------- Additional Comments From kazu at cs dot umass dot edu 2004-12-10 22:25 ------- Subject: Re: [4.0 regression] loop miscompilation at -O1 (-ftree-ch)
Hi Jeff, > > Can you come up with a hypothetical scenario? > No need. It's fundamentally broken in that it's recording > an invalid equivalence. It's recording that tmp_1 = next_2 > which is totally bogus. > > The only reason it works is because we don't walk through > all the SSA_NAME_VALUEs back to the oldest. Yes, I was relying on that fact. Ah, now I see that we have two different definitions of SSA_NAME_VALUE. Rather than pushing what I think is the right definition, let's get a correct definition from the author of tree-ssa-dom.c. :-) Is the following more or less correct? SSA_NAME_VALUE(X) gives you a gimple min invariant or an SSA_NAME that has the same value as X. If we do not find a gimple min invariant for X, we make every effort to make SSA_NAME_VALUE(X) evaluate to the oldest SSA_NAME that has the same value as X (via record_const_or_copy, etc), but that is not guaranteed. For the best result, you may have to walk SSA_NAME_VALUE, but it's questionable how profitable that is. The only comment I can find for SSA_NAME_VALUE is this: /* Get the value of this SSA_NAME, if available. */ #define SSA_NAME_VALUE(N) \ SSA_NAME_CHECK (N)->ssa_name.value_handle I think it would be helpful to expand the comment for a future reference. To be honest, given that tree-ssa-dom.c never walks SSA_NAME_VALUE and that record_const_or_copy makes a transitive closure of "copy-of" relations, I was under the impression that SSA_NAME_VALUE gives you the oldest value. Kazu Hirata -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18694 ------- You are receiving this mail because: ------- You are on the CC list for the bug, or are watching someone who is.
