llvm dev - Jun 2020 - Target specific named address spaces

Hi,
Is there a way to implement named address spaces with clang/llvm as it is
possible with gcc ?

We would like to have our own named address space that would be recognized
by the frontend.

Thanks in advance!
Regards,
Sebastien

Hi Sebastien,

Clang does have support for numbered address spaces via the
__attribute__((address_space(N))) syntax. These address spaces map down to
addrspace() in LLVM IR, and addrspaces in LLVM IR have arbitrary semantics
unknown to LLVM itself.

However, there is no way to define custom address space keywords without adding
them explicitly to the lexing and parsing, and there is also no way for a target
to express how its address spaces are supposed to be related (subspaces,
conversions etc). I have an initial patch to add support for target AS
compatibility here (https://reviews.llvm.org/D62574) but hit some snags and have
not had the bandwidth to finish it. I don't think a lot of people really
require this, so it's hard to justify working on it.

/ Bevin
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Sebastien
Le Duc via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Wednesday, June 24, 2020 4:25 PM
To: 'llvm-dev' <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Target specific named address spaces

Hi,
Is there a way to implement named address spaces with clang/llvm as it is
possible with gcc ?

We would like to have our own named address space that would be recognized
by the frontend.

Thanks in advance!
Regards,
Sebastien



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From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
    Sebastien Le Duc via llvm-dev <llvm-dev at lists.llvm.org> Sent:

    > Hi, Is there a way to implement named address spaces with
    > clang/llvm as it is possible with gcc ?

    > We would like to have our own named address space that would
    > be recognized by the frontend.
>>>>> On Thu, 25 Jun 2020 08:27:10 +0000, Bevin Hansson via
llvm-dev <llvm-dev at lists.llvm.org> said:
    Bevin> Hi Sebastien, Clang does have support for numbered address
    Bevin> spaces via the __attribute__((address_space(N)))
    Bevin> syntax. These address spaces map down to addrspace() in LLVM
    Bevin> IR, and addrspaces in LLVM IR have arbitrary semantics
    Bevin> unknown to LLVM itself.

In the meantime, I wonder whether something like

#define __mppa_cluster __attribute__((address_space(42)))

plus a few LLVM-only passes could not do most of the job while avoiding
diving into Clang...
Then you have some decent minimal __mppa_cluster named address space. :-)

Otherwise, if you can use C++, you can also use wrapper
classes to add address spaces around some data types,
such as

  mppa::cluster<int> v;

to declare some int variable inside your MPPA cluster (just an imaginative
example to give the idea).

I used to do this in SYCL and also OpenCL C++ was using this trick
https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_Cxx.html#address-spaces-library
At some point if the C++ proposal about the . (dot) operator overloading
lands, it should be easier to implement this way.
-- 
  Ronan KERYELL