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Date: Fri, 09 Jan 2015 11:35:00 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] x86_64: optimise muldiv64 for x86_64
	architecture
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To: Frediano Ziglio <freddy77@gmail.com>, Anthony Liguori <aliguori@amazon.com>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Frediano Ziglio <frediano.ziglio@huawei.com>, qemu-devel@nongnu.org



On 09/01/2015 11:27, Frediano Ziglio wrote:
> 
> 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
>  

Good idea.  However, if you have __int128, you can just do

   return (__int128)a * b / c

and the compiler should generate the right code.  Conveniently, there is
already CONFIG_INT128 that you can use.

Paolo