When a loop like the following:
=============================
#define N 8
int main ()
{
int b[N] = {0,3,6,9,12,15,18,21};
int a[N];
int i;
for (i = 0; i < N; i++){
a[i] = b[i];
}
/* check results: */
for (i = 0; i < N; i++){
if (a[i] != b[i])
abort ();
}
return 0;
}
=============================
is compiled with:
-O2 -ftree-vectorize -maltivec -funroll-loops
it gets vectorized and then completely unrolled. The complete unroller looks at
loop->nb_iterations to get the number of iterations; the problem is that loop-
>nb_iterations is not always updated after vectorization, so the loop ends up
being unrolled by 8 instead of by 2, and we get an execution failure.
The following patch will fix the problem:
Index: tree-vectorizer.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/tree-vectorizer.c,v
retrieving revision 2.25
diff -c -3 -p -r2.25 tree-vectorizer.c
*** tree-vectorizer.c 8 Nov 2004 13:54:41 -0000 2.25
--- tree-vectorizer.c 9 Nov 2004 13:16:54 -0000
*************** make_loop_iterate_ntimes (struct loop *l
*** 635,640 ****
--- 635,642 ----
if (vect_debug_stats (loop) || vect_debug_details (loop))
print_generic_expr (dump_file, cond_stmt, TDF_SLIM);
+
+ loop->nb_iterations = niters;
}
*************** vect_transform_loop_bound (loop_vec_info
*** 2881,2886 ****
--- 2883,2890 ----
if (vect_debug_details (NULL))
print_generic_expr (dump_file, cond_stmt, TDF_SLIM);
+
+ loop->nb_iterations = new_loop_bound;
}
--
Summary: wrong unrolling after vectorization due to invalid loop-
>nb_iterations
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: tree-optimization
AssignedTo: dorit at il dot ibm dot com
ReportedBy: dorit at il dot ibm dot com
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: powerpc-apple-darwin7.0.0
GCC host triplet: powerpc-apple-darwin7.0.0
GCC target triplet: powerpc-apple-darwin7.0.0
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18400
