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llvm-abi

This is a library for generating LLVM IR that complies with platform ABIs.

Typically this is useful for frontends generating code that needs to comply with a target's language-specific ABI, such as a C ABI. There's a generic interface for an 'ABI' and implementations for each target (e.g. x86, ARM etc.).

Why?

LLVM IR doesn't currently have any way for representing language type information, however platform ABIs are typically expressed in terms of source language types.

This means backends don't have all the available information for generating ABI-correct calls/returns so it's up to frontends to emit code that provides the necessary information to the backend.

Unfortunately this task is quite involved and target-specific, hence this library has been created to handle this and abstract away the details.

Who's developing this?

This is currently being developed by Stephen Cross and was created due to the need for good ABI support in the Loci compiler; ultimately the aim is that this library is useful to all LLVM frontends that are interested in ABI compliance.

It is worth noting that many significant pieces of functionality have been pulled out of Clang.

If you'd like to contribute, that would be awesome! Also any suggestions or queries are very welcome (raise a GitHub issue).

Furthermore, it'd be great to add support for languages beyond C.

Current status

Currently the following ABIs are supported:

  • x86_32
  • x86_64 (note: no Win64 support yet)

Clearly the aim is to also add support for the other LLVM targets by extracting the necessary parts from Clang.

In terms of features, there are at least the following gaping holes in the library:

  • CPU selection - There's a lot of functionality in Clang for selecting a generic CPU (we typically don't select the native CPU because similar CPUs with the same architecture may not have the same processor features) and this needs to be brought into the library.
  • Encoding user-specified alignment for types
  • Receiving varargs parameters
  • Marking arguments as already in-memory - There are excessive loads/stores being generated due to not recognising the arguments are already in stack memory.
  • inalloca support - There are some aspects of functionality in various places but it's very incomplete.

Testing

The approach to testing is currently to create LLVM IR files that specify an ABI function type (e.g. int (double, int)), which generates some code that's then checked against the file.

Two functions will be generated: a caller and a callee, both with the ABI-encoded function signature. The caller function then receives arguments, decodes them, then immediately re-encodes them and passes them to the callee. Further it then decodes the return value and then re-encodes it to return.

This testing strategy makes it fairly simple to check that the ABI implementation is encoding and decoding arguments as expected.

If a test fails then Clang is run to see what it outputs. Essentially the aim is to always produce output that (functionally) matches Clang.