> Hi,
>
> We would like to know if there is some way to find the true and false
> branches of a conditional jump in RTL. In the tree CFG, we have two
> edge flags for that, EDGE_{TRUE,FALSE}_VALUE, but those flags have no
> meaning for the RTL CFG. So our question is, is there some other way
> to tell what edge will be taken in a conditional jump if the condition
> is true?
>
> It seems that some passes assume a canonical form of IF-THEN-ELSE even
> on RTL. From ifcvt.c:find_if_header:
>
> /* The THEN edge is canonically the one that falls through. */
> if (then_edge->flags & EDGE_FALLTHRU)
> ;
> else if (else_edge->flags & EDGE_FALLTHRU)
> {
> edge e = else_edge;
> else_edge = then_edge;
> then_edge = e;
> }
> else
> /* Otherwise this must be a multiway branch of some sort. */
> return NULL;
>
> On the other hand, in cfgexpand.c:expand_gimple_cond_expr we have,
>
> false_edge->flags |= EDGE_FALLTHRU;
>
> and loop-unswitch.c assumes that the BRANCH_EDGE is the true_edge:
>
> true_edge = BRANCH_EDGE (unswitch_on_alt);
> false_edge = FALLTHRU_EDGE (unswitch_on);
>
> So which is it? Is BRANCH_EDGE always taken if the condition is true,
> or FALLTHRU_EDGE, or do you have to look at the condition to know?
> Who knows an answer? :-)
:) It depends on how the conditional is constructed.
If you use get_condition the edge taken when conditional is true is
always BRANCH_EDGE if some exists (it is possible to have conditional
jump to the following instruction where you have only one edge with
EDGE_FALLTHRU flag).
Otherwise you have to look into conditional jump RTL yourself to figure
out if it has form
(set (pc) (if_then_else (cond) (pc) (label_ref xxx))
or
(set (pc) (if_then_else (cond) (label_ref xxx) (pc))
In the first case we are taking barnch edge when conditional is false.
Honza
>
> Gr.
> Steven
