[Bug c/111857] New: RISC-V: Failed to vectorize small GNU vector if zvl4096b with fixed-vlmax

[juzhe.zhong at rivai dot ai via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111857

            Bug ID: 111857
           Summary: RISC-V: Failed to vectorize small GNU vector if
                    zvl4096b with fixed-vlmax
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juzhe.zhong at rivai dot ai
  Target Milestone: ---

typedef char vnx16i __attribute__ ((vector_size (16)));

#define MASK_16 12, 13, 14, 15, 12, 13, 14, 15, 12, 13, 14, 15, 12, 13, 14, 15

vnx16i __attribute__ ((noinline, noclone))
test_1 (vnx16i x, vnx16i y)
{
  return __builtin_shufflevector (x, y, MASK_16);
}

If we specify it as -march=rv64gcv
--param=riscv-autovec-preference=fixed-vlmax.
it can vectorize.

But failed to vectorize it with -march=rv64gcv_zvl4096b
--param=riscv-autovec-preference=fixed-vlmax

https://godbolt.org/z/zx816ao5o