Re: [Qemu-devel] [PATCH] target/arm: Fix SMMLS argument order

[Laurent Desnogues]

Hi,

On Thu, Aug 29, 2019 at 3:33 AM Richard Henderson
<address@hidden> wrote:
>
> The previous simplification got the order of operands to the
> subtraction wrong.  Since the 64-bit product is the subtrahend,
> we must use a 64-bit subtract to properly compute the borrow
> from the low-part of the product.
>
> Fixes: 5f8cd06ebcf5 ("target/arm: Simplify SMMLA, SMMLAR, SMMLS, SMMLSR")
> Reported-by: Laurent Desnogues <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>

Tested-by: Laurent Desnogues <address@hidden>

Thanks,

Laurent

> ---
>  target/arm/translate.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/target/arm/translate.c b/target/arm/translate.c
> index cbe19b7a62..a0f7577f47 100644
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -8824,7 +8824,16 @@ static void disas_arm_insn(DisasContext *s, unsigned 
> int insn)
>                          if (rd != 15) {
>                              tmp3 = load_reg(s, rd);
>                              if (insn & (1 << 6)) {
> -                                tcg_gen_sub_i32(tmp, tmp, tmp3);
> +                                /*
> +                                 * For SMMLS, we need a 64-bit subtract.
> +                                 * Borrow caused by a non-zero multiplicand
> +                                 * lowpart, and the correct result lowpart
> +                                 * for rounding.
> +                                 */
> +                                TCGv_i32 zero = tcg_const_i32(0);
> +                                tcg_gen_sub2_i32(tmp2, tmp, zero, tmp3,
> +                                                 tmp2, tmp);
> +                                tcg_temp_free_i32(zero);
>                              } else {
>                                  tcg_gen_add_i32(tmp, tmp, tmp3);
>                              }
> @@ -10068,7 +10077,14 @@ static void disas_thumb2_insn(DisasContext *s, 
> uint32_t insn)
>                      if (insn & (1 << 20)) {
>                          tcg_gen_add_i32(tmp, tmp, tmp3);
>                      } else {
> -                        tcg_gen_sub_i32(tmp, tmp, tmp3);
> +                        /*
> +                         * For SMMLS, we need a 64-bit subtract.
> +                         * Borrow caused by a non-zero multiplicand lowpart,
> +                         * and the correct result lowpart for rounding.
> +                         */
> +                        TCGv_i32 zero = tcg_const_i32(0);
> +                        tcg_gen_sub2_i32(tmp2, tmp, zero, tmp3, tmp2, tmp);
> +                        tcg_temp_free_i32(zero);
>                      }
>                      tcg_temp_free_i32(tmp3);
>                  }
> --
> 2.17.1
>