feat(riscv): WasmOp::Call lowering — leaf-call subset#116
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v0.3.1 minimum-viable cross-function call support in the RISC-V
selector. WasmOp::Call(idx) no longer errors with `Unsupported` for
leaf-call shapes — it now lowers to a label-based RiscVOp::Call that
the ELF builder resolves to a PC-relative `auipc + jalr` when the
callee is in the same compilation unit.
Behavior:
* Move top N vstack values (capped at 8) into a0..a(N-1).
* Emit `RiscVOp::Call { label: format!("synth_func_{idx}") }`.
* Push a fresh `a0` vreg as the return value.
What's deliberately deferred (documented in the lower_call doc + the
#[ignore]-marked `recursive_self_call_emits_two_call_ops` test):
* Function-signature plumbing from the decoder. Without it, the
selector can't know how many args to pop, so the v0.3.1 cut
over-consumes the vstack on back-to-back calls with surviving
results. v0.4 will pipe `FuncSig` through and lift this restriction.
* Args beyond 8 (RV psABI says spill to stack at fixed offsets — not
implemented).
* Caller-side a0..a7 invalidation across the BL — callers wanting to
survive a call should `drop` or `local.tee` their live values
explicitly until v0.4 models this properly.
* Multi-result returns (wasm 2.0).
* Cross-`.text` relocations for multi-unit linking.
Tests:
* `call_emits_label_and_argument_marshalling` — single-arg call, label
encodes `synth_func_{idx}`.
* `call_two_args_marshals_to_a0_a1` — two-arg call from i32.const seq.
* `recursive_self_call_emits_two_call_ops` — #[ignore]'d documentation
of the back-to-back-calls gap, to be flipped when v0.4 plumbing
lands.
Total: 100 passing tests in synth-backend-riscv (was 99); 1 ignored
that documents the next milestone.
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Summary
Minimum-viable cross-function call support in the RISC-V instruction selector.
WasmOp::Call(idx)now lowers to a label-basedRiscVOp::Callinstead of returningSelectorError::Unsupported. The ELF builder already resolves call labels via PC-relativeauipc + jalr, so the only missing piece was the selector arm — this PR adds it.This is the third Track A item for v0.3.1 after PRs #113, #114, #115. RV32 i64 lowering is the next one.
What it does
lower_call(func_idx, op)method insynth-backend-riscv/src/selector.rs.What's deferred (deliberately, with tracking tests)
The v0.3.1 cut is the leaf-call subset. The following are known gaps, each carrying a comment and (where applicable) an `#[ignore]`'d test:
The ignored test `recursive_self_call_emits_two_call_ops` is the canary that v0.4's plumbing work will flip back on.
Tests
Local run before push:
Test plan
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