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Date: Thu, 22 Aug 2013 11:48:48 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 1/3] int128: add int128_exts64()
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>, Alex Williamson <alex.williamson@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

Il 22/08/2013 11:47, Peter Maydell ha scritto:
> On 22 August 2013 10:09, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> Il 22/08/2013 10:20, Alexey Kardashevskiy ha scritto:
>>> +static inline Int128 int128_exts64(int64_t a)
>>> +{
>>> +    return (Int128) { .lo = a, .hi = (a >> 63) ? -1 : 0 };
>>> +}
>>
>> The "? -1 : 0" is not necessary, but the compiler will remove it at -O1
>> or more (interestingly, or -O0 it will remove the shift and leave the
>> conditional!).
> 
> We can avoid relying on implementation defined
> behaviour here by using
>   .hi = (a < 0) ? -1 : 0;
> 
> (I know we allow ourselves to assume right-shift of signed
> ints is arithmetic shift, but I think it's nicer to avoid it unless
> it really makes the code better.)

This is what Alexey proposed.  I suggested (a >> 63) without the ?: but
he misunderstood my (probably not clear enough) suggestion.

Paolo