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

Fri Sep 21 13:01:39 PDT 2012

Here is an updated patch which moves TargetData out of Target and into Support/VMCore so that there is no circular dependencies.

> -----Original Message-----
> From: Hal Finkel [mailto:hfinkel at anl.gov]
> Sent: Thursday, September 20, 2012 4:02 PM
> To: Eli Friedman
> Cc: Villmow, Micah; 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
> 
> On Thu, 20 Sep 2012 15:34:52 -0700
> Eli Friedman <eli.friedman at gmail.com> wrote:
> 
> > On Thu, Sep 20, 2012 at 3:30 PM, Villmow, Micah
> > <Micah.Villmow at amd.com> wrote:
> > > 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.
> >
> > And that's why I didn't like this approach in the first place.
> 
> I don't think that TargetData belongs in Target. TargetData represents
> target information that can be known without linking to the target
> descriptions, and thus needs to be available outside of Target.
> Furthermore, TargetData has no dependencies to the rest of Target (as
> far as I can tell). Let's move it into VMCore.
> 
>  -Hal
> 
> >
> > -Eli
> >
> > >> -----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
> > >> >
> > >> >
> > >> > _______________________________________________
> > >> > LLVM Developers mailing list
> > >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > >> >
> > >
> > >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 
> 
> --
> Hal Finkel
> Postdoctoral Appointee
> Leadership Computing Facility
> Argonne National Laboratory

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