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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/112760] [14 Regression] wrong code with -O2
 -fno-dce -fno-guess-branch-probability -m8bit-idiv -mavx
 --param=max-cse-insns=0 and __builtin_add_overflow_p()
Date: Wed, 29 Nov 2023 10:45:23 +0000
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X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: wrong-code
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112760

Uro=C5=A1 Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|target                      |rtl-optimization
   Last reconfirmed|                            |2023-11-29
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Target Milestone|---                         |14.0

--- Comment #2 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
With the original testcase, ce1 pass is if-converting:

   20: flags:CCZ=3Dcmp(r110:SI,r111:SI)
      REG_DEAD r111:SI
      REG_DEAD r110:SI
   21: pc=3D{(flags:CCZ=3D=3D0)?L23:pc}
      REG_DEAD flags:CCZ
   39: NOTE_INSN_BASIC_BLOCK 5
   22: r103:HI=3D0x1
   23: L23:

with:

IF-THEN-JOIN block found, pass 2, test 2, then 5, join 6
scanning new insn with uid =3D 45.
scanning new insn with uid =3D 44.
scanning new insn with uid =3D 46.
if-conversion succeeded through noce_try_cmove
Removing jump 21.
deleting insn with uid =3D 21.
deleting insn with uid =3D 22.

to:

   20: flags:CCZ=3Dcmp(r110:SI,r111:SI)
      REG_DEAD r111:SI
      REG_DEAD r110:SI
   45: r118:HI=3D0x1
   44: flags:CCZ=3Dcmp(r110:SI,r111:SI)
   46: r103:HI=3D{(flags:CCZ=3D=3D0)?r103:HI:r118:HI}

And things go downhill from here. Before postreload we have:

   20: flags:CCZ=3Dcmp(ax:SI,dx:SI)
      REG_UNUSED flags:CCZ
   44: flags:CCZ=3Dcmp(ax:SI,dx:SI)
      REG_DEAD dx:SI
      REG_DEAD ax:SI
   62: ax:HI=3D0x1
      REG_EQUIV 0x1
   46: bx:HI=3D{(flags:CCZ=3D=3D0)?bx:HI:ax:HI}
      REG_DEAD flags:CCZ
      REG_DEAD ax:HI

and in posteload pass (insn 44) is removed:

   20: flags:CCZ=3Dcmp(ax:SI,dx:SI)
      REG_UNUSED flags:CCZ
   62: ax:HI=3D0x1
      REG_EQUIV 0x1
   46: bx:HI=3D{(flags:CCZ=3D=3D0)?bx:HI:ax:HI}
      REG_DEAD flags:CCZ
      REG_DEAD ax:HI

here comes pro_and_epilogue pass that detects "unused" (insn 20) and removes
it:

df_analyze called
deleting insn with uid =3D 20.

Confirmed as RTL optimization problem.=