[LLVMdev] Proposal: intp type

[Talin]

I realize that most users of LLVM aren't affected by this, because most
frontends aren't target-neutral, and thus know in advance how big a pointer
is. At least, that's my impression.

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.

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.)

On Tue, Nov 10, 2009 at 8:24 AM, Kenneth Uildriks <kennethuil at gmail.com>wrote:

> On Tue, Nov 10, 2009 at 8:10 AM, me22 <me22.ca at gmail.com> wrote:
> > 2009/11/9 Kenneth Uildriks <kennethuil at gmail.com>:
> >>
> >> 1. Conversions to/from other integer types: right now, integer type
> >> conversions are always explicity specified as either a trunc, a sext,
> >> or a zext.  Since the size of intp is not known at IR generation time,
> >> you can't know whether a conversion to/from intp truncates or extends.
> >>
> >
> > Now that there are arbitrary-sized integers, couldn't you zext to i256
> > then trunc down again, and later let the folder simplify as
> > appropriate?
>
> I suppose that would work, but I wouldn't like to see two cast
> instructions for every conversion.
>
> Perhaps every conversion to/from intp could be represented as a zext,
> whether or not it actually performs an extension.  Is there anything
> in LLVM that depends on a zext actually increasing the size of the
> integer?
>
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-- 
-- Talin
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