archived-ballistic/docs/PROGRAMMING_RULES.md

Compiler Aliasing Fear

Pointer aliasing forces compilers to avoid optimizing code as best as they should. We use local variables to fix this problem.

For all examples shown below, assume context is a pointer to a heap allocated struct.

Rule 1: Scalars inside a loop

If booleans or integers gets modified inside a loop, always assign them to local variables before doing so. Yes there are other ways to make the compiler optimize this specific scenario, but we do this to easily guarantee speed.

// SLOW
//
for (int i = 0; i < 100; i++) {
    // Forces a write to memory 100 times.
    //
    context->counter++;

    // This stopped context->counter from being kept in a CPU Register..
    //
    // The compiler doesnt know what do_something() does. It might write to
    // context->counter. 
    // 
    // The compiler is forced to:
    //
    // 1. Save `counter` from register to RAM.
    // 2. Calls do_something().
    // 3. Reloads `counter` fron ram to register.
    //
    do_something(context)

}

Register Only Approach

// FASTEST

// Load from memory to register once.
//
uint32_t count = context->counter;

for (int i = 0; i < 100; i++) {
    // Modify register directly. Zero memory access here.
    //
    count++; 

    // Pass the register value to the helper. We pass 'count' directly to avoid
    // the memory store.
    //
    do_something(context, count);
}

// Store from register back to memory once.
// 
context->counter = count;

Hybrid Aprpoach (Optimized Load, Forced Store)

We do this if we cannot change the function signature.

// FAST

uint32_t count = context->counter;

for (int i = 0; i < 100; i++) {
    count++; 

    // We must sync memory because do_something() reads it. This costs us 1 
    // STORE per loop, but we still save the LOAD.
    // 
    context->counter = count;

    do_something(context);
}

// No final store needed, we kept it in sync the whole time.

Rule 2: Arrays inside a loop

struct->array[i] forces the CPU to calculate Base + Offset + (i * 4) every time. *cursor++ is just Pointer + 4.

// SLOW
//
for (int i = 0; i < 100; ++i) {
    context->instructions[i] = ...; // Math overhead + Aliasing fear.
}

// FAST
//
bal_instruction_t *cursor = context->instructions;
for (int i = 0; i < 100; ++i) {
    *cursor++ = ...; // Simple increment.
}