Re: [Patch]: Uncouple size_t and sizetype

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, Mar 16, 2012 at 12:33 PM, Tristan Gingold <gingold@adacore.com> wrote:
>
> On Mar 16, 2012, at 12:02 PM, Richard Guenther wrote:
>
>> On Fri, Mar 16, 2012 at 11:39 AM, Tristan Gingold <gingold@adacore.com> wrote:
>>> Hi,
>>>
>>> currently sizetype precision (cf store-layout.c:initialize_sizetypes) is the same as size_t.
>>> This is an issue on VMS, where size_t is 'unsigned int', but we'd like to have a 64 bit sizetype
>>> for Ada. ?My understanding is that ISO-C doesn't require size_t precision to match the one of
>>> void *.
>>>
>>> We can't really lie about size_t because it is exposed in API (such as writev).
>>>
>>> I don't see any reason (other than historic one) to have an exact match between sizetype and size_t.
>>> So this patch adds an hook to allow targets to define sizetype.
>>
>> Well, there is at least "common sense" that couples size_t and sizetype.
>> As you can at most allocate size_t memory via malloc (due to its size_t
>> use for the size) sizes larger than what fits into size_t do not make much
>> sense. ?Thus, a sizetype larger than size_t does not make much sense.
>
> Agreed, but malloc() is not the only way to get memory. ?At least on VMS, there are
> some syscalls to allocate memory with a 64 bit length argument.
>
>> The middle-end of course would not care much what you use for sizetype.
>> But be warned - if the mode for sizetype is different of ptr_mode things
>> are going to be interesting for you (yes, ptr_mode, not Pmode).
>
> That's the issue. ?POINTER_SIZE is 64 bits (when -mpointer-size=64) but size_t should always be 32 bit.

Ok.

>>> I initially thought about using Pmode precision for sizetype precision, but there are a few machines
>>> (m32c, sh, h8300) where the precisions aren't the same. ?I don't know wether this is on purpose or
>>> unintentional.
>>
>> At least for m32c it is IIRC because 24bit computations are soo expensive
>> on that target, so HImode is chosen for sizetype.
>
> That's a good reason!
>
>> So - why do you need a 64bit sizetype again? ;)
>>
>> Can it be that you don't really need 64bit sizes but you hit issues with
>> sizetype != ptr_mode size?
>
> I don't have an urgent need for 64bit sizes (although would be nice to have them).
>
> I remember that the first build with sizetype=32 but ptr_mode =DImode was a failure.
> Maybe I should first investigate this path, as m32c could use "unsigned int" (16 bits)
> for size_type alongside 32 for POINTER_SIZE ?

Well, this setup is not well supported by the middle-end (and indeed m32c
has existing issues with that).  So in your case decoupling sizetype from
size_t sounds like the more appropriate solution.

>> Btw, while we are transitioning to target hooks in this case I'd prefer
>> a target macro alongside the existing SIZE_TYPE, etc. ones.
>
> Ok.

I'd choose SIZETYPE (for confusion, heh), defaulting to SIZE_TYPE.

Richard.

> Tristan.
>