Re: [Qemu-devel] [PATCH 2/2] qemu-common.h: optimise muldiv64 for x86_64 architecture

[Peter Maydell]

On 9 January 2015 at 11:25, Frediano Ziglio <address@hidden> wrote:
> As this platform can do multiply/divide using 128 bit precision use
> these instructions to implement it.
>
> Signed-off-by: Frediano Ziglio <address@hidden>
> ---
>  include/qemu-common.h | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index f3033ae..880659d 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -370,11 +370,23 @@ static inline uint8_t from_bcd(uint8_t val)
>  }
>
>  /* compute with 96 bit intermediate result: (a*b)/c */
> -#ifdef CONFIG_INT128
> +#if defined(CONFIG_INT128) && !defined(__x86_64__)
>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>  {
>      return (__int128)a * b / c;
>  }
> +#elif defined(__x86_64__)
> +/* Optimised x64 version. This assume that a*b/c fits in 64 bit */

This assumption isn't necessarily true, and this implementation
will dump core on overflow. For instance:

Inputs: a = 8000000000000000 b = 80000000 c = 1
fn muldiv64 result 0
fn muldiv64_with_int128 result 0
fn muldiv64_with_uint128 result 0
Floating point exception (core dumped)

-- PMM