Re: [Qemu-devel] [PATCH v2] target-ppc: Fix narrow-mode add/sub carry output

[Aurelien Jarno]

On Wed, Apr 03, 2013 at 03:56:45PM -0500, Richard Henderson wrote:
> Broken in b5a73f8d8a57e940f9bbeb399a9e47897522ee9a, the carry itself was
> fixed in 79482e5ab38a05ca8869040b0d8b8f451f16ff62.  But we still need to
> produce the full 64-bit addition.
> 
> Simplify the conditions at the top of the functions for when we need a
> new temporary.  Only plain addition is important enough to warrent avoiding
> the temporary, and the extra tcg move op that would come with it.
> 
> Signed-off-by: Richard Henderson <address@hidden>
> ---
> Version 2 fixes the temporary creation problem spotted by Aurelien.
> ---
> 
>  target-ppc/translate.c | 35 ++++++++++++++++++++++-------------
>  1 file changed, 22 insertions(+), 13 deletions(-)
> 
> diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> index 5e741d1..fa1778a 100644
> --- a/target-ppc/translate.c
> +++ b/target-ppc/translate.c
> @@ -768,22 +768,25 @@ static inline void gen_op_arith_add(DisasContext *ctx, 
> TCGv ret, TCGv arg1,
>  {
>      TCGv t0 = ret;
>  
> -    if (((compute_ca && add_ca) || compute_ov)
> -        && (TCGV_EQUAL(ret, arg1) || TCGV_EQUAL(ret, arg2)))  {
> +    if (compute_ca || compute_ov) {
>          t0 = tcg_temp_new();
>      }
>  
>      if (compute_ca) {
>          if (NARROW_MODE(ctx)) {
> +            /* Caution: a non-obvious corner case of the spec is that we
> +               must produce the *entire* 64-bit addition, but produce the
> +               carry into bit 32.  */
>              TCGv t1 = tcg_temp_new();
> -            tcg_gen_ext32u_tl(t1, arg2);
> -            tcg_gen_ext32u_tl(t0, arg1);
> -            tcg_gen_add_tl(t0, t0, t1);
> -            tcg_temp_free(t1);
> +            tcg_gen_xor_tl(t1, arg1, arg2);        /* add without carry */
> +            tcg_gen_add_tl(t0, arg1, arg2);
>              if (add_ca) {
>                  tcg_gen_add_tl(t0, t0, cpu_ca);
>              }
> -            tcg_gen_shri_tl(cpu_ca, t0, 32);
> +            tcg_gen_xor_tl(cpu_ca, t0, t1);        /* bits changed w/ carry 
> */
> +            tcg_temp_free(t1);
> +            tcg_gen_shri_tl(cpu_ca, cpu_ca, 32);   /* extract bit 32 */
> +            tcg_gen_andi_tl(cpu_ca, cpu_ca, 1);
>          } else {
>              TCGv zero = tcg_const_tl(0);
>              if (add_ca) {
> @@ -1122,24 +1125,30 @@ static inline void gen_op_arith_subf(DisasContext 
> *ctx, TCGv ret, TCGv arg1,
>  {
>      TCGv t0 = ret;
>  
> -    if (compute_ov && (TCGV_EQUAL(ret, arg1) || TCGV_EQUAL(ret, arg2)))  {
> +    if (compute_ca || compute_ov) {
>          t0 = tcg_temp_new();
>      }
>  
>      if (compute_ca) {
>          /* dest = ~arg1 + arg2 [+ ca].  */
>          if (NARROW_MODE(ctx)) {
> +            /* Caution: a non-obvious corner case of the spec is that we
> +               must produce the *entire* 64-bit addition, but produce the
> +               carry into bit 32.  */
>              TCGv inv1 = tcg_temp_new();
> +            TCGv t1 = tcg_temp_new();
>              tcg_gen_not_tl(inv1, arg1);
> -            tcg_gen_ext32u_tl(t0, arg2);
> -            tcg_gen_ext32u_tl(inv1, inv1);
>              if (add_ca) {
> -                tcg_gen_add_tl(t0, t0, cpu_ca);
> +                tcg_gen_add_tl(t0, arg2, cpu_ca);
>              } else {
> -                tcg_gen_addi_tl(t0, t0, 1);
> +                tcg_gen_addi_tl(t0, arg2, 1);
>              }
> +            tcg_gen_xor_tl(t1, arg2, inv1);         /* add without carry */
>              tcg_gen_add_tl(t0, t0, inv1);
> -            tcg_gen_shri_tl(cpu_ca, t0, 32);
> +            tcg_gen_xor_tl(cpu_ca, t0, t1);         /* bits changes w/ carry 
> */
> +            tcg_temp_free(t1);
> +            tcg_gen_shri_tl(cpu_ca, cpu_ca, 32);    /* extract bit 32 */
> +            tcg_gen_andi_tl(cpu_ca, cpu_ca, 1);
>          } else if (add_ca) {
>              TCGv zero, inv1 = tcg_temp_new();
>              tcg_gen_not_tl(inv1, arg1);

Reviewed-by: Aurelien Jarno <address@hidden>
Tested-by: Aurelien Jarno <address@hidden>

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