[LLVMdev] Proposal: New IR instruction for casting between address spaces

[Villmow, Micah]

> -----Original Message-----
> From: Chris Lattner [mailto:clattner at apple.com]
> Sent: Thursday, September 13, 2012 11:53 PM
> To: Mon Ping Wang
> Cc: Villmow, Micah; llvmdev at cs.uiuc.edu Mailing List
> Subject: Re: [LLVMdev] Proposal: New IR instruction for casting between
> address spaces
> 
> 
> On Sep 13, 2012, at 5:55 PM, Mon Ping Wang <monping at apple.com> wrote:
> >>> The pointer size is target dependent so it seems strange to choose
> an arbitrary size to convert to and from. Are you making a practical
> argument that 64b is sufficient on all machines so all targets can use
> that?  In other words, pointers > 64 doesn't make any sense in terms of
> the address space? (A pointer to be > 64 if clients want to use some
> upper bits to track some state I guess).
> >>>
> >>> In terms of the three new instructions, one could argue that
> ptrtoint and intoptr has the same issue or those can also explode in a
> similar way.  To use them, this seems target dependent so unless we
> really want to support all the various addressing structures, I rather
> not have them.
> >>
> >> My point is that any producer of this sort of pointer cast is
> already necessarily target specific (it is generating target-specific
> address space numbers!).  If the front-end knows the address space to
> use, it can know a safe integer size to use.
> >>
> >
> > It depends on what the address space is used for. If I'm logically
> partitioning an address space that overlap my pointer size may all be
> the same size so this issue doesn't come up other than I know the
> pointer size are the same.
> 
> Sure, in that case, use bitcast.
> 
> > My understanding is that is becoming an issue since a pointer type
> size could be different for different address space.  I agree for the
> case where the pointer size is address space dependent that the client
> has to understand the size and the properties to decide if they need to
> do truncation, sign extension or zero extensions.
> 
> Right.
> 
> > This is a problem for auto upgrade as well.  Today, we have bit cast
> between same size pointers for different address space.  We would need
> to do something special for auto upgrade here since the proposal is to
> not allow bit cast between pointers of different address spaces.
> 
> I haven't followed the details of the proposal, but I think it makes
> perfect sense to continue using bitcast for ptr/ptr casts within the
> same pointer size.  If you do that, then there is no auto-upgrade
> issue: all existing bc files can just be assumed to have the same
> pointer size.
[Villmow, Micah] So basically we don't need a new IR instructions, but instead
1) bitcasts between pointers of different size is illegal, the proper approach is inttoptr/ptrtoint.
2) bitcasts between pointers of the same size stays legal.
3) No new IR instruction is needed, as converting between pointers of different sizes requires inttoptr/ptrtoint.

The only issues are then to update the verifier to assert on bitcasts between pointers of different sizes and add in auto-upgrade of binaries to switch to inttoptr/ptrtoint. By doing this, I then can clear the way for allowing LLVM to support multiple pointer sizes.

Sound good?

Micah
> 
> -Chris