Re: [Qemu-devel] [PATCH v2 1/3] int128: add int128_exts64()

[Peter Maydell]

On 22 August 2013 10:09, Paolo Bonzini <address@hidden> 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.)

-- PMM