[llvm-dev] [RFC] Changing X86 data layout for address spaces

[Philip Reames via llvm-dev]

Please review the properties of an address space which are configurable 
via the data layout.  For example, bitwidth is one of those parameters.  
If that parameter space covers your needs, then you do not need LLVM 
side support.

Philip

On 7/30/19 2:59 PM, Amy Huang wrote:
> Thanks for the info--
> It seems like the way to do this is for clang to use address spaces to 
> represent different sized pointers in the IR. If that's the case, then 
> LLVM still has to know about those specific address spaces. I don't 
> see a clear way to avoid adding knowledge of the address spaces to LLVM.
> I think that's the approach we'll go with, unless there are more 
> objections.
>
> On Sun, Jul 28, 2019 at 8:23 AM John Reagan 
> <john.reagan at vmssoftware.com <mailto:john.reagan at vmssoftware.com>> wrote:
>
>     That is basically what we do today to provide mixed sized pointers
>     with
>     our legacy frontends.  They generate IR to our old code generator
>     which
>     has ADDR32 and ADDR64 datatypes.  We use a 64-bit address data layout
>     and then typecast the 32-bit forms to/from the underlying 64-bit
>     addresses.  I have been warned that such rampant typecasting might
>     interfere with certain optimizations or TBAA data.  We haven't
>     investigated that yet.  Since clang doesn't go through our IR-to-IR
>     converter, we'll have to teach clang to do the same.
>
>     On 7/26/2019 6:35 PM, Amy Huang wrote:
>     >       *Clang* can assign whatever meaning to an AS it wishes,
>     and their
>     >     properties are configurable via data layout configuration.
>     >
>     >
>     > I think Clang checks that its data layout matches the LLVM
>     target data
>     > layout, so I'm not sure how to go about implementing the address
>     spaces
>     > as a change in Clang.
>     > Thoughts?
>     >
>     > The other issue I was running into is how to do the address
>     space casts
>     > in clang - my current thought was to lower the address space
>     casts in
>     > LLVM to the sign extension / zero extension / truncation.
>     >
>     > Thanks,
>     > Amy
>     >
>     > On Mon, Jul 22, 2019 at 5:06 PM Philip Reames via llvm-dev
>     > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     <mailto:llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>>
>     wrote:
>     >
>     >     Please don't bake in knowledge *in LLVM* of a address space
>     unless
>     >     there's a strong need.  *Clang* can assign whatever meaning
>     to an AS
>     >     it wishes, and their properties are configurable via data layout
>     >     configuration.
>     >
>     >     The standards of review are much lower for a Clang only
>     change as it
>     >     can be revised arbitrarily without breaking other uses of LLVM.
>     >     Building in first class knowledge to LLVM itself is a breaking
>     >     change (potentially) for other frontends.
>     >
>     >     Philip
>     >
>     >     On 7/22/19 1:19 PM, Reid Kleckner via llvm-dev wrote:
>     >>     Yes, the address spaces are intended to be generally useful for
>     >>     other applications. Glad to hear that you would find it
>     useful. :)
>     >>
>     >>     For now we were only going to add them for x86, but it
>     should be
>     >>     easy to add such address spaces to other targets.
>     >>
>     >>     On Mon, Jul 22, 2019 at 1:13 PM John Reagan via llvm-dev
>     >>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>     <mailto:llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>>
>     wrote:
>     >>
>     >>         Amy, when you say "implement MSVC's extensions", is that
>     >>         MSVC/LLVM or
>     >>         are you going to add these to clang?  OpenVMS has
>     dual-sized
>     >>         pointers as
>     >>         well and want to see them in clang.  One of my engineers
>     >>         noticed that
>     >>         address space 4 was a 32-bit pointer address space (or at
>     >>         least it used
>     >>         to be).  I haven't seen any formal discussion of that
>     over in
>     >>         the cfe
>     >>         dev list.  I'm happy to help out where we can.
>     >>
>     >>         John
>     >>
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