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

Thu Sep 20 15:30:47 PDT 2012

If I don't bring in TargetData, then there is no way for me to verify the address space size in the verifier or in the auto-upgrade mechanisms. 

> -----Original Message-----
> From: Eli Friedman [mailto:eli.friedman at gmail.com]
> Sent: Thursday, September 20, 2012 2:32 PM
> To: Villmow, Micah
> Cc: Chris Lattner; Mon Ping Wang; llvm-commits at cs.uiuc.edu;
> llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Proposal: New IR instruction for casting between
> address spaces
> 
> We can't add a circular dependency between Target and VMCore.
> 
> -Eli
> 
> On Thu, Sep 20, 2012 at 8:21 AM, Villmow, Micah <Micah.Villmow at amd.com>
> wrote:
> > Ping!
> >
> >> -----Original Message-----
> >> From: Villmow, Micah
> >> Sent: Tuesday, September 18, 2012 4:12 PM
> >> To: 'Chris Lattner'; 'Mon Ping Wang'
> >> Cc: 'llvm-commits at cs.uiuc.edu'; 'llvmdev at cs.uiuc.edu'
> >> Subject: RE: [LLVMdev] Proposal: New IR instruction for casting
> >> between address spaces
> >>
> >> Resending since I got an error.
> >>
> >> > -----Original Message-----
> >> > From: Villmow, Micah
> >> > Sent: Tuesday, September 18, 2012 4:04 PM
> >> > To: Villmow, Micah; 'Chris Lattner'; 'Mon Ping Wang'
> >> > Cc: 'llvm-commits at cs.uiuc.edu'; 'llvmdev at cs.uiuc.edu'
> >> > Subject: RE: [LLVMdev] Proposal: New IR instruction for casting
> >> > between address spaces
> >> >
> >> > Added a new patch after some feedback. Also make sure all of the
> >> > tools/examples build correctly.
> >> >
> >> > > -----Original Message-----
> >> > > From: Villmow, Micah
> >> > > Sent: Tuesday, September 18, 2012 11:24 AM
> >> > > To: Villmow, Micah; Chris Lattner; Mon Ping Wang
> >> > > Cc: llvm-commits at cs.uiuc.edu; llvmdev at cs.uiuc.edu
> >> > > Subject: RE: [LLVMdev] Proposal: New IR instruction for casting
> >> > > between address spaces
> >> > >
> >> > > Here is the patch that i've developed that implements the below
> >> > points.
> >> > > The test itself won't work until the target data changes are
> added.
> >> > >
> >> > > > -----Original Message-----
> >> > > > From: llvmdev-bounces at cs.uiuc.edu
> >> > > > [mailto:llvmdev-bounces at cs.uiuc.edu]
> >> > > > On Behalf Of Villmow, Micah
> >> > > > Sent: Friday, September 14, 2012 8:16 AM
> >> > > > To: Chris Lattner; Mon Ping Wang
> >> > > > Cc: llvmdev at cs.uiuc.edu Mailing List
> >> > > > Subject: Re: [LLVMdev] Proposal: New IR instruction for casting
> >> > > > between address spaces
> >> > > >
> >> > > >
> >> > > >
> >> > > > > -----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
> >> > > >
> >> > > >
> >> > > >
> >> > > > _______________________________________________
> >> > > > LLVM Developers mailing list
> >> > > > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
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