Re: [Qemu-devel] [PATCH v7 21/52] tcg: Use offsets not indices for TCGv_*

[Philippe Mathieu-Daudé]

On 10/20/2017 08:19 PM, Richard Henderson wrote:
> Using the offset of a temporary, relative to TCGContext, rather than
> its index means that we don't use 0.  That leaves offset 0 free for
> a NULL representation without having to leave index 0 unused.
> 
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  tcg/tcg.h | 37 ++++++++++++++++++++-----------------
>  1 file changed, 20 insertions(+), 17 deletions(-)
> 
> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index 8f692bc6cf..7fe0fb9e07 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -429,13 +429,13 @@ typedef TCGv_ptr TCGv_env;
>  #endif
>  
>  /* Dummy definition to avoid compiler warnings.  */
> -#define TCGV_UNUSED_I32(x) (x = (TCGv_i32)-1)
> -#define TCGV_UNUSED_I64(x) (x = (TCGv_i64)-1)
> -#define TCGV_UNUSED_PTR(x) (x = (TCGv_ptr)-1)
> +#define TCGV_UNUSED_I32(x) (x = (TCGv_i32)NULL)
> +#define TCGV_UNUSED_I64(x) (x = (TCGv_i64)NULL)
> +#define TCGV_UNUSED_PTR(x) (x = (TCGv_ptr)NULL)
>  
> -#define TCGV_IS_UNUSED_I32(x) ((x) == (TCGv_i32)-1)
> -#define TCGV_IS_UNUSED_I64(x) ((x) == (TCGv_i64)-1)
> -#define TCGV_IS_UNUSED_PTR(x) ((x) == (TCGv_ptr)-1)
> +#define TCGV_IS_UNUSED_I32(x) ((x) == (TCGv_i32)NULL)
> +#define TCGV_IS_UNUSED_I64(x) ((x) == (TCGv_i64)NULL)
> +#define TCGV_IS_UNUSED_PTR(x) ((x) == (TCGv_ptr)NULL)
>  
>  /* call flags */
>  /* Helper does not read globals (either directly or through an exception). It
> @@ -454,7 +454,7 @@ typedef TCGv_ptr TCGv_env;
>  #define TCG_CALL_NO_WG_SE       (TCG_CALL_NO_WG | TCG_CALL_NO_SE)
>  
>  /* used to align parameters */
> -#define TCG_CALL_DUMMY_ARG      ((TCGArg)(-1))
> +#define TCG_CALL_DUMMY_ARG      ((TCGArg)0)
>  
>  /* Conditions.  Note that these are laid out for easy manipulation by
>     the functions below:
> @@ -701,17 +701,20 @@ static inline size_t temp_idx(TCGTemp *ts)
>  
>  static inline TCGArg temp_arg(TCGTemp *ts)
>  {
> -    return temp_idx(ts);
> +    ptrdiff_t a = (void *)ts - (void *)&tcg_ctx;
> +    tcg_debug_assert(a >= offsetof(TCGContext, temps)
> +                     && a < offsetof(TCGContext, temps[tcg_ctx.nb_temps]));
> +    return a;
>  }
>  
>  static inline TCGTemp *arg_temp(TCGArg a)
>  {
> -    return a == TCG_CALL_DUMMY_ARG ? NULL : &tcg_ctx.temps[a];
> -}
> -
> -static inline size_t arg_index(TCGArg a)
> -{
> -    return a;
> +    if (a == TCG_CALL_DUMMY_ARG) {
> +        return NULL;
> +    }
> +    tcg_debug_assert(a >= offsetof(TCGContext, temps)
> +                     && a < offsetof(TCGContext, temps[tcg_ctx.nb_temps]));
> +    return (void *)&tcg_ctx + a;

Hmmm why not cast it as TCGTemp*?

>  }
>  
>  static inline TCGArg tcgv_i32_arg(TCGv_i32 t)
> @@ -746,17 +749,17 @@ static inline TCGTemp *tcgv_ptr_temp(TCGv_ptr t)
>  
>  static inline TCGv_i32 temp_tcgv_i32(TCGTemp *t)
>  {
> -    return (TCGv_i32)temp_idx(t);
> +    return (TCGv_i32)temp_arg(t);
>  }
>  
>  static inline TCGv_i64 temp_tcgv_i64(TCGTemp *t)
>  {
> -    return (TCGv_i64)temp_idx(t);
> +    return (TCGv_i64)temp_arg(t);
>  }
>  
>  static inline TCGv_ptr temp_tcgv_ptr(TCGTemp *t)
>  {
> -    return (TCGv_ptr)temp_idx(t);
> +    return (TCGv_ptr)temp_arg(t);
>  }
>  
>  #if TCG_TARGET_REG_BITS == 32
>