Optimize pass examples in C in llvm 15

[hstk30-hw]

Hope to test the special optimize pass easily

[hstk30-hw]

Tail Recursion Elimination

https://godbolt.org/z/YM7v4n3G9

code:

int factorial (int x, int y){
  if (x==0)
    return y;
  else
    return factorial(x-1,y*x);
}

cmd: -O2 -fno-vectorize

[hstk30-hw]

GlobalOpt

SROA(scalar replacement of aggregates)

https://godbolt.org/z/13jKve8Ef

#include <stdio.h>

typedef struct {
    int x;
    int y;
} Point;

static Point p = {10, 20};

void updatePoint() {
    p.x += 5;
    p.y += 10;
}

void printPoint() {
    printf("Point: (%d, %d)\n", p.x, p.y);
}

cmd: -O2

OptimizeGlobalAddressOfAllocation

body

https://godbolt.org/z/qxx1aMWhM

extern void *malloc(unsigned long size);
extern void f1();

static int *g_ptr;

int f() {
    g_ptr = malloc(sizeof(int) * 1);
    *g_ptr = -1;
    return 0;
}

int main() {
    f();
    *g_ptr = 1;
    f1();
    *((char*)g_ptr) = 2;

    return *g_ptr;
}

init

https://godbolt.org/z/37hevohzd

extern void *malloc(unsigned long size);

static int *g_ptr;

static void init_g() {
    g_ptr = malloc(sizeof(int) * 1);
    return ;
}

static int is_init() {
    int* local_ptr = g_ptr;
    init_g();
    if (local_ptr == 0)
        return 1;
    else
        return -1;
}

int main() {
    g_ptr = 0;
    int res = is_init();

    return res;
}

ShrinkGlobalToBoolean

https://godbolt.org/z/n4rqj6Tec

static int flag = 0;

void store() {
    flag = 5;
    return ;
}

int bar() {
    return flag;
}

[hstk30-hw]

PartialInlining

https://godbolt.org/z/76dvM8MKn

__attribute__((noinline)) int notinline(int* align_val){
  *align_val = 1;
  return 0;
}

int inlinedFunc(int cond, int* align_val) {
  if (cond) {
    *align_val = 10;
  }
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);
  notinline(align_val);

  return 0;
}

extern void disableTailCallEli(int *);
int dummyCaller(int cond, int* align_val) {
  int l = 3;
  disableTailCallEli(&l);
  int val = inlinedFunc(cond, align_val);
  return val;
}

cmd: -O1 -mllvm -enable-partial-inlining=1 -mllvm -skip-partial-inlining-cost-analysis=1

[hstk30-hw]

Constant Propagation and Dead Argument Elimination

https://godbolt.org/z/fd3M1s5Yq

extern void use(int a);

__attribute__((noinline))
static void test1(int a) {
    use(a);
}

__attribute__((noinline))
static int test2(int a) {
    use(a);
    return a + 1;
}

int main() {
    test1(1);
    // test1(2); 
    test2(2);
}