ESP32-C3: Implement software atomic operations for missing A extension

[luoliwoshang]

Problem

ESP32-C3 doesn't support RISC-V A extension (atomic operations). When code uses atomic operations, linking fails with undefined symbol errors:

llgo run -a -target esp32c3 .

ld.lld: error: undefined symbol: __atomic_store_4
>>> referenced by write
>>>               .../write.o:(github.com/goplus/llgo/_demo/embed/esp32c3/write.main)

Root Cause

1. ESP32-C3 only supports RV32IMC instruction set, no A extension (atomic operations)
2. PR fix(esp32c3): correct -march to rv32imc to prevent illegal instruction crashes #1443 fixed -march=rv32imac → -march=rv32imc and excluded atomic.c compilation
3. However, when Go code uses atomic operations (e.g., sync/atomic package or compiler-generated atomic instructions), it generates calls to __atomic_* functions
4. Without hardware support or software implementation, linker cannot find these symbols

Proposed Solution

Reference TinyGo's approach (tinygo#2146) - implement software atomic operations by disabling interrupts:

//export __atomic_load_4
func __atomic_load_4(ptr *uint32, ordering int32) uint32 {
    mask := riscv.DisableInterrupts()
    value := *ptr
    riscv.EnableInterrupts(mask)
    return value
}

//export __atomic_store_4
func __atomic_store_4(ptr *uint32, value uint32, ordering int32) {
    mask := riscv.DisableInterrupts()
    *ptr = value
    riscv.EnableInterrupts(mask)
}

Rationale: On single-core systems (like ESP32-C3), disabling interrupts guarantees atomicity since no other code can interrupt current execution.

Functions to Implement

Common atomic operation functions:

• __atomic_load_1/2/4/8 - Atomic load
• __atomic_store_1/2/4/8 - Atomic store
• __atomic_exchange_1/2/4/8 - Atomic exchange
• __atomic_compare_exchange_1/2/4/8 - Atomic compare-and-swap (CAS)
• __atomic_fetch_add_1/2/4/8 - Atomic add
• __atomic_fetch_sub_1/2/4/8 - Atomic subtract
• __atomic_fetch_and/or/xor_1/2/4/8 - Atomic bitwise operations

Simplified Approach

For llgo, we can start with the most commonly used functions (like __atomic_store_4, __atomic_load_4) using simple assignment:

// targets/device/esp/esp32c3_atomic.c

void __atomic_store_4(uint32_t *ptr, uint32_t value, int ordering) {
    *ptr = value;  // Simple assignment (safe on single-core)
}

uint32_t __atomic_load_4(uint32_t *ptr, int ordering) {
    return *ptr;
}

Add interrupt disabling logic later if stricter atomicity guarantees are needed.

Related Links

• TinyGo PR #2146 - Software atomic operations for RISC-V
• PR #1443 - Fix ESP32-C3 -march and atomic.c