The testcase below gets misscompiled at -O2.
The alias info looks this way:
Dereferenced pointers
xa, UID 1527, struct test1 *, symbol memory tag: SMT.4, default def: xa_4
xb, UID 1528, struct test2 *, symbol memory tag: SMT.5, default def: xb_3
Symbol memory tags
SMT.4, UID 1547, struct test1, is addressable, is global, call clobbered, may
aliases: { global }
SMT.5, UID 1548, struct test2, is addressable, is global, call clobbered, may
aliases: { global }
The alias sets for SMT.4 and SMT.5 intersect, so everything looks OK here.
However, access_can_touch_variable (correctly) determines that the dereferences
of xa and xb cannot touch "global"; hence we create the following virual
operands:
# SMT.5_8 = V_MAY_DEF <SMT.5_7>;
xb_3->sub.a = 1;
# SMT.4_10 = V_MAY_DEF <SMT.4_9>;
xa_4->a = 8;
# VUSE <SMT.5_8>;
D.1531_5 = xb_3->sub.a;
The accesses to xa and xb appear to be independent, which leads to a
misscompilation.
struct test1
{
int a;
int b;
};
struct test2
{
float d;
struct test1 sub;
};
int global;
int bla(struct test1 *xa, struct test2 *xb)
{
global = 1;
xb->sub.a = 1;
xa->a = 8;
return xb->sub.a;
}
int main(void)
{
struct test2 pom;
if (bla (&pom.sub, &pom) != 8)
abort ();
return 0;
}
--
Summary: Misscompilation due to alias analysis
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: rakdver at gcc dot gnu dot org
GCC build triplet: i686-pc-linux
GCC host triplet: i686-pc-linux
GCC target triplet: i686-pc-linux
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29156
