A simple program that shows the effect of word alignment on the performance of programs.

data_alignment.c

/**
 * File: data_alignment.c
 * This is a collection of simple functions that attempt to read
 * from a 5MB file in varying sizes and with different buffer 
 * alignments.
 * 
 * The munge* functions are derived from Jonathan Rentzsch's article
 * https://www.ibm.com/developerworks/library/pa-dalign/
 *
 * This work is inspired by the same article referenced above.
 * Before running the examples, create a data file by running:
 *
 *     base64 /dev/urandom | head -c 5000000 > datafile
 *
 * This will create a 5MB file with random textual data in it.
 * However, a file with any size can be used.
 *
 * The results of a sample run can be viewed at
 * https://docs.google.com/spreadsheets/d/17tfu_kl4w5Ad8Sqpdh4nl5GEenH8VF6vZY0lh7MDxhI/edit?usp=sharing
 * The test was run on a Macbook Pro with a quad-core 2.8 GHz Intel Core i7 processor and 16GB of RAM.
 */


#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>

#define USAGE_ERROR         1
#define INPUT_FILE_ERROR    2
#define MEM_ERROR           3
#define FILE_READ_ERROR     4
#define NUM_MUNGE_FXNS      4
#define NUM_TRIES_PER_RUN   10
#define NUM_INCREMENTS      128

//
// `size` is the amount of data we want to process, in this case 5MB
// `*data` is a pointer to the buffer we want to read.
// We're going to stagger the alignment to the data buffer by incrementing
// the pointer to the buffer and running each test again.
//


static void munge8(void *data, uint32_t size)
{
    uint8_t *data8 = (uint8_t *) data;
    uint8_t *data8End = data8 + size;
    
    while (data8 != data8End)
    {
        *data8 = -*data8;
        data8++;
    }
}

static void munge16(void *data, uint32_t size)
{
    uint16_t *data16 = (uint16_t *) data;
    uint16_t *data16End = data16 + (size >> 1); /* divide size by 2 */
    uint8_t *data8 = (uint8_t *) data16End;
    /* strip away upper 31 bits */
    uint8_t *data8End = data8 + (size & 0x00000001);
    
    while (data16 != data16End)
    {
        *data16 = -*data16;
        data16++;
    }
    while (data8 != data8End)
    {
        *data8 = -*data8;
        data8++;
    }
}

static void munge32(void *data, uint32_t size)
{
    uint32_t *data32 = (uint32_t*) data;
    uint32_t *data32End = data32 + (size >> 2); /* Divide size by 4. */
    uint8_t *data8 = (uint8_t*) data32End;
    uint8_t *data8End = data8 + (size & 0x00000003); /* Strip upper 30 bits. */
    
    while(data32 != data32End)
    {
        *data32 = -*data32;
        data32++;
    }
    while(data8 != data8End)
    {
        *data8 = -*data8;
        data8++;
    }
}

static void munge64(void *data, uint32_t size)
{
    double *data64 = (double*) data;
    double *data64End = data64 + (size >> 3); /* Divide size by 8. */
    uint8_t *data8 = (uint8_t*) data64End;
    /* Strip upper 29 bits. */
    uint8_t *data8End = data8 + (size & 0x00000007);
    
    while(data64 != data64End)
    {
        *data64 = -*data64;
        data64++;
    }
    while(data8 != data8End)
    {
        *data8 = -*data8;
        data8++;
    }
}

static float run(void(*mungeFxn)(void *, uint32_t), void *data, uint32_t size)
{
    clock_t start;
    clock_t stop;
    float duration;
    start = clock();
    mungeFxn(data, size);
    stop = clock();
    duration = (float)(stop - start)/CLOCKS_PER_SEC;
    return duration;
}

static float calculateAverage(float *runtimes, int len)
{
    float sum = 0;
    for (int i = 0; i < len; i++) {
        sum += runtimes[i];
    }
    float average = sum/len;
    //fprintf(stderr, "Sum: %f; len: %d, Average: %f ms\n", sum, len, average);
    return average;
}

static float performRuns(void(*func)(void *, uint32_t), char *buffer, long bufSize)
{
    float runtimes[NUM_TRIES_PER_RUN];
    float duration;
    for (int i = 0; i < NUM_TRIES_PER_RUN; i++) {
        duration= run(func, buffer, (uint32_t)bufSize);
        runtimes[i] = duration;
    }
    
    duration = calculateAverage(runtimes, NUM_TRIES_PER_RUN);
    return duration;
}

static float *incrementPointerAndRun(void(*func)(void *, uint32_t), char *buffer, long bufSize)
{
    float *result = (float *) malloc(sizeof(float) * NUM_INCREMENTS);
    for (size_t j = 0; j < NUM_INCREMENTS; j++) {
        result[j] = performRuns(func, buffer++, bufSize--);
    }
    return result;
}

static void runAll(char *buffer, long bufSize, float **result)
{
    void(*func[])(void *, uint32_t) = {&munge8, &munge16, &munge32, &munge64};
    int len = sizeof(func)/sizeof(func[0]);
    for (int i = 0; i < len; i++) {
        result[i] = incrementPointerAndRun(func[i], buffer, bufSize);
    }
}

static void getFileDataAndSize(FILE *file, char **buffer, long *fileSize)
{
    size_t lengthRead;
    
    // obtain file size
    fseek(file , 0 , SEEK_END);
    *fileSize = ftell(file);
    rewind(file);
    
    // allocate memory to contain whole file
    *buffer = (char *) malloc(sizeof(char) * *fileSize);
    if (*buffer == NULL)
    {
        fprintf(stderr, "Memory error\n");
        exit(MEM_ERROR);
    }
    
    // copy file into buffer
    lengthRead = fread(*buffer, 1, *fileSize, file);
    if (lengthRead != *fileSize)
    {
        fprintf(stderr, "File read error\n");
        exit(FILE_READ_ERROR);
    }
    
}

static void printResults(float **result)
{
    for (size_t i = 0; i < NUM_MUNGE_FXNS; i++) {
        fprintf(stderr, "munge%lu: ", (size_t)pow(2, (3 + i)));
        float *row = result[i];
        for (size_t j = 0; j < NUM_INCREMENTS; j++) {
            fprintf(stderr, "%f | ", row[j]);
        }
        fprintf(stderr, "\n");
    }
}

int main(int argc, char const *argv[])
{
    if (argc < 2)
    {
        fprintf(stderr, "Usage: data_alignment <10MB file>\n");
        exit(USAGE_ERROR);
    }
    const char *filename = argv[1];
    FILE *file = fopen(filename, "r");
    if (!file)
    {
        fprintf(stderr, "Unable to open file: '%s'\n", filename);
        exit(INPUT_FILE_ERROR);
    }
    
    char *buffer;
    long fileSize;
    getFileDataAndSize(file, &buffer, &fileSize);
    
#ifdef TESTRUN
    for (size_t i = 0; i < fileSize; i++) {
        fprintf(stderr, "%c", buffer[i]);
    }
    fprintf(stderr, "\n");
#endif
    
    float **result = (float **)malloc(sizeof(float *) * NUM_MUNGE_FXNS);
    if (result == NULL)
    {
        fprintf(stderr, "Memory error\n");
        exit(MEM_ERROR);
    }
    runAll(buffer, fileSize, result);
    printResults(result);
    // clean up
    if (buffer)
    {
        free(buffer);
    }
    if (result)
    {
        for (size_t i = 0; i < NUM_MUNGE_FXNS; i++) {
            free(result[i]);
        }
        free(result);
    }
    fclose(file);
    return 0;
}