riscv-backend

name	riscv-backend
description	Implement issue

RISC-V Backend

Use this skill to execute issue #89 in incremental, test-first steps.

Workflow

1. Bootstrap llvm-target-riscv crate and workspace wiring.
2. Implement register model and ABI helpers.
3. Implement instruction opcodes + encoder with format-focused unit tests.
4. Implement lowering for the same InstrKind coverage as x86/aarch64 backends.
5. Validate object emission and end-to-end RISC-V ELF generation.
6. Review PR + run full tests.
7. If problems are found, open issue(s) and fix in the same PR.
8. Post review feedback summary and merge.

Step 1: Bootstrap

• Add src/llvm-target-riscv crate.
• Add modules: regs.rs, abi.rs, instructions.rs, lower.rs, encode.rs, lib.rs.
• Export RiscVBackend and RiscVEmitter with interfaces matching existing target crates.

Step 2: Registers + ABI

• Integer regs: x0..x31 (x0 hardwired zero).
• ABI mapping: arg regs a0..a7, return reg a0.
• Keep allocatable and callee-saved sets explicit and tested.

Step 3: Encoding Core

• Implement helpers for R/I/S/B/U/J type encodings.
• Add at least 30 encoding tests across formats and key opcodes.
• Emit ELF object sections via llvm_codegen::emit::Emitter.

Step 4: Lowering Coverage

• Match lowering coverage expectations used in current backends.
• Unsupported operations must fail deterministically with explicit errors/TODO comments, never panic silently.

Step 5: Validation

Run at minimum:

cargo +stable test -p llvm-target-riscv
cargo +stable test

If riscv64-linux-gnu-objdump is unavailable, document blocker and keep object-shape tests deterministic.

Step 6: Review + Full Test

• Review implementation PR for correctness, ABI compliance, and missing tests.
• Run targeted tests and a full suite (cargo +stable test unless blocked).

Step 7: Issue+Fix Loop (Same PR)

• If a concrete bug/regression is found, open a GitHub issue documenting it.
• Fix findings in the same PR branch and push follow-up commits.

Step 8: Post Review Summary

• Post PR review feedback (gh pr review --comment or gh pr comment) with findings and fixes.
• Include issue links in the summary comment.

Resources

• Use references/issue-89-plan.md for acceptance checklist.
• Use scripts/riscv_coverage_audit.sh to track lowering/encoding progress.