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Date: Thu, 20 Jun 2013 09:46:24 -0700
From: Richard Henderson <rth@twiddle.net>
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Subject: Re: [Qemu-devel] [PATCH] int128: optimize
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: peter.maydell@linaro.org, qemu-devel@nongnu.org

On 06/20/2013 08:00 AM, Paolo Bonzini wrote:
>  static inline Int128 int128_sub(Int128 a, Int128 b)
>  {
> -    return int128_add(a, int128_neg(b));
> +    uint64_t lo = a.lo - b.lo;
> +    return (Int128) { lo, (lo < a.lo) + a.hi - b.hi };

This one isn't right.  Consider { 2, 0 } - { 2, 0 }

  lo = 2 - 2 = 0;
  = { 0, (0 < 2) + 0 - 0 }
  = { 0, 1 }

I'd be happier with a more traditional

  (Int128){ a.lo - b.lo, a.hi - b.hi - (a.lo < b.lo) };


r~