[LLVMdev] Language-specific vs target-specific address spaces (was Re: [PATCH] OpenCL support - update on keywords)

[David Neto]

On Mon, Feb 28, 2011 at 4:41 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>
> The more I think about it, the more I become uncomfortable with the
> concept of language-specific address spaces in LLVM.  These are the
> main issues I see with language-specific address spaces:

...

> Instead of language-specific address spaces, each target should
> concentrate on exposing all of its address spaces as target-specific
> address spaces, and frontends should use a language -> target mapping
> in target-specific code.  We can continue to expose the target's main
> shared writable address space as address space 0 as we do now.
>
> For example, Clang could define a set of internal address space
> constants for OpenCL and use TargetCodeGenInfo to provide the mapping
> to target address spaces.

In principle this is a fine idea.

I think the difficulty is that LLVM and Clang provide an
infrastructure for numbered address spaces, but no standard assignment
on top of that infrastructure.  The trick is define some conventions,
e.g. what the numbers might mean for a language front-end, and whether
the interpretation of the numbers change as the IR moves to later
stages.  We're working in a bit of a vacuum.

For example, you're proposing a remapping step somewhere along the
line: that could be entirely inside a back-end code generator.  Or it
could conceivably be an LLVM pass itself, which then could be used
with multiple backends that understand the new convention.

So I think we need a couple of things:
- proposals for number assignments and their associated semantics.
- code to flesh out and embody those semantics. e.g. a sample
implementation / translation layer

Basically Anton got the ball rolling: his code patch was a bit of
both.  And I think he's planning to post a number of OpenCL proposals
in general.

As it is, I hope that backends that do not understand address spaces
at all know to error out when they receive IR that uses address
spaces.


david