[Embedded] LLGO Compiler Support for RISC-V CSR Operations

[luoliwoshang]

Background

During the migration of TinyGo device packages to goplus/lib/emb, we discovered that RISC-V CSR (Control and Status Register) operations require built-in compiler support.

Problem Description

Current Status

• File: emb/device/riscv/csr.go
• Functions: CSR.Get(), CSR.Set(), CSR.SetBits(), CSR.ClearBits()
• Error: missing function body

TinyGo Implementation

In TinyGo, these CSR operations are implemented as compiler intrinsics:

• File: tinygo/compiler/inlineasm.go
• Function: func (b *builder) emitCSROperation(call *ssa.CallCommon) (llvm.Value, error)

Current Temporary Solution

func (csr CSR) Get() uintptr {
    // TODO(zzy): implement tinygo/compiler/inlineasm.go emitCSROperation
    panic("TODO: CSR.Get()")
    return 0
}

Requirements

Option 1: LLGO Compiler Built-in Support

• Add built-in support for RISC-V CSR operations in LLGO compiler
• Similar to TinyGo's approach, generate corresponding LLVM IR at compiler level

Option 2: Use AsmFull to implement

Related Files

• emb/device/riscv/csr.go - CSR operation functions that need implementation
• TinyGo reference implementation: tinygo/compiler/inlineasm.go
• Validation targets: riscv-qemu, hifive1b and other RISC-V targets

Priority

This is a blocking issue for complete TinyGo device package migration, affecting embedded development support for RISC-V platforms.

Technical Details

The CSR operations are fundamental for RISC-V embedded programming:

• CSR.Get(): Read CSR register value
• CSR.Set(): Write CSR register value
• CSR.SetBits(): Atomically set specific bits
• CSR.ClearBits(): Atomically clear specific bits

These operations typically compile to RISC-V CSR instructions like csrr, csrw, csrs, csrc.