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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/99905] [8/9/10/11 Regression] wrong code with -mno-mmx
 -mno-sse since r7-4540-gb229ab2a712ccd44
Date: Wed, 07 Apr 2021 08:48:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99905

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Trying 7, 11, 13 -> 14:
    7: r109:TI=3D[argp:DI]
      REG_EQUIV [argp:DI]
   11: {r110:TI=3Dr109:TI 0>>0x3f;clobber flags:CC;}
      REG_DEAD r109:TI
      REG_UNUSED flags:CC
   13: {r85:HI=3Dr110:TI#0<-<0x8;clobber flags:CC;}
      REG_DEAD r110:TI
      REG_UNUSED flags:CC
   14: r86:SI=3Dzero_extend(r85:HI#0)
      REG_DEAD r85:HI
Successfully matched this instruction:
(set (reg:SI 86 [ _6 ])
    (const_int 0 [0]))

looks like the bug.
We have
(set (reg:SI 86 [ _6 ])
    (zero_extend:SI (subreg:QI (rotate:HI (subreg:HI (lshiftrt:TI (reg/v:TI=
 109
[ i ])
                        (const_int 63 [0x3f])) 0)
                (const_int 8 [0x8])) 0)))
and are substing (mem/c:TI (reg/f:DI 16 argp) [3 i+0 S16 A128]) into
(reg/v:TI 109 [ i ]) and that gives the SET_SRC of const0_rtx.
subst first makes
(set (reg:SI 86 [ _6 ])
    (and:SI (subreg:SI (rotate:HI (subreg:HI (lshiftrt:TI (mem/c:TI (reg/f:=
DI
16 argp) [3 i+0 S16 A128])
                        (const_int 63 [0x3f])) 0)
                (const_int 8 [0x8])) 0)
        (const_int 255 [0xff])))
which still LGTM.=