phiopt: Improve value_replacement [PR104645]

phiopt: Improve value_replacement [PR104645]

The following patch fixes the P1 regression by reusing existing
value_replacement code.  That function already has code to
handle simple preparation statements (casts, and +,&,|,^ binary
assignments) before a final binary assignment (which can be
much wider range of ops).  When we have e.g.
      if (y_3(D) == 0)
        goto <bb 4>;
      else
        goto <bb 3>;
     <bb 3>:
      y_4 = y_3(D) & 31;
      _1 = (int) y_4;
      _6 = x_5(D) r<< _1;
     <bb 4>:
      # _2 = PHI <x_5(D)(2), _6(3)>
the preparation statements y_4 = y_3(D) & 31; and
_1 = (int) y_4; are handled by constant evaluation, passing through
y_3(D) = 0 initially and propagating that through the assignments
with checking that UB isn't invoked.  But the final
_6 = x_5(D) r<< _1; assign is handled differently, either through
neutral_element_p or absorbing_element_p.
In the first function below we now have:
  <bb 2> [local count: 1073741824]:
  if (i_2(D) != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870913]:
  _3 = i_2(D) & 1;
  iftmp.0_4 = (int) _3;

  <bb 4> [local count: 1073741824]:
  # iftmp.0_1 = PHI <iftmp.0_4(3), 0(2)>
where in GCC 11 we had:
  <bb 2> :
  if (i_3(D) != 0)
    goto <bb 3>; [INV]
  else
    goto <bb 4>; [INV]

  <bb 3> :
  i.1_1 = (int) i_3(D);
  iftmp.0_5 = i.1_1 & 1;

  <bb 4> :
  # iftmp.0_2 = PHI <iftmp.0_5(3), 0(2)>
Current value_replacement can handle the latter as the last
stmt of middle_bb is a binary op that in this case satisfies
absorbing_element_p.
But the former we can't handle, as the last stmt in middle_bb
is a cast.

The patch makes it work in that case by pretending all of middle_bb
are the preparation statements and there is no binary assign at the
end, so everything is handled through the constant evaluation.
We simply set at the start of middle_bb the lhs of comparison
virtually to the rhs, propagate it through and at the end
see if virtually the arg0 of the PHI is equal to arg1 of it.

For GCC 13, I think we just should throw away all the neutral/absorbing
element stuff and do the constant evaluation of the whole middle_bb
and handle that way all the ops we currently handle in neutral/absorbing
element.

2022-04-01  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/104645
	* tree-ssa-phiopt.cc (value_replacement): If assign has
	CONVERT_EXPR_CODE_P rhs_code, treat it like a preparation
	statement with constant evaluation.

	* gcc.dg/tree-ssa/pr104645.c: New test.