Oh Dear

[Walter Bright]

Michiel Helvensteijn wrote:
> Walter Bright wrote:
> 
>>> In other words, every bug in DMD is really a feature of the language.
>>> Until you fix it. That kind of permanent instability may be part of the
>>> reason people have been so negative around here lately.
>> I don't agree with that characterization. Bugs get reported to bugzilla,
>> and get fixed in a regular cycle.
> 
> Sure they do. But if it's really true that the DMD implementation *is* the D
> language specification, any bug in DMD would also be part of that
> specification. And as the compiler changes, so does the spec. It's the
> danger of that approach.

Looking at the kinds of issues reported in bugzilla makes it pretty 
plain that D users well understand the difference between a bug and a 
problem with the specification. I don't believe there is any danger 
there. Nobody is going to mistake a compiler crash, for example, as 
specified behavior.


> 
>>> I will file those corrections.
>> Thank you, I look forward to it.
> 
> Issue 3165, Issue 3166.

Thank you.

>>> By the way, what kind of integer division *does* D use?
>> To be frank, it is what the x86 DIV instruction does.
> After a quick Google search, it would seem DIV is the x86 "unsigned divide"
> instruction. Are you sure you aren't using IDIV ("signed divide") for
> negative operands? If not, you'd actually be interpreting 2's complement
> signed ints as unsigned ints. (And surely such a thing would have been
> noticed before now?)

If there's any question about it, you can do a quick test and check what 
the compiler generates with obj2asm. If it wasn't doing things 
correctly, you bet people would have noticed by now. A divide that 
doesn't work is going to be very, very obvious.


>> But I agree that 
>> the modulus should be defined, regardless of whether that makes it less
>> efficient on some machines.
> 
> Good. Make sure that your divide operation and your modulo operation are
> consistent. The following identity should hold:
> 
> if  a / n = q
> and a % n = r
> 
> a = q*n + r