https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112774
Bug ID: 112774
Summary: Vectorize the loop by inferring nonwrapping
information from arrays
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hliu at amperecomputing dot com
Target Milestone: ---
This case extracted from another benchmark and it is simpler than the case in
PR101450, as it has the additional boundary information from the array:
int A[1024 * 2];
int foo (unsigned offset, unsigned N)
{
int sum = 0;
for (unsigned i = 0; i < N; i++)
sum += A[i + offset];
return sum;
}
The Gimple before the vectorization pass is:
<bb 3> [local count: 955630224]:
# sum_12 = PHI <sum_9(6), 0(5)>
# i_14 = PHI <i_10(6), 0(5)>
_1 = offset_8(D) + i_14;
_2 = A[_1];
sum_9 = _2 + sum_12;
i_10 = i_14 + 1;
GCC failed to vectorize it as it the chrec "{offset_8, +, 1}_1" may
overflow/wrap. I summarized more details in the email:
https://gcc.gnu.org/pipermail/gcc/2023-November/242854.html
Actually, GCC already knows it won't by inferring the range from the array
(in estimate_numbers_of_iterations -> infer_loop_bounds_from_undefined ->
infer_loop_bounds_from_array):
Induction variable (unsigned int) offset_8(D) + 1 * iteration does not wrap
in statement _2 = A[_1];
in loop 1.
Statement _2 = A[_1];
is executed at most 2047 (bounded by 2047) + 1 times in loop 1.
We can use re-use this information to vectorize this case. I already have a
simple patch to achieve this, and will send it out later (after doing more
tests).
