[RISC-V] FP structs larger than 16 bytes in interops

Open
#107,386 6 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Quiet
Tech stack
csharp
Domain
backend

Research direction

Start with the ABI rationale in the linked discussion and trace the RISC-V interop handling for floating-point structs. Determine where structs larger than 16 bytes are classified, then add the requested exception behavior and coverage for the described empty-struct and aligned-field cases.

Written by the indexing model from the issue text.

Description

arch-riscv area-Interop-coreclr

.NET currently supports passing/returning structs according to hardware floating-point calling convention up to 16 bytes for reasons stated in https://github.com/dotnet/runtime/pull/107286#discussion_r1743260919

Native ABI does not limit size of FP structs so this won't work:

struct Empty {}
struct S { double; Empty; long; }

[DllImport("SomeLib")]
public static extern void func(S s);

.NET will pass S according to integer calling convention (by implicit reference) and the native side will expect it in two registers, fa0 and a0.

Such structs are acheivable via empty structs, which are undefined in C, which ABI .NET interops follow. However, it would be nice to throw an exception if FP structs > 16 bytes appear in interops. (Things like struct { _Alignas(16) double; long; } are legal in C and .NET users may use empty structs to match the layout.)

Dominant language
C#
Stars
18.3k
Forks
5.6k
PR metrics pending

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

More from dotnet/runtime

All issues in dotnet/runtime

Similar issues

More C# issues

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.