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

Villmow, Micah
Wed, 12 Sep 2012 21:45:27 -0000

> -----Original Message-----
> From: Mon P Wang [mailto:monping at apple.com]
> Sent: Wednesday, September 12, 2012 1:12 PM
> To: Villmow, Micah
> Cc: Dan Gohman; llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Proposal: New IR instruction for casting between
> address spaces
>
> Hi,
>
> On Sep 11, 2012, at 2:30 PM, Villmow, Micah wrote:
>
> >
> >
> >> -----Original Message-----
> >> From: Dan Gohman [mailto:gohman at apple.com]
> >> Sent: Tuesday, September 11, 2012 1:28 PM
> >> To: Villmow, Micah
> >> Cc: llvmdev at cs.uiuc.edu
> >> Subject: Re: [LLVMdev] Proposal: New IR instruction for casting
> between
> >> address spaces
> >>
> >> On Sep 11, 2012, at 1:03 PM, "Villmow, Micah"
> <Micah.Villmow at amd.com>
> >> wrote:
> >>
> >>>
> >>> From: Villmow, Micah
> >>> Sent: Tuesday, September 11, 2012 12:51 PM
> >>> To: llvm-commits at cs.uiuc.edu
> >>> Subject: Proposal: New IR instruction for casting between
address
> >>> spaces
> >>>
> >>> Problem:
> >>> Bit casting between pointers of different address spaces only
works
> if
> >> all address space pointers are the same size. With changes from
> email
> >> chain [1][2], support for different pointer sizes breaks the
bitcast
> >> instruction since there is no guarantee that the pointer size for
> the
> >> address space is on the source and destination arguments are of
the
> same
> >> size.
> >>
> >> Can you comment on whether the need for this seems like a
> fundamental
> >> need, in your field, or more of a limitation of the current
> generation
> >> of architectures?
> > [Villmow, Micah] I think this is a little of both. While the current
> and previous generation of GPU architectures are limited in what they
> are capable of doing based on hardware restrictions, I also think there
> is a fundamental need. Not all devices will run with 64bit operations
> at full speed(or even have native instructions in any case), but memory
> sizes will soon eclipse what is addressable with 32bit pointers on non-
> PC systems. What this is causing is 32bit systems requiring addressing
> into 64bit memory and switching over to 64bit for address calculations
> destroys the performance advantage that the segmented memory provides.
> > In the CPU world, this isn't that much of a problem that I can
tell
> as they have already been solved(32bit vs 64bit math is 1-1 in most
> cases), but in non-CPU architectures, this is a huge performance
> penalty(64bit mul runs 6x slower than 32bit mul). So being able to
> switch to 32bit in the cases where it is required and switch to 64bit
> where it is required is a fundamental need that I don't think will go
> away even if the architectures improve their memory infrastructure.
> >>
> >>> Solution:
> >>> Remove the ability of bitcast to cast between pointers of
different
> >> address spaces and replace with an instruction that handles this
> case
> >> explicitely.
> >>>
> >>> Proposed changes:
> >>> *         Add restriction to the verifier on the bitcast
> instruction
> >> making bitcasting between address spaces illegal.
> >>> *         Change documentation[3] to state the bitcast to
pointers
> of
> >> different address spaces is illegal.
> >>> *         Add in a new IR node, addrspacecast, that allows
> conversions
> >> between address spaces
> >>> *         Updated the reader/writer to handle these cases
> >>> *         Update the documentation to insert the new IR node.
> >>> *         Add the following documentation:
> >>> 'addrspacecast .. to' Instruction
> >>>
> >>> Syntax:
> >>>
> >>>  <result> = addrspacecast <ty> <value> to
<ty2>             ;
> yields
> >> ty2
> >>> Overview:
> >>>
> >>> The ' addrspacecast ' instruction converts value to
type ty2
> without
> >> changing any bits.
> >>
> >> This is mildly imprecise, because the whole point of this
> instruction is
> >> that it can change the bit width.
> > [Villmow, Micah] Doh, cut and paste error, will fix it.
> >>
> >>>
> >>> Arguments:
> >>>
> >>> The ' addrspacecast ' instruction takes a value to
cast, which must
> be
> >> a non-aggregate first class value with a pointer type, and a type
to
> >> cast it to, which must also be a pointer type. The pointer types
of
> >> value and the destination type, ty2, must be identical, except for
> the
> >> address space.
> >>
> >> Having a "pointer type" is sufficient to imply that it
is a "non-
> >> aggregate first class value".
> >>
> >>>
> >>> Semantics:
> >>>
> >>> The ' addrspacecast ' instruction converts value to
type ty2. It
> >> converts the type to the type that is implied by the address space
> of
> >> the destination pointer. If the destination pointer is smaller
than
> the
> >> source pointer, the upper bits are truncated. If the inverse is
> true,
> >> the upper bits are sign extended, otherwise the operation is a no-
> op.
> >>
> >> Why sign-extended? Ptrtoint/inttoptr are zero-extended, and
it's
> >> surprising that addrspacecast would be different from them.
> > [Villmow, Micah] Take for example a pointer representing a negative
> pointer offset into a 16 bit address space, if this is converted to a
> 64bit address space, the upper 48 bits would be zero and your negative
> offset just became positive. The difference between these two
> instruction types is that addrspacecast does not explicitly convert to
> any size, only implicitly, so the bits would need to be filled
> correctly.
> >>
>
> I view a pointer as pointing to a location in memory and not as an
> offset relative to some base register.  I think the proper semantic
> here is the same as inttoptr where it does a zero-extension.
[Villmow, Micah] Yeah, but the pointer won't point to the same location if
the conversion from a smaller pointer to a larger pointer is zero extended.
Take two address spaces(1 and 2) that are 16 and 64 bits in size.
int(1) *a = 0xFFFFFFF9;
int(2) *b = *a;
Is b -10(SExt), or is it 4294967289(ZExt)?
This works for inttoptr and ptrtoint because there is an assumption that the
pointer is always the same size. Maybe we even need to extend ptrtoint/inttoptr
to handle this case by adding unsigned versions?

>
>   -- Mon Ping
>
>
>
>
> >> Dan
> >>
> >
> >
> >
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