[Qemu-devel] Re: [PATCH 4/6] tcg-i386: Implement small forward branches.

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

On Thu, Dec 17, 2009 at 6:55 PM, Richard Henderson <rth@twiddle.net> wrote:
> There are places, like brcond2, where we know that the destination
> of a forward branch will be within 127 bytes.  Add the R_386_PC8
> relocation type to support this, and add a flag to tcg_out_jxx to
> generate it.  Set the flag in the small forward branches in brcond2.
>
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  elf.h                 |    2 ++
>  tcg/i386/tcg-target.c |   36 +++++++++++++++++++++++++-----------
>  2 files changed, 27 insertions(+), 11 deletions(-)
>
> diff --git a/elf.h b/elf.h
> index 11674d7..c84c8ab 100644
> --- a/elf.h
> +++ b/elf.h
> @@ -243,6 +243,8 @@ typedef struct {
>  #define R_386_GOTOFF   9
>  #define R_386_GOTPC    10
>  #define R_386_NUM      11
> +/* Not a dynamic reloc, so not included in R_386_NUM.  Used in TCG.  */
> +#define R_386_PC8      23
>
>  #define R_MIPS_NONE            0
>  #define R_MIPS_16              1
> diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
> index 972b102..cc3d28f 100644
> --- a/tcg/i386/tcg-target.c
> +++ b/tcg/i386/tcg-target.c
> @@ -61,6 +61,9 @@ static void patch_reloc(uint8_t *code_ptr, int type,
>     case R_386_PC32:
>         *(uint32_t *)code_ptr = value - (long)code_ptr;
>         break;
> +    case R_386_PC8:
> +        *(uint8_t *)code_ptr = value - (long)code_ptr;
> +        break;
>     default:
>         tcg_abort();
>     }
> @@ -305,7 +308,8 @@ static void tcg_out_addi(TCGContext *s, int reg, tcg_target_long val)
>         tgen_arithi(s, ARITH_ADD, reg, val, 0);
>  }
>
> -static void tcg_out_jxx(TCGContext *s, int opc, int label_index)
> +/* Use SMALL != 0 to force a short forward branch.  */
> +static void tcg_out_jxx(TCGContext *s, int opc, int label_index, int small)
>  {
>     int32_t val, val1;
>     TCGLabel *l = &s->labels[label_index];
> @@ -320,6 +324,7 @@ static void tcg_out_jxx(TCGContext *s, int opc, int label_index)
>                 tcg_out8(s, 0x70 + opc);
>             tcg_out8(s, val1);
>         } else {
> +            assert (!small);

To be consistent with the rest I'd use:

  if (small)
    tcg_abort();

>             if (opc == -1) {
>                 tcg_out8(s, 0xe9);
>                 tcg_out32(s, val - 5);
> @@ -329,6 +334,15 @@ static void tcg_out_jxx(TCGContext *s, int opc, int label_index)
>                 tcg_out32(s, val - 6);
>             }
>         }
> +    } else if (small) {
> +        if (opc == -1) {
> +            tcg_out8(s, 0xeb);
> +        } else {
> +            tcg_out8(s, 0x0f);

I don't think this prefix should be output.


Laurent

> +            tcg_out8(s, 0x70 + opc);
> +        }
> +        tcg_out_reloc(s, s->code_ptr, R_386_PC8, label_index, -1);
> +        s->code_ptr += 1;
>     } else {
>         if (opc == -1) {
>             tcg_out8(s, 0xe9);
> @@ -355,7 +369,7 @@ static void tcg_out_brcond(TCGContext *s, int cond,
>     } else {
>         tcg_out_modrm(s, 0x01 | (ARITH_CMP << 3), arg2, arg1);
>     }
> -    tcg_out_jxx(s, tcg_cond_to_jcc[cond], label_index);
> +    tcg_out_jxx(s, tcg_cond_to_jcc[cond], label_index, 0);
>  }
>
>  /* XXX: we implement it at the target level to avoid having to
> @@ -376,42 +390,42 @@ static void tcg_out_brcond2(TCGContext *s,
>         break;
>     case TCG_COND_LT:
>         tcg_out_brcond(s, TCG_COND_LT, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_LTU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_LE:
>         tcg_out_brcond(s, TCG_COND_LT, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_LEU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_GT:
>         tcg_out_brcond(s, TCG_COND_GT, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_GTU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_GE:
>         tcg_out_brcond(s, TCG_COND_GT, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_GEU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_LTU:
>         tcg_out_brcond(s, TCG_COND_LTU, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_LTU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_LEU:
>         tcg_out_brcond(s, TCG_COND_LTU, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_LEU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_GTU:
>         tcg_out_brcond(s, TCG_COND_GTU, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_GTU, args[0], args[2], const_args[2], args[5]);
>         break;
>     case TCG_COND_GEU:
>         tcg_out_brcond(s, TCG_COND_GTU, args[1], args[3], const_args[3], args[5]);
> -        tcg_out_jxx(s, JCC_JNE, label_next);
> +        tcg_out_jxx(s, JCC_JNE, label_next, 1);
>         tcg_out_brcond(s, TCG_COND_GEU, args[0], args[2], const_args[2], args[5]);
>         break;
>     default:
> @@ -913,7 +927,7 @@ static inline void tcg_out_op(TCGContext *s, int opc,
>         }
>         break;
>     case INDEX_op_br:
> -        tcg_out_jxx(s, JCC_JMP, args[0]);
> +        tcg_out_jxx(s, JCC_JMP, args[0], 0);
>         break;
>     case INDEX_op_movi_i32:
>         tcg_out_movi(s, TCG_TYPE_I32, args[0], args[1]);
> --
> 1.6.5.2
>
>