[Bug tree-optimization/45685] [4.6 Regression] missed conditional move opportunity in loop

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45685

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org,
                   |                            |spop at gcc dot gnu.org

--- Comment #14 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-11-25 
11:13:06 UTC ---
First of all tree if-conversion goes crazy here.

What Jakub refers to (optimizing the constants) should be done while we
still have the PHIs around:

<bb 5>:
  # val_4 = PHI <1(4), -1(3)>
  # prephitmp.26_45 = PHI <-1(4), 1(3)>
  # prephitmp.27_46 = PHI <4294967295(4), 1(3)>
  # prephitmp.27_48 = PHI <1(4), 4294967295(3)>

here at least prephitmp.26_45 = -val_4, prephitmp.27_46 = -prephitmp.27_48,
etc.

Now - we can teach PRE not to do PHI insertions for sign changes or
negations, but it's hard to tell if the insertion involved further
simplifications that are worthwhile.

We go from

<bb 5>:
  # val_4 = PHI <1(4), -1(11)>
...
  if (D.4323_24 != i_19)
    goto <bb 6>;
  else
    goto <bb 12>;

<bb 12>:
  goto <bb 7>;

<bb 6>:
  val_25 = -val_4;

<bb 7>:
  # val_3 = PHI <val_4(12), val_25(6)>
...
  D.4326_28 = (unsigned int) val_3;

to the above, inserting for -val_4 and (unsigned int) val_3.

That said, if-conversion should be improved for the case of generating
constants.