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Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
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Date: Sun, 02 Jun 2013 16:36:58 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 10/21] memory: make section size a 128-bit
	integer
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

Il 02/06/2013 16:18, Peter Maydell ha scritto:
> On 31 May 2013 23:18, Richard Henderson <rth@twiddle.net> wrote:
>> On 05/30/2013 02:16 PM, Paolo Bonzini wrote:
>>> +static inline Int128 int128_rshift(Int128 a, int n)
>>> +{
>>> +    return (Int128) { (a.lo >> n) | (a.hi << (64 - n)), (a.hi >> n) };
>>> +}
>>
>> Produces wrong results for n == 0, since (a.hi << 64) is undefined.
> 
> It produces wrong results for shifts by more than 64,
> for that matter.

This should work:

    int64_t h;
    if (!n) {
        return a;
    }
    h = a.hi >> n;
    if (n >= 64) {
        return (Int128) { h, h >> 63 };
    } else {
       return (Int128) { (a.lo >> n) | (a.hi << (64 - n)), h };
    }

Paolo