chfang@pathscale:~/gcc$ cat foo.c int a[100], b[100]; void foo(int n, int mid) { int i, t = 0; for(i=0; i<n; i++) { a[i] = b[i] + t; t = b[i]; } } chfang@pathscale:~/gcc$ gcc -O3 -ftree-vectorizer-verbose=7 -c foo.c foo.c:6: note: not vectorized: unsupported use in stmt. foo.c:3: note: vectorized 0 loops in function. scalar expansion of t into array to carry the values accross iteration.
Basically undoing predictive commoning (which we switched the order for 4.5 to fix a different issue). Confirmed.
I think PR 35229 covers this issue.
I just found that the test case in the same as (similar to) bug 35229. The subject of this bug is wrong. Scalar expansion is not appropriate for this case. Actually the loop can be transform to: void foo(int n) { int i; a[0] = b[0]; /* + t if t live before this point */ for(i=1; i<n; i++) { a[i] = b[i] + b[i-1]; } /* t = b[n-1]; is t live after this point */ } The this loop can be vectorized. In open64, this optimization is called forward (backward) substitution, i.e. substitute t with b[i-1]. I am not clear whether bug 35229 addresses the same issue. Maybe we should close one of them.
Link to vectorizer missed-optimization meta-bug.
Duplicate of 35229. *** This bug has been marked as a duplicate of bug 35229 ***