able to directly use z24 - z27 to perform mad
oberation and remove the redundant moves in the prologue.

I tried to implement register coalescing in IRA to fix it, but I am not sure
whether it's a good idea to fix it?=