Re: [Qemu-devel] [PATCH 1/2] tcg: Fold unspecified opcode test into tcg_can_emit_vec_op

[Peter Maydell <peter.maydell@linaro.org>]

On 17 February 2018 at 16:40, Richard Henderson
<richard.henderson@linaro.org> wrote:
> This releases the callers from having to check themselves,
> which tidies up the code a bit.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  tcg/aarch64/tcg-target.inc.c |  4 ++++
>  tcg/i386/tcg-target.inc.c    |  4 ++++
>  tcg/tcg-op-gvec.c            | 28 ++++++++++++----------------
>  3 files changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/tcg/aarch64/tcg-target.inc.c b/tcg/aarch64/tcg-target.inc.c
> index be3192078d..9b0a803d79 100644
> --- a/tcg/aarch64/tcg-target.inc.c
> +++ b/tcg/aarch64/tcg-target.inc.c
> @@ -2217,6 +2217,10 @@ static void tcg_out_vec_op(TCGContext *s, TCGOpcode opc,
>  int tcg_can_emit_vec_op(TCGOpcode opc, TCGType type, unsigned vece)
>  {
>      switch (opc) {
> +    case 0:
> +        /* Unspecified opcode */
> +        return 1;
> +
>      case INDEX_op_add_vec:
>      case INDEX_op_sub_vec:
>      case INDEX_op_mul_vec:

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
since there's clearly no behaviour change here.

But it isn't clear to me why tcg_can_emit_vec_op() should return 1
for unspecified opcodes -- shouldn't the default be "we can't vectorize
something we don't know about", not "no idea what this is so let's
assume it's vectorizable" ?

A comment about why unspecified opcodes are always OK might be helpful.

thanks
-- PMM