[Bug target/115795] RISC-V: vsetvl step causes wrong codegen after fusing info

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115795

--- Comment #7 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
(In reply to Jordi Sala from comment #6)
> Perfect, that's what I was looking for. I'm thinking of adding a way to tell
> GCC to minimize, maximize or preserve SEW on vsetvl expand. Like
> -mrvv-vsetvl-sew={maximize,minimize,preserve}

No. I don't think you need to add an option here.

I think you should change the default according to the vsetvl intrinsic builtin
sematic. For example, set SEW/LMUL as e32m2 if it is __riscv_vsetvl_e32m2, or
set it as e64m1 if it is __riscv_vsetvl_e64m1 which will make codegen more
straightforward.


    if (TARGET_XTHEADVECTOR)
      {
        machine_mode inner_mode = GET_MODE_INNER (mode);
        /* SEW.  */
        e.add_input_operand (Pmode,
          gen_int_mode (GET_MODE_BITSIZE (inner_mode), Pmode));
        /* LMUL.  */
        e.add_input_operand (Pmode,
          gen_int_mode (get_vlmul (mode), Pmode));
      }
    else
      {
        /* Normalize same RATO (SEW/LMUL) into same vsetvl instruction.

             - e8,mf8/e16,mf4/e32,mf2/e64,m1 --> e8mf8
             - e8,mf4/e16,mf2/e32,m1/e64,m2  --> e8mf4
             - e8,mf2/e16,m1/e32,m2/e64,m4   --> e8mf2
             - e8,m1/e16,m2/e32,m4/e64,m8    --> e8m1
             - e8,m2/e16,m4/e32,m8           --> e8m2
             - e8,m4/e16,m8                  --> e8m4
             - e8,m8                         --> e8m8
        */
        /* SEW.  */
        e.add_input_operand (Pmode, gen_int_mode (8, Pmode));

        /* LMUL.  */
        machine_mode e8_mode
          = get_vector_mode (QImode, GET_MODE_NUNITS (mode)).require ();
        e.add_input_operand (Pmode, gen_int_mode (get_vlmul (e8_mode), Pmode));
      }

into:

        machine_mode inner_mode = GET_MODE_INNER (mode);
        /* SEW.  */
        e.add_input_operand (Pmode,
          gen_int_mode (GET_MODE_BITSIZE (inner_mode), Pmode));
        /* LMUL.  */
        e.add_input_operand (Pmode,
          gen_int_mode (get_vlmul (mode), Pmode));