------- Comment #6 from rguenth at gcc dot gnu dot org 2010-01-30 16:54 ------- (In reply to comment #4) > Eh, ignore comment #3. My understanding of CD-DCE really *is* rusty :-). > > Being more careful now: IIRC the statement "j = 0" would have been control > dependent on "if (b)" and marking "j = 0" would result in marking "if (b)" > necessary. > > But since the value "0" is propagated into the PHI, there is no statement left > to mark "if (b)" necessary. So ignoring the edge with the constant (like the > very wrong patch of comment #3) results in *nothing* needing "if (b)" anymore. > So the function is incorrectly transformed to just "return j". > > What is necessary here, is a reason to mark "if (b)" necessary, but not the > loop. But since the loop now falls through directly to the PHI for "j_1", I > don't see how...
You have the other PHI argument for that which _is_ control-dependent on if (b) (like the constant PHI arg should be). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42906
