Re: [Qemu-devel] [Qemu-arm] [PATCH 6/7] target/arm: Simplify SMMLA, SMMLAR, SMMLS, SMMLSR

[Laurent Desnogues]

Hi Richard,

On Thu, Aug 8, 2019 at 10:28 PM Richard Henderson
<address@hidden> wrote:
>
> All of the inputs to these instructions are 32-bits.  Rather than
> extend each input to 64-bits and then extract the high 32-bits of
> the output, use tcg_gen_muls2_i32 and other 32-bit generator functions.
>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  target/arm/translate.c | 72 +++++++++++++++---------------------------
>  1 file changed, 26 insertions(+), 46 deletions(-)
>
> diff --git a/target/arm/translate.c b/target/arm/translate.c
> index ddc54e77e4..77154be743 100644
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -391,34 +391,6 @@ static void gen_revsh(TCGv_i32 var)
>      tcg_gen_ext16s_i32(var, var);
>  }
>
> -/* Return (b << 32) + a. Mark inputs as dead */
> -static TCGv_i64 gen_addq_msw(TCGv_i64 a, TCGv_i32 b)
> -{
> -    TCGv_i64 tmp64 = tcg_temp_new_i64();
> -
> -    tcg_gen_extu_i32_i64(tmp64, b);
> -    tcg_temp_free_i32(b);
> -    tcg_gen_shli_i64(tmp64, tmp64, 32);
> -    tcg_gen_add_i64(a, tmp64, a);
> -
> -    tcg_temp_free_i64(tmp64);
> -    return a;
> -}
> -
> -/* Return (b << 32) - a. Mark inputs as dead. */
> -static TCGv_i64 gen_subq_msw(TCGv_i64 a, TCGv_i32 b)
> -{
> -    TCGv_i64 tmp64 = tcg_temp_new_i64();
> -
> -    tcg_gen_extu_i32_i64(tmp64, b);
> -    tcg_temp_free_i32(b);
> -    tcg_gen_shli_i64(tmp64, tmp64, 32);
> -    tcg_gen_sub_i64(a, tmp64, a);
> -
> -    tcg_temp_free_i64(tmp64);
> -    return a;
> -}
> -
>  /* 32x32->64 multiply.  Marks inputs as dead.  */
>  static TCGv_i64 gen_mulu_i64_i32(TCGv_i32 a, TCGv_i32 b)
>  {
> @@ -8872,23 +8844,27 @@ static void disas_arm_insn(DisasContext *s, unsigned 
> int insn)
>                             (SMMUL, SMMLA, SMMLS) */
>                          tmp = load_reg(s, rm);
>                          tmp2 = load_reg(s, rs);
> -                        tmp64 = gen_muls_i64_i32(tmp, tmp2);
> +                        tcg_gen_muls2_i32(tmp2, tmp, tmp, tmp2);
>
>                          if (rd != 15) {
> -                            tmp = load_reg(s, rd);
> +                            tmp3 = load_reg(s, rd);
>                              if (insn & (1 << 6)) {
> -                                tmp64 = gen_subq_msw(tmp64, tmp);
> +                                tcg_gen_sub_i32(tmp, tmp, tmp3);

Shouldn't you subtract tmp from tmp3?

>                              } else {
> -                                tmp64 = gen_addq_msw(tmp64, tmp);
> +                                tcg_gen_add_i32(tmp, tmp, tmp3);
>                              }
> +                            tcg_temp_free_i32(tmp3);
>                          }
>                          if (insn & (1 << 5)) {
> -                            tcg_gen_addi_i64(tmp64, tmp64, 0x80000000u);
> +                            /*
> +                             * Adding 0x80000000 to the 64-bit quantity
> +                             * means that we have carry in to the high
> +                             * word when the low word has the high bit set.
> +                             */
> +                            tcg_gen_shri_i32(tmp2, tmp2, 31);
> +                            tcg_gen_add_i32(tmp, tmp, tmp2);
>                          }
> -                        tcg_gen_shri_i64(tmp64, tmp64, 32);
> -                        tmp = tcg_temp_new_i32();
> -                        tcg_gen_extrl_i64_i32(tmp, tmp64);
> -                        tcg_temp_free_i64(tmp64);
> +                        tcg_temp_free_i32(tmp2);
>                          store_reg(s, rn, tmp);
>                          break;
>                      case 0:
> @@ -10114,22 +10090,26 @@ static void disas_thumb2_insn(DisasContext *s, 
> uint32_t insn)
>                    }
>                  break;
>              case 5: case 6: /* 32 * 32 -> 32msb (SMMUL, SMMLA, SMMLS) */
> -                tmp64 = gen_muls_i64_i32(tmp, tmp2);
> +                tcg_gen_muls2_i32(tmp2, tmp, tmp, tmp2);
>                  if (rs != 15) {
> -                    tmp = load_reg(s, rs);
> +                    tmp3 = load_reg(s, rs);
>                      if (insn & (1 << 20)) {
> -                        tmp64 = gen_addq_msw(tmp64, tmp);
> +                        tcg_gen_add_i32(tmp, tmp, tmp3);
>                      } else {
> -                        tmp64 = gen_subq_msw(tmp64, tmp);
> +                        tcg_gen_sub_i32(tmp, tmp, tmp3);

Same here.

Also the way you do the computation means you don't propagate the
borrow from the lower 32-bit of the 64-bit product when doing the
subtraction.

Thanks,

Laurent

>                      }
> +                    tcg_temp_free_i32(tmp3);
>                  }
>                  if (insn & (1 << 4)) {
> -                    tcg_gen_addi_i64(tmp64, tmp64, 0x80000000u);
> +                    /*
> +                     * Adding 0x80000000 to the 64-bit quantity
> +                     * means that we have carry in to the high
> +                     * word when the low word has the high bit set.
> +                     */
> +                    tcg_gen_shri_i32(tmp2, tmp2, 31);
> +                    tcg_gen_add_i32(tmp, tmp, tmp2);
>                  }
> -                tcg_gen_shri_i64(tmp64, tmp64, 32);
> -                tmp = tcg_temp_new_i32();
> -                tcg_gen_extrl_i64_i32(tmp, tmp64);
> -                tcg_temp_free_i64(tmp64);
> +                tcg_temp_free_i32(tmp2);
>                  break;
>              case 7: /* Unsigned sum of absolute differences.  */
>                  gen_helper_usad8(tmp, tmp, tmp2);
> --
> 2.17.1
>
>