https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96281
Bug ID: 96281
Summary: TBAA does not work as expected for a simple test case
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Keywords: alias
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: felix.yang at huawei dot com
Target Milestone: ---
Target: aarch64
Test case: foo.c
typedef struct state_t {
int threadid;
} state_t;
int history_h[8][12][64];
void history_good (state_t *s) {
int i, j;
if (s->threadid >= 0 && s->threadid < 8) {
for (i = 0; i < 12; i++) {
for (j = 0; j < 64; j++) {
history_h[s->threadid][i][j] = (history_h[s->threadid][i][j] +
1) >> 1;
}
}
}
}
$ gcc -S -O2 -ftree-loop-vectorize -funroll-loops foo.c -fopt-info
foo.c:14:13: optimized: loop unrolled 6 times
When the input parameter s is specified to be unaliased with __restrict__ type
qualifier like:
void history_good (state_t * __restrict__ s)
The inner loop could be auto-vectorized:
$ gcc -S -O2 -ftree-loop-vectorize -funroll-loops foo.c -fopt-info
foo.c:14:13: optimized: loop vectorized using 16 byte vectors
foo.c:8:6: optimized: loop with 15 iterations completely unrolled (header
execution count 16535624)
Looks like TBAA is not working here for this case. Then I noticed the
following logic in tree-ssa-alias.c:
1939 /* When we are trying to disambiguate an access with a pointer
dereference
1940 as base versus one with a decl as base we can use both the size
1941 of the decl and its dynamic type for extra disambiguation.
1942 ??? We do not know anything about the dynamic type of the decl
1943 other than that its alias-set contains base2_alias_set as a subset
1944 which does not help us here. */
1945 /* As we know nothing useful about the dynamic type of the decl just
1946 use the usual conflict check rather than a subset test.
1947 ??? We could introduce -fvery-strict-aliasing when the language
1948 does not allow decls to have a dynamic type that differs from their
1949 static type. Then we can check
1950 !alias_set_subset_of (base1_alias_set, base2_alias_set) instead. */
1951 if (base1_alias_set != base2_alias_set
1952 && !alias_sets_conflict_p (base1_alias_set, base2_alias_set))
1953 return false;
This was introduced by: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42834
>From the comments, this depends on the language of the source code.
So at least for C & C++, could we check !alias_set_subset_of (base1_alias_set,
base2_alias_set) instead here?
Any other languages supported by GCC that makes a difference?
