https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80215
Bug ID: 80215
Summary: VN has trouble with conditional equivalences (DOM/FRE)
Product: gcc
Version: 7.0.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rguenth at gcc dot gnu.org
Target Milestone: ---
Currently DOM doesn't handle either redundancy in foo or bar while FRE only
handles the first.
int d, e;
int foo (int a, int c)
{
e = a * c;
if (a == c)
return a * c;
return 0;
}
int bar (int a, int c)
{
e = a * c;
if (a == c)
return c * c;
return 0;
}
another example is
int e;
int baz (int a, int c)
{
e = a + c;
if (a == c)
return c + c;
return 0;
}
with the additional complication of c + c -> 2 * c canonicalization.
The issue is unifying of two values (plus eventual followup simplifications
that might be possible). VN algorithms generally do not handle this well.
For DOM-VRP I played with re-propagating starting from uses of the
newly recorded equivalency.
