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- Subject: Re: undefined behaviour in lua source
- From: Daurnimator <quae@...>
- Date: Tue, 21 Jun 2016 15:41:36 +1000
On 21 June 2016 at 02:30, Roberto Ierusalimschy <firstname.lastname@example.org> wrote:
> Last time I checked, in all machines that implement the IEEE standard
> (that seems to be all machines), a float division by zero was a valid
> operation, which may result in -inf, inf, or NaN. We try to follow
> the IEEE standard as much as possible (in other words, we try to not
> interfere with it)
The issue is that it relies on undefined behaviour in C. This means
the compiler can do *anything*.
> If there was a test for (b == 0), what should it do
> in a positive case?
1/0 returns inf
-1/0 returns -inf
1/-0 returns -inf
-1/-0 returns inf
0/0 returns nan
To get the above desited behaviour, I think we end up with something like:
#define luai_numdiv(L,a,b) ((void)L,
However, this doesn't seem to get optimized away by compilers as I
Perhaps someone else can weigh in here?
Without the various standard libary functions introduced in C99 (such
as signbit(), copysign(), nan(), etc) it's trickier too.