Implement memory alignment prediction

[jserv]

Extending #268, alignment prediction involves using branch prediction techniques to predict whether a memory address is correctly aligned. If the prediction is correct, we can skip unnecessary alignment checks, which can ultimately improve emulation performance. Below is an example in C code:

#include <stdbool.h>
#include <stdint.h>

/* Define a structure for the Branch History Table (BHT).
 * The BHT stores PC-based predictions for memory alignment.
 */
typedef struct {
    uint32_t pc; /* Program Counter */
    /* Predicted alignment (true for aligned, false for unaligned). */
    bool prediction;
} bht_entry_t;

#define BHT_SIZE 1024
static bht_entry_t bht[BHT_SIZE];

/* Predict memory alignment based on PC.
 * In this case, we predict every other instruction to be misaligned.
 */
bool predict_alignment(uint32_t pc)
{
    /* If the lowest bit of PC is 0, we predict alignment. */
    return !(pc & 1);
}

/* Emulate a load instruction. */
void emulate_load(uint32_t address)
{
    /* Fetch the current PC (for demonstration purposes). */
    uint32_t pc = 0x1000;

    /* Check if we have a prediction in the BHT. */
    uint32_t index = pc % BHT_SIZE;
    if (bht[index].pc == pc) {
        /* We have a prediction, use it. */
        if (bht[index].prediction) {
            /* Aligned access; perform the load operation.
             * Here, we can access memory efficiently since we predicted
             * alignment.
             */
            /* ... */
        } else {
            /* Misaligned access; handle it accordingly.
             * This allows us to optimize handling of misaligned loads.
             */
            /* ... */
        }
    } else {
        /* No prediction found, check alignment and update the BHT. */
        bool is_aligned = predict_alignment(pc);
        if (is_aligned) {
            /* Aligned access; perform the load operation.
             * Since we accurately predicted alignment, this results in
             * efficient memory access.
             */
            /* ... */
        } else {
            /* Misaligned access; handle it accordingly.
             * We can efficiently address misaligned loads thanks to the
             * prediction.
             */
            /* ... */
        }

        /* Update the BHT with the prediction. */
        bht[index].pc = pc;
        bht[index].prediction = is_aligned;
    }
}

int main()
{
    /* Emulate a load instruction with a memory address. */
    emulate_load(0x1004); /* Replace with the actual address. */
    return 0;
}

Explain:

• We define a Branch History Table (BHT) to store PC-based alignment predictions. Each entry in the BHT contains the PC and the predicted alignment.
• The predict_alignment function predicts memory alignment based on the PC. In this simplified example, we assume that every other instruction is misaligned. In a real-world scenario, we would use more sophisticated prediction logic based on the given workload.
• In the emulate_load function, we first check if we have a prediction in the BHT for the current PC. If a prediction exists, we use it. If not, we predict alignment using the predict_alignment function, perform the load operation, and update the BHT with the prediction.
• This approach minimizes the number of alignment checks by predicting alignment based on the PC, which can significantly improve load instruction emulation performance.

However, the effectiveness of this technique depends on the accuracy of our alignment prediction logic. In practice, more complex and accurate prediction strategies can be employed based on the observed behavior of the target workload.

[jserv]

An alternative approach is to implement a memory cache to store frequently accessed memory regions. This can significantly reduce the number of memory accesses and improve performance. Consider the following:

#define CACHE_SIZE 64
uint8_t memory_cache[CACHE_SIZE];

uint8_t load_byte(uint32_t address)
{
    if (address < CACHE_SIZE) {
        return memory_cache[address];
    return memory[address]; /* Fallback to main memory */
}