Re: [Qemu-devel] [PATCH 09/10] tcg: Optimize half-dead add2/sub2

[Aurelien Jarno <aurelien@aurel32.net>]

On Tue, Oct 02, 2012 at 11:32:29AM -0700, Richard Henderson wrote:
> When x86_64 guest is not in 64-bit mode, the high-part of the 64-bit
> add is dead.  When the host is 32-bit, we can simplify to 32-bit
> arithmetic.
> 
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/tcg.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index c069e44..21c1074 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -1306,8 +1306,39 @@ static void tcg_liveness_analysis(TCGContext *s)
>              break;
>          case INDEX_op_end:
>              break;
> -            /* XXX: optimize by hardcoding common cases (e.g. triadic ops) */
> +
> +        case INDEX_op_add2_i32:
> +        case INDEX_op_sub2_i32:
> +            args -= 6;
> +            nb_iargs = 4;
> +            nb_oargs = 2;
> +            /* Test if the high part of the operation is dead, but not
> +               the low part.  The result can be optimized to a simple
> +               add or sub.  This happens often for x86_64 guest when the
> +               cpu mode is set to 32 bit.  */
> +            if (dead_temps[args[1]]) {
> +                if (dead_temps[args[0]]) {
> +                    goto do_remove;
> +                }
> +                /* Create the single operation plus nop.  */
> +                if (op == INDEX_op_add2_i32) {
> +                    op = INDEX_op_add_i32;
> +                } else {
> +                    op = INDEX_op_sub_i32;
> +                }
> +                gen_opc_buf[op_index] = op;
> +                args[1] = args[2];
> +                args[2] = args[4];
> +                assert(gen_opc_buf[op_index + 1] == INDEX_op_nop);
> +                tcg_set_nop(s, gen_opc_buf + op_index + 1, args + 3, 3);
> +                /* Fall through and mark the single-word operation live.  */
> +                nb_iargs = 2;
> +                nb_oargs = 1;
> +            }
> +            goto do_not_remove;
> +
>          default:
> +            /* XXX: optimize by hardcoding common cases (e.g. triadic ops) */
>              args -= def->nb_args;
>              nb_iargs = def->nb_iargs;
>              nb_oargs = def->nb_oargs;
> @@ -1321,6 +1352,7 @@ static void tcg_liveness_analysis(TCGContext *s)
>                      if (!dead_temps[arg])
>                          goto do_not_remove;
>                  }
> +            do_remove:
>                  tcg_set_nop(s, gen_opc_buf + op_index, args, def->nb_args);
>  #ifdef CONFIG_PROFILER
>                  s->del_op_count++;

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net