http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55936
Bug #: 55936
Summary: Missed VRP optimization
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: l...@redhat.com
gcc.dg/tree-ssa/vrp06.c looks like:
foo (int i, int j, int a)
{
if (i >= 10)
if (i <= 30)
if (i == j)
{
a--;
/* This should fold to 'if (0)'. */
if (i < 0)
i = baz ();
/* This should fold to 'if (1)'. */
if (j > 0)
a--;
/* This should fold to 'if (0)'. */
if (i != j)
return 0;
}
return i + a + j;
}
VRP is supposed to optimize away the 3 innermost conditions in a single pass;
the key being to realize that the test i < 0 is always false and thus i does
not vary (nor does j vary). Once i & j are determined to have non-varying
values, we should be able to optimize away the i != j test because we're in the
true clause if the i == j test.
Unfortunately, VRP drops i to varying and it's unable to optimize away the last
test during the first VRP pass. It does get optimized later.
