[Bug tree-optimization/18694] [4.0 regression] loop miscompilation at -O1 (-ftree-ch)
------- Additional Comments From kazu at cs dot umass dot edu 2004-12-10 19:57 -------
Subject: Re: [4.0 regression] loop
miscompilation at -O1 (-ftree-ch)
Hi Jeff,
> > I think so. :-)
> I don't. :( I think it'll record tmp_1 = next_2, which is actually
> wrong, even though it doesn't actually cause problems with this
> testcase.
IMHO, you should really think of it as tmp_1 -> next_2 or "tmp_1 is a
copy-of next_2". It is a one-way relation, not an equivalence because
it is not symmetric.
In other words, tmp_1 -> next_2 (or SSA_NAME_VALUE (tmp_1) == next_2)
means that tmp_1 should be replaced with next_2 if you were moving a
statement in E->dest to E->src. The other direction does not hold
because next_2 should *not* be replaced with tmp_1. It should be
replaced with 0 instead.
Let me also think hard to see if I can prove my patch is correct or
come up with a counterexample. I certainly don't intend to push my
patch blindly.
Oh, by the way, let's wait on putting the testcase. I am pretty sure
people will complain about a "regression" in their test results.
Kazu Hirata
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18694
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