[Bug c/111931] New: RISC-V: Trivial optimization of VSETVL PASS

["juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111931

            Bug ID: 111931
           Summary: RISC-V: Trivial optimization of VSETVL PASS
           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: ---

#include "riscv_vector.h"

void foo9 (int8_t *base, int8_t* out, size_t vl, size_t m)
{
    vint8mf8_t v0;
    size_t avl = __riscv_vsetvl_e8mf8 (vl);

    for (size_t i = 0; i < m; i++)
    {
        v0 = __riscv_vle8_v_i8mf8 (base + i, avl);
        __riscv_vse8_v_i8mf8 (out + i, v0, avl);
    }
}

ASM:

        vsetvli a2,a2,e8,mf8,ta,ma
        beq     a3,zero,.L8
        add     a3,a0,a3
.L3:
        vle8.v  v1,0(a0)
        addi    a0,a0,1
        vse8.v  v1,0(a1)
        addi    a1,a1,1
        bne     a0,a3,.L3
.L8:
        ret

The vsetvl should be optimized into "vsetvl zero, a2" instead of "vsetvl a2,a2"

The reason we failed to optimize it is because we set the entry block as
unknown block when computing reaching_def. So preds_has_same_avl_p is false.

As long as there are predecessors block before the user vsetvl, we can optimize
it.

This is an trivial issue. I will fix it if I find the time.