Re: SPIR-V and GCC

[Thomas Schwinge <thomas at codesourcery dot com>]

Hi!

On Fri, 6 Mar 2015 15:23:21 +0000 (UTC), BogDan <bog_dan_ro@yahoo.com> wrote:
> > This http://www.g-truc.net/post-0714.html great article explans better
> > what SPIR-V is.

Note that I don't know anything about SPIR-V, and I'm currently too busy
to learn about it, but:

> I checked them a little bit and I'm afraid it might be a a few problems to create a SPIR-V backend:

These look -- at least superficially -- similar to issues we (that is,
primarily Bernd, CCed) faced, when creating a Nvidia PTX backend
(primarily for offloading use, as part of the OpenACC project we're
currently working on).

> 1. SPIR-V a binary Intermediate Language format but it's still not machine code. It seems gcc backends produces a text machine code, so I think will not be possible to produce SPIR-V a binary format directly from GCC backend.

(Not a problem for text-based PTX.)  Indeed I don't know if it's feasible
to have GCC backends generate output in a binary format -- but then, I
don't think this is a show-stopper?  I'm assuming that any binary format
can be represented in a textual format, and then converted into the
desired binary format by an "assembler"?

> 2. Because SPIR-V is not a machine format, it doesn't have any registers. Instead it's using IDs for everything [1]. I didn't find a way to create IDs instead of registers in GCC.

> [1] https://www.khronos.org/registry/spir-v/specs/1.0/SPIRV.html#_instructions

In the nvptx backend, we currently disable all register allocation, under
the assumption that there is an unbounded register file, with the
registers having arbitrary names -- does that match the SPIR-V model?

> 3. SPIR-V supports some "non-standard" types[2], will it be a problems to support these types in GCC frontend/backend?

> [2] https://www.khronos.org/registry/spir-v/specs/1.0/SPIRV.html#_types

Certainly GCC can be taught about "non-standard" types, but you'll have
to see at which stage of the compilation process this is best done.


GrÃÃe,
 Thomas

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