combine: Discard REG_UNUSED note in i2 when register is also referenced in i3 [PR118739]

combine: Discard REG_UNUSED note in i2 when register is also referenced in i3 [PR118739]

The combine pass is trying to combine:

Trying 16, 22, 21 -> 23:
   16: r104:QI=flags:CCNO>0
   22: {r120:QI=r104:QI^0x1;clobber flags:CC;}
      REG_UNUSED flags:CC
   21: r119:QI=flags:CCNO<=0
      REG_DEAD flags:CCNO
   23: {r110:QI=r119:QI|r120:QI;clobber flags:CC;}
      REG_DEAD r120:QI
      REG_DEAD r119:QI
      REG_UNUSED flags:CC

and creates the following two insn sequence:

modifying insn i2    22: r104:QI=flags:CCNO>0
      REG_DEAD flags:CC
deferring rescan insn with uid = 22.
modifying insn i3    23: r110:QI=flags:CCNO<=0
      REG_DEAD flags:CC
deferring rescan insn with uid = 23.

where the REG_DEAD note in i2 is not correct, because the flags
register is still referenced in i3.  In try_combine() megafunction,
we have this part:

--cut here--
    /* Distribute all the LOG_LINKS and REG_NOTES from I1, I2, and I3.  */
    if (i3notes)
      distribute_notes (i3notes, i3, i3, newi2pat ? i2 : NULL,
			elim_i2, elim_i1, elim_i0);
    if (i2notes)
      distribute_notes (i2notes, i2, i3, newi2pat ? i2 : NULL,
			elim_i2, elim_i1, elim_i0);
    if (i1notes)
      distribute_notes (i1notes, i1, i3, newi2pat ? i2 : NULL,
			elim_i2, local_elim_i1, local_elim_i0);
    if (i0notes)
      distribute_notes (i0notes, i0, i3, newi2pat ? i2 : NULL,
			elim_i2, elim_i1, local_elim_i0);
    if (midnotes)
      distribute_notes (midnotes, NULL, i3, newi2pat ? i2 : NULL,
			elim_i2, elim_i1, elim_i0);
--cut here--

where the compiler distributes REG_UNUSED note from i2:

   22: {r120:QI=r104:QI^0x1;clobber flags:CC;}
      REG_UNUSED flags:CC

via distribute_notes() using the following:

--cut here--
	  /* Otherwise, if this register is used by I3, then this register
	     now dies here, so we must put a REG_DEAD note here unless there
	     is one already.  */
	  else if (reg_referenced_p (XEXP (note, 0), PATTERN (i3))
		   && ! (REG_P (XEXP (note, 0))
			 ? find_regno_note (i3, REG_DEAD,
					    REGNO (XEXP (note, 0)))
			 : find_reg_note (i3, REG_DEAD, XEXP (note, 0))))
	    {
	      PUT_REG_NOTE_KIND (note, REG_DEAD);
	      place = i3;
	    }
--cut here--

Flags register is used in I3, but there already is a REG_DEAD note in I3.
The above condition doesn't trigger and continues in the "else" part where
REG_DEAD note is put to I2.  The proposed solution corrects the above
logic to trigger every time the register is referenced in I3, avoiding the
"else" part.

	PR rtl-optimization/118739

gcc/ChangeLog:

	* combine.cc (distribute_notes) <case REG_UNUSED>: Correct the
	logic when the register is used by I3.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr118739.c: New test.