https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66739
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2015-07-02
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
Target Milestone|--- |6.0
Ever confirmed|0 |1
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is
x = a - b;
if (x != 0)
vs.
if (a != b)
which we now more aggressively produce (the choice is not obvious and
we are missing the reverse transform).
The usual kind of action is to stick a single_use guard on the minus for
/* Transform comparisons of the form X - Y CMP 0 to X CMP Y.
??? The transformation is valid for the other operators if overflow
is undefined for the type, but performing it here badly interacts
with the transformation in fold_cond_expr_with_comparison which
attempts to synthetize ABS_EXPR. */
(for cmp (eq ne)
(simplify
(cmp (minus @0 @1) integer_zerop)
(cmp @0 @1)))
but I really don't like that solution (it will cause SCCVN regressions).
value-numbering can also perform the reverse transform (though late
forwprop will kill that again). I suppose we should look into
finding a more general solution for the forwprop issues from inside
forwprop.
