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Re: [Qemu-devel] [PATCH] int128: optimize
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From: |
Richard Henderson |
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Subject: |
Re: [Qemu-devel] [PATCH] int128: optimize |
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Date: |
Thu, 20 Jun 2013 09:46:24 -0700 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130514 Thunderbird/17.0.6 |
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~