Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64 architecture

[Richard Henderson <rth@twiddle.net>]

On 01/09/2015 01:53 AM, Frediano Ziglio wrote:
> As this platform can do multiply/divide using 128 bit precision use
> these instruction to implement it.
> 
> Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
> ---
>  include/qemu-common.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index f862214..5366220 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -370,6 +370,7 @@ static inline uint8_t from_bcd(uint8_t val)
>  }
>  
>  /* compute with 96 bit intermediate result: (a*b)/c */
> +#ifndef __x86_64__
>  static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>  {
>      union {
> @@ -392,6 +393,18 @@ static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
>      res.l.low = (((rh % c) << 32) + (rl & 0xffffffff)) / c;
>      return res.ll;
>  }
> +#else
> +static inline uint64_t muldiv64(uint64_t a, uint32_t b, uint32_t c)
> +{
> +    uint64_t res;
> +
> +    asm ("mulq %2\n\tdivq %3"
> +         : "=a"(res)
> +         : "a"(a), "qm"((uint64_t) b), "qm"((uint64_t)c)
> +         : "rdx", "cc");
> +    return res;
> +}
> +#endif

Honestly, this ought to move into qemu/host-utils.h, and it should
use __int128 for targets that support it.  Which includes x86_64,
but also other 64-bit hosts.


r~