How can I generate SVE vectors with LLVM - clang

Question

clang version 11.0.0
example.c:
#define ARRAYSIZE 1024
int a[ARRAYSIZE];
int b[ARRAYSIZE];
int c[ARRAYSIZE];
void subtract_arrays(int *restrict a, int *restrict b, int *restrict c)
{
for (int i = 0; i < ARRAYSIZE; i++)
{
a[i] = b[i] - c[i];
}
}
int main()
{
subtract_arrays(a, b, c);
}
command:
clang --target=aarch64-linux-gnu -march=armv8-a+sve -O3 -S example.c
LLVM always generate NEON vectors, but I want it generate SVE vectors.
How can I do this?

Answer 1

Unfortunately Clang version 11 does not support SVE auto-vectorization.
This will come with LLVM 13: Architecture support in LLVM
You can however generate SVE code with intrinsic functions or inline assembly.
Your code with intrinsic functions would look something along the lines of:
#include <arm_sve.h>
void subtract_arrays(int *restrict a, int *restrict b, int *restrict c) {
int i = 0;
svbool_t pg = svwhilelt_b32(i, ARRAYSIZE);
do
{
svint32_t db_vec = svld1(pg, &b[i]);
svint32_t dc_vec = svld1(pg, &c[i]);
svint32_t da_vec = svsub_z(pg, db_vec, dc_vec);
svst1(pg, &a[i], da_vec);
i += svcntw();
pg = svwhilelt_b32(i, ARRAYSIZE);
}
while (svptest_any(svptrue_b32(), pg));
}
I had a similar problem thinking that SVE auto-vectorization is supported.
When targeting SVE with Clang, optimization reports show successful vectorization despite only vectorizing for Neon.