[Qemu-devel] Re: [PATCH 3/6] tcg-x86_64: Implement setcond and movcond.

[Laurent Desnogues <laurent.desnogues@gmail.com>]

On Thu, Dec 17, 2009 at 6:32 PM, Richard Henderson <rth@twiddle.net> wrote:
> Implement conditional moves in the x86_64 backend.
>
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/x86_64/tcg-target.c |   65 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 files changed, 62 insertions(+), 3 deletions(-)
>
> diff --git a/tcg/x86_64/tcg-target.c b/tcg/x86_64/tcg-target.c
> index 2339091..e411755 100644
> --- a/tcg/x86_64/tcg-target.c
> +++ b/tcg/x86_64/tcg-target.c
> @@ -491,9 +491,8 @@ static void tcg_out_jxx(TCGContext *s, int opc, int label_index)
>     }
>  }
>
> -static void tcg_out_brcond(TCGContext *s, int cond,
> -                           TCGArg arg1, TCGArg arg2, int const_arg2,
> -                           int label_index, int rexw)
> +static void tcg_out_cond(TCGContext *s, int cond, TCGArg arg1,
> +                         TCGArg arg2, int const_arg2, int rexw)
>  {
>     if (const_arg2) {
>         if (arg2 == 0) {
> @@ -508,9 +507,45 @@ static void tcg_out_brcond(TCGContext *s, int cond,
>     } else {
>         tcg_out_modrm(s, 0x01 | (ARITH_CMP << 3) | rexw, arg2, arg1);
>     }
> +}
> +
> +static void tcg_out_brcond(TCGContext *s, int cond,
> +                           TCGArg arg1, TCGArg arg2, int const_arg2,
> +                           int label_index, int rexw)
> +{
> +    tcg_out_cond(s, cond, arg1, arg2, const_arg2, rexw);
>     tcg_out_jxx(s, tcg_cond_to_jcc[cond], label_index);
>  }
>
> +static void tcg_out_setcond(TCGContext *s, int cond, TCGArg arg0,
> +                            TCGArg arg1, TCGArg arg2, int const_arg2, int rexw)

Perhaps renaming arg0 to dest would make things slightly
more readable.

> +{
> +    int use_xor = (arg0 != arg1 && (const_arg2 || arg0 != arg2));
> +
> +    if (use_xor)
> +        tcg_out_movi(s, TCG_TYPE_I32, arg0, 0);
> +    tcg_out_cond(s, cond, arg1, arg2, const_arg2, rexw);
> +    tcg_out_modrm(s, 0x90 | tcg_cond_to_jcc[cond] | P_EXT | P_REXB, 0, arg0);

A comment saying this is a setcc would be nice.

Also note that tcg_out_modrm will generate an unneeded prefix
for some registers. cf. the patch I sent to the list months ago.

> +    if (!use_xor)
> +        tgen_arithi32(s, ARITH_AND, arg0, 0xff);

Wouldn't movzbl be better?

Regarding the xor optimization, I tested it on my i7 and it was
(very) slightly slower running a 64-bit SPEC2k gcc.

> +}
> +
> +static void tcg_out_movcond(TCGContext *s, int cond, TCGArg arg0,
> +                            TCGArg arg1, TCGArg arg2, int const_arg2,
> +                            TCGArg arg3, TCGArg arg4, int rexw)

Perhaps renaming arg0 to dest would make things slightly
more readable.

You should also add a note stating that arg3 != arg4.

> +{
> +    if (arg0 == arg3) {
> +        cond = tcg_invert_cond(cond);
> +        arg3 = arg4;
> +        arg4 = arg0;
> +    }
> +
> +    tcg_out_cond(s, cond, arg1, arg2, const_arg2, rexw);
> +    if (arg0 != arg4)
> +        tcg_out_mov(s, arg0, arg4);
> +    tcg_out_modrm(s, 0x40 | tcg_cond_to_jcc[cond] | P_EXT | rexw, arg0, arg3);

A comment saying this is cmovcc would be nice.

Note, I didn't check the correctness, though it looks OK. I'll
make real tests at the next iteration :-)

> +}
> +
>  #if defined(CONFIG_SOFTMMU)
>
>  #include "../../softmmu_defs.h"
> @@ -1197,6 +1232,24 @@ static inline void tcg_out_op(TCGContext *s, int opc, const TCGArg *args,
>         tcg_out_modrm(s, 0x8b, args[0], args[1]);
>         break;
>
> +    case INDEX_op_setcond_i32:
> +        tcg_out_setcond(s, args[3], args[0], args[1], args[2],
> +                        const_args[2], 0);
> +        break;
> +    case INDEX_op_setcond_i64:
> +        tcg_out_setcond(s, args[3], args[0], args[1], args[2],
> +                        const_args[2], P_REXW);
> +        break;
> +
> +    case INDEX_op_movcond_i32:
> +        tcg_out_movcond(s, args[5], args[0], args[1], args[2],
> +                        const_args[2], args[3], args[4], 0);
> +        break;
> +    case INDEX_op_movcond_i64:
> +        tcg_out_movcond(s, args[5], args[0], args[1], args[2],
> +                        const_args[2], args[3], args[4], P_REXW);
> +        break;
> +
>     case INDEX_op_qemu_ld8u:
>         tcg_out_qemu_ld(s, args, 0);
>         break;
> @@ -1376,6 +1429,12 @@ static const TCGTargetOpDef x86_64_op_defs[] = {
>     { INDEX_op_ext16u_i64, { "r", "r"} },
>     { INDEX_op_ext32u_i64, { "r", "r"} },
>
> +    { INDEX_op_setcond_i32, { "r", "r", "re" } },
> +    { INDEX_op_setcond_i64, { "r", "r", "re" } },
> +
> +    { INDEX_op_movcond_i32, { "r", "r", "re", "r", "r" } },
> +    { INDEX_op_movcond_i64, { "r", "r", "re", "r", "r" } },

For the i32 variants, "ri" instead of "re" is enough.


Laurent

> +
>     { INDEX_op_qemu_ld8u, { "r", "L" } },
>     { INDEX_op_qemu_ld8s, { "r", "L" } },
>     { INDEX_op_qemu_ld16u, { "r", "L" } },
> --
> 1.6.5.2
>
>