Stream: git-wasmtime

Topic: wasmtime / PR #13913 feat: GHC calling convention

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:28):

playX18 opened PR #13913 from playX18:ap/ghc-callconv to bytecodealliance:main:

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This PR adds LLVM’s GHC (ghccc) calling convention to Cranelift as CallConv::Ghc / CLIF ghc, for x86_64, aarch64, and riscv64.

Why?

People writing compilers for functional languages often want "register
pinning": keep runtime / VM state in fixed hardware registers, and jump
between functions instead of doing normal C calls. Haskell does this with
the STG machine and LLVM's ghccc. SML/NJ (and similar CPS systems) do
heap-allocated continuations and need something like jump-with-arguments.
Scheme compilers in CPS style want the same kind of thing.

In Cranelift today, enable_pinned_reg only gives you one reserved
register. tail helps with return_call, but it is still basically a
SysV-like ABI: it still builds native frames, and it does not pin a big
set of STG-style registers for you.

So this PR adds LLVM's GHC calling convention. Then you can map many
virtual machine registers to real regs through the callconv, with no
callee-saves, and use return_call between ghc functions.

Summary

• Mirror LLVM STG register pinning: args only in fixed regs, no callee-saves, no return values, no stack spill of overflow args (Unsupported).
• return_call allowed between ghc functions (supports_tail_calls); does not use Tail’s callee-pops-stack-args behavior.
• x86_64: Sp is %rbp, so ghc frames are FP-less (RSP-relative spills only). Entry from SysV/Rust that needs Sp must use a naked asm trampoline (documented in cranelift/docs/ir.md); Cranelift system_v cannot place Sp in %rbp while using it as FP.
• aarch64 / riscv64: Sp is not the hardware FP, so full STG arg lists work from system_v without asm.
• Unsupported on s390x/pulley.
• Exceptions not supported for ghc.

Test plan

• [x] Verifier: filetests/filetests/verifier/ghc-abi.clif
• [x] Precise-output: isa/{x64,aarch64,riscv64}/ghc.clif
• [x] cargo run -p cranelift-tools -- test ./filetests/filetests/isa/*/ghc.clif ./filetests/filetests/verifier/ghc-abi.clif

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:32):

playX18 updated PR #13913.

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:33):

playX18 has marked PR #13913 as ready for review.

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:33):

playX18 requested wasmtime-compiler-s390x-reviewers for a review on PR #13913.

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:33):

playX18 requested cfallin for a review on PR #13913.

Wasmtime GitHub notifications bot (Jul 21 2026 at 00:33):

playX18 requested wasmtime-compiler-reviewers for a review on PR #13913.

Last updated: Jul 29 2026 at 05:03 UTC