Re: [Qemu-devel] [PATCH 1/2] tcg-x86_64: Special-case all 32-bit AND operands.

[Aurelien Jarno <aurelien@aurel32.net>]

On Tue, Jan 05, 2010 at 04:03:00PM -0800, Richard Henderson wrote:
> This avoids an unnecessary REX.W prefix when dealing with AND
> operands that fit into a 32-bit quantity.  The most common change
> actually seen is movz[wb]q -> movz[wb]l.
> 
> Similarly, avoid REXW in ext{8,16}u_i64 tcg opcodes.
> 
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/x86_64/tcg-target.c |   26 ++++++++------------------
>  1 files changed, 8 insertions(+), 18 deletions(-)
> 
> diff --git a/tcg/x86_64/tcg-target.c b/tcg/x86_64/tcg-target.c
> index 2339091..f584c94 100644
> --- a/tcg/x86_64/tcg-target.c
> +++ b/tcg/x86_64/tcg-target.c
> @@ -426,24 +426,18 @@ static inline void tgen_arithi64(TCGContext *s, int c, int r0, int64_t val)
>      } else if ((c == ARITH_ADD && val == -1) || (c == ARITH_SUB && val == 1)) {
>          /* dec */
>          tcg_out_modrm(s, 0xff | P_REXW, 1, r0);
> -    } else if (val == (int8_t)val) {
> -        tcg_out_modrm(s, 0x83 | P_REXW, c, r0);
> -        tcg_out8(s, val);
> -    } else if (c == ARITH_AND && val == 0xffu) {
> -        /* movzbl */
> -        tcg_out_modrm(s, 0xb6 | P_EXT | P_REXW, r0, r0);
> -    } else if (c == ARITH_AND && val == 0xffffu) {
> -        /* movzwl */
> -        tcg_out_modrm(s, 0xb7 | P_EXT | P_REXW, r0, r0);
>      } else if (c == ARITH_AND && val == 0xffffffffu) {
>          /* 32-bit mov zero extends */
>          tcg_out_modrm(s, 0x8b, r0, r0);
> +    } else if (c == ARITH_AND && (uint64_t)val <= 0xffffffffu) {
> +        /* AND with no high bits set can use a 32-bit operation.  */
> +        tgen_arithi32(s, c, r0, val);

Do we really want to call tgen_arithi32() here, that will redo part of
the above tests again? It might be better to simply remove the REX.W
prefix above instead.

> +    } else if (val == (int8_t)val) {
> +        tcg_out_modrm(s, 0x83 | P_REXW, c, r0);
> +        tcg_out8(s, val);
>      } else if (val == (int32_t)val) {
>          tcg_out_modrm(s, 0x81 | P_REXW, c, r0);
>          tcg_out32(s, val);
> -    } else if (c == ARITH_AND && val == (uint32_t)val) {
> -        tcg_out_modrm(s, 0x81, c, r0);
> -        tcg_out32(s, val);
>      } else {
>          tcg_abort();
>      }
> @@ -1182,16 +1176,12 @@ static inline void tcg_out_op(TCGContext *s, int opc, const TCGArg *args,
>          tcg_out_modrm(s, 0x63 | P_REXW, args[0], args[1]);
>          break;
>      case INDEX_op_ext8u_i32:
> +    case INDEX_op_ext8u_i64:
>          tcg_out_modrm(s, 0xb6 | P_EXT | P_REXB, args[0], args[1]);
>          break;
>      case INDEX_op_ext16u_i32:
> -        tcg_out_modrm(s, 0xb7 | P_EXT, args[0], args[1]);
> -        break;
> -    case INDEX_op_ext8u_i64:
> -        tcg_out_modrm(s, 0xb6 | P_EXT | P_REXW, args[0], args[1]);
> -        break;
>      case INDEX_op_ext16u_i64:
> -        tcg_out_modrm(s, 0xb7 | P_EXT | P_REXW, args[0], args[1]);
> +        tcg_out_modrm(s, 0xb7 | P_EXT, args[0], args[1]);
>          break;
>      case INDEX_op_ext32u_i64:
>          tcg_out_modrm(s, 0x8b, args[0], args[1]);

This part looks fine.

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