size_t + ptrdiff_t

[Manu]

On 19 February 2012 20:07, Timon Gehr <timon.gehr at gmx.ch> wrote:

> On 02/19/2012 03:59 PM, Manu wrote:
>
>> Okay, so it came up a couple of times, but the questions is, what are we
>> going to do about it?
>>
>> size_t and ptrdiff_t are incomplete, and represent non-complimentary
>> signed/unsigned halves of the requirement.
>> There are TWO types needed, register size, and pointer size. Currently,
>> these are assumed to be the same, which is a false assumption.
>>
>> I propose size_t + ssize_t should both exist, and represent the native
>> integer size. Also something like ptr_t, and ptrdiff_t should also
>> exist, and represent the size of the pointer.
>>
>> Personally, I don't like the _t notation at all. It doesn't fit the rest
>> of the D types, but it's established, so I don't expect it can change.
>> But we do need the 2 missing types.
>>
>> There is also the problem that there is lots of code written using the
>> incorrect types. Some time needs to be taken to correct phobos too I
>> guess.
>>
>
> Currently, size_t is defined to be what you call ptr_t, ptrdiff_t is
> present, and what you call size_t/ssize_t does not exist. Under which
> circumstances is it important to have a distinct type that denotes the
> register size? What kind of code requires such a type? It is unportable.
>

It is just as unportable as size_t its self. The reason you need it is to
improve portability, otherwise people need to create arbitrary version
mess, which will inevitably be incorrect.
Anything from calling convention code, structure layout/packing, copying
memory, basically optimising for 64bits at all... I can imagine static
branches on the width of that type to select different paths.
Even just basic efficiency, using 32bit ints on many 64bit machines require
extra sign-extend opcodes after every single load... total waste of cpu
time.

Currently, if you're running a 64bit system with 32bit pointers, there is
absolutely nothing that exists at compile time to tell you you're running a
64bit system, or to declare a variable of the machines native type, which
you're crazy if you say is not important information. What's the point of a
64bit machine, if you treat it exactly like a 32bit machine in every aspect?
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