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

[Paolo Bonzini]

On 09/01/2015 12:25, Frediano Ziglio wrote:
>  /* 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 */
> +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;
> +}

Reorder it the other way, and you can simplify the first #if.

I applied patch 1 locally, and will send a pull request once the tree is
thawed.

Paolo