On Fri, 25 Nov 2016, Torbjörn Granlund wrote:
Marc Glisse <marc.gli...@inria.fr> writes:
Fixed. It was unhappy about (-13) << 2. I am a bit surprised it
doesn't complain about (-13) >> 2 on the next line, we'll see if it
ever becomes an issue.
This must be a compiler problem. At least in C, shifting negative
integers is undefined. It is a pity that it is, but the standard say
so.
I am less sure about C++. Some googling did not provide a definitive
answer.
But if left shift of negative integers is undefined, surely right shift
is too!
Surprisingly, shifting negative numbers left is undefined, while
shifting them right it implementation-defined.
The value of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are
zero-filled. If E1 has an unsigned type, the value of the result is E1 × 2^E2 ,
reduced modulo one more than the maximum value representable in the result type.
Otherwise, if E1 has a signed type and non-negative value, and E1 × 2^E2 is
representable in the corresponding unsigned type of the result type, then that value,
converted to the result type, is the resulting value; otherwise, the behavior is
undefined.
The value of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an
unsigned type or if E1 has a signed type and a non-negative value, the value of the
result is the integral part of the quotient of E1/2^E2 . If E1 has a signed type and
a negative value, the resulting value is implementation-defined.
--
Marc Glisse
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