[LLVMdev] Proposal: intp type

[Kenneth Uildriks]

On Wed, Nov 11, 2009 at 2:10 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> Kenneth Uildriks wrote:
>>
>> On Tue, Nov 10, 2009 at 6:10 PM, Talin <viridia at gmail.com> wrote:
>>>
>>> In my case, I've been attempting to build a target-neutral frontend. In
>>> my
>>> tool chain, the target is specified at link time, not at compile time.
>>> Among
>>> other things, that means that the same IR file can be used for multiple
>>> targets.
>>
>> That's the direction I'm going in too.
>>
>>> What strikes me is how tantalizingly close LLVM comes to being able to do
>>> this. I am surprised, for example, that I can general all of the DWARF
>>> debugging structures without ever having to choose a target machine. Most
>>> things can be done quite easily without knowing the exact size of a
>>> pointer.
>>> When it comes to being able to "generate once, run anywhere", LLVM is
>>> like
>>> 99.5% of the way there. Which makes that last remaining .5% particularly
>>> vexing.
>>>
>>> There's only a tiny handful of fairly esoteric cases which require
>>> selecting
>>> a target before you generate IR. Unfortunately, the "pointer the same
>>> size
>>> as an int" is one of these rare cases -  it is something that is very
>>> painful to try and work around. (A similar esoteric use case is: "which
>>> of
>>> the following two types is larger, 3 x int32 or 2 x {}*? -- i.e. the
>>> union
>>> problem.)
>>
>> I'm willing to spend some time on adding intp to LLVM... my
>> front-end's standard libraries would be cleaner and more portable that
>> way.
>
> Sorry, but I'm still opposed. From your description of 'intp' it sounds like
> it's a strict subset of pointers. You can't sext it, zext it or trunc it,
> like you can with integers. You can bitcast it, but only to another pointer.

You can do integer arithmetic & bitwise operations with it.  You can
convert it to other types of integers, although you wouldn't be able
to tell whether you were truncating or zexting them at IR-generation
time (at least not without target data).  You can create literal
values of intp type.  You can, of course, safely convert it to/from a
pointer.

intp is an integer, not a pointer.  It's sized the same as a pointer,
so you can use it as a pointer offset, a size parameter, or something
along those lines, without having to know how big a pointer.

>
> The use case you mentioned was that some native system APIs want integers
> that are the same size as pointers. So why not just declare those arguments
> or fields with a pointer type in LLVM? Then you've got a field with the
> right size.

But not necessarily the right alignment.  Some platforms align
pointers differently from ints.