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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/104054] [9/10/11/12 Regression]
 '-fcompare-debug' failure (length) w/ -Os -funroll-loops since
 r8-3946-gfe94440235cfaa57
Date: Mon, 17 Jan 2022 12:22:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104054

--- Comment #3 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
The first difference is in rnreg pass, w/o -g:

   28: L28:
   29: NOTE_INSN_BASIC_BLOCK 4
   30: [`i']=3D0
   63: di:DI=3Dr9:DI       <--- here
   64: dx:DI=3Dr10:DI
    9: r8:HI=3D0x5
      REG_EQUAL 0x5
   98: {cx:DI=3D0;clobber flags:CC;}
      REG_UNUSED flags:CC
   84: si:DI=3D0x1
   99: {r11:DI=3D0;clobber flags:CC;}
      REG_UNUSED flags:CC
   79: pc=3DL41


and w/ -g:

   34: L34:
   35: NOTE_INSN_BASIC_BLOCK 4
   36: debug begin stmt marker
   37: [`i']=3D0
   77: ax:DI=3Dr9:DI
   78: dx:DI=3Dr10:DI
    9: r8:HI=3D0x5
      REG_EQUAL 0x5
  112: {cx:DI=3D0;clobber flags:CC;}
      REG_UNUSED flags:CC
   98: si:DI=3D0x1
  113: {di:DI=3D0;clobber flags:CC;}
      REG_UNUSED flags:CC
   93: pc=3DL53

And indeed, the -fcompare-debug test succeeds with -fno-rename-registers=