[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()

["ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112760

Uroš 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 Uroš Bizjak <ubizjak at gmail dot com> ---
With the original testcase, ce1 pass is if-converting:

   20: flags:CCZ=cmp(r110:SI,r111:SI)
      REG_DEAD r111:SI
      REG_DEAD r110:SI
   21: pc={(flags:CCZ==0)?L23:pc}
      REG_DEAD flags:CCZ
   39: NOTE_INSN_BASIC_BLOCK 5
   22: r103:HI=0x1
   23: L23:

with:

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

to:

   20: flags:CCZ=cmp(r110:SI,r111:SI)
      REG_DEAD r111:SI
      REG_DEAD r110:SI
   45: r118:HI=0x1
   44: flags:CCZ=cmp(r110:SI,r111:SI)
   46: r103:HI={(flags:CCZ==0)?r103:HI:r118:HI}

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

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

and in posteload pass (insn 44) is removed:

   20: flags:CCZ=cmp(ax:SI,dx:SI)
      REG_UNUSED flags:CCZ
   62: ax:HI=0x1
      REG_EQUIV 0x1
   46: bx:HI={(flags:CCZ==0)?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 = 20.

Confirmed as RTL optimization problem.