[LLVMdev] Emitting IR in older formats (for NVVM)

[Tobias Grosser]

On 12.01.2015 05:48, Jonathan Ragan-Kelley wrote:
> This question is specifically motivated by the practical constraints of
> NVVM, but I don't know anywhere better to ask (hopefully, e.g.,
> @jholewinski is still following), and I believe it concerns general LLVM
> issues:
>
> NVIDIA's libNVVM is built on LLVM 3.2. This means its bitcode and LL
> text parsers are from that generation. It's interface calls for adding
> modules as either bitcode blobs or LL text buffers. LLVM's bitcode and
> assembly formats have never been intended to maintain strong
> cross-version compatibility. However, this means that a compiler built
> on more recent LLVMs struggles mightily to emit even simple IR for NVVM
> to compile.
>
> Specifically, I find 3.4 (the official Ubuntu package) generates both
> bitcode streams and LL text which are incompatible with the 3.2 parser
> in the current NVVM release (6.5). I'm hardly surprised that the binary
> format changes, but simple examples generally manage fine in LL text.
> Nontrivial modules, however, do not. Specifically, the first issue I
> notice is that `#i` attribute references (as opposed to inline
> attributes) appear to be a recent addition to the assembly syntax; they
> are always used by the assembly serializer (when, e.g., streaming out a
> Module) for any function attributes in generated assembly, but cannot be
> parsed by the NVVM/3.2 parser. This seems to be the main compatibility
> issue, but it's hard to tell without first eliminating it and then
> proceeding further.
>
> So that's the challenge.
>
> The obvious questions are:
>
> 1. Narrowly, is it possible to coerce the standard LLVM bitcode or text
> writers to emit more conservative/backwards-compatible output
> (specifically with an eye towards NVVM/3.2)? Am I just going to have to
> resort to brittle string rewrites on the generated text to inline all
> #attr values?
>
> 2. More generally, is there another accepted way to create NVVM programs
> from LLVM-based compilers which use versions more recent than 3.2? I
> can't imagine I'm the first one to run into this—3.2 is fairly old at
> this point, and NVVM seems wedded to its interface for stability reasons.

Dear Jonathan,

the following link may help:

https://github.com/KhronosGroup/SPIR-Tools/tree/master/spir-encoder/llvm_3.5_spir_encoder

 From the documentation:

spir-encoder
------------
This utility provides a mechanism to produce LLVM bitcode compatible
with the SPIR 1.2 specification when working with recent versions of
LLVM. Specifically, this tool takes a SPIR module generated using LLVM
3.5 and outputs bitcode using LLVM 3.2 encoding. This tool assumes
that the SPIR module is valid, and does not perform any
transformations to the module.

Even though spir is just a subset of LLVM-IR, it may actually be generic 
enough for your purposes.

Cheers,
Tobias