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- Subject: Re: [ANN] Lua 5.2.0 (work1) now available
- From: Vaughan McAlley <ockegheim@...>
- Date: Tue, 19 Jan 2010 11:32:59 +1100
2010/1/19 Leo Razoumov <email@example.com>:
> On 2010-01-18, Roberto Ierusalimschy <firstname.lastname@example.org> wrote:
>> > > You do not present any arguments. Actual use cases show that there
>> > > is a need. Existing bit libraries offer this functionality for
>> > > that reason, too.
>> > I agree with you on this. Arithmetic right shift is a primitive
>> > operation and I couldn't live without it.
>> Just a remind: at least we would be in good company, as ANSI C does not
>> offer this operation too :)
>> ISO/IEC 9899:1999 (E)
>> The result 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 nonnegative
>> value, the value of the result is the integral part of the quotient of
>> E1 / 2E2. If E1 has a signed type and a negative value, the resulting
>> value is implementation-defined.
>> -- Roberto
> I think there is a good reason ANSI C does not mess up with right
> shifting of *negative* values. In two's complement system being
> negative is a matter of interpretation. 0xFFFFFFFF and -1 have the
> same bit-pattern just two different interpretations. Actually, it is
> easier to restrict all bit operations to *unsigned* types so that
> issues of "sign extension" never come up. In this case, the functions
> that take results of bit operations as their arguments should be
> properly declared as
> foo(unsigned int n) instead of foo(int n). Taking FP apart can also be
> done treating bit patterns as unsigned.
> Bits were all born equal and at birth they all were unsigned:-)
Yes, shifting a sign bit can turn it into a value bit and vice-versa.
Bit operations seem to suit positive numbers better, and the C89
standard reflects that.
The standard Lua bit library should only attempt operations that are
predictable and portable over many platforms. It should be happy with
catering to a proportion (80%?) of users’ needs. People requiring
fancy or platform-specific operations can use a library or roll their
own. Roberto et al won’t be offended.