Hans Petter Selasky
2021-04-29 21:06:42 UTC
Hi,
Can someone please explain what C-compiler flag I'm missing, to make
this simple C-program terminate?
I have a function _abs() which at some point is equal to one, but the
compiler thinks it is not required to emit the test for the while() at
all, looking at the assembly code! A printf() clearly verifies that the
_abs() function has returned one.
I even added an own if() check, but apparently the compiler is lost!
cat << EOF > test.c
#include <stdio.h>
#include <stdint.h>
typedef struct {
int32_t x;
int32_t y;
} c32_t;
static c32_t
_mul(c32_t a, c32_t b)
{
return (c32_t){
a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x
};
}
static uint32_t
_abs(c32_t a)
{
return (a.x * a.x + a.y * a.y);
}
int
main()
{
c32_t a = {4, 1};
c32_t b = {};
uint32_t l1 = 0;
while (_abs(a) != 1) {
uint32_t l0 = _abs(a) & ~1;
/* figure out least significant bit */
l0 &= ~(l0 - 1);
if (l0 == 0 || l0 > l1) {
l1 = l0;
if (_abs(a) == 1)
printf("VALUE IS ONE\n");
printf("Y:0x%08x ABS:0x%08x\n", a.y, _abs(a));
}
a.y += 2;
}
size_t count = 0;
do {
count++;
b = _mul(b, a);
} while (b.x != 1 || b.y != 0);
printf("%d,%d,0x%x COUNT=%zd\n", a.x, a.y, _abs(a), count);
return (0);
}
EOF
clang11 -O2 test.c && ./a.out
--HPS
Can someone please explain what C-compiler flag I'm missing, to make
this simple C-program terminate?
I have a function _abs() which at some point is equal to one, but the
compiler thinks it is not required to emit the test for the while() at
all, looking at the assembly code! A printf() clearly verifies that the
_abs() function has returned one.
I even added an own if() check, but apparently the compiler is lost!
cat << EOF > test.c
#include <stdio.h>
#include <stdint.h>
typedef struct {
int32_t x;
int32_t y;
} c32_t;
static c32_t
_mul(c32_t a, c32_t b)
{
return (c32_t){
a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x
};
}
static uint32_t
_abs(c32_t a)
{
return (a.x * a.x + a.y * a.y);
}
int
main()
{
c32_t a = {4, 1};
c32_t b = {};
uint32_t l1 = 0;
while (_abs(a) != 1) {
uint32_t l0 = _abs(a) & ~1;
/* figure out least significant bit */
l0 &= ~(l0 - 1);
if (l0 == 0 || l0 > l1) {
l1 = l0;
if (_abs(a) == 1)
printf("VALUE IS ONE\n");
printf("Y:0x%08x ABS:0x%08x\n", a.y, _abs(a));
}
a.y += 2;
}
size_t count = 0;
do {
count++;
b = _mul(b, a);
} while (b.x != 1 || b.y != 0);
printf("%d,%d,0x%x COUNT=%zd\n", a.x, a.y, _abs(a), count);
return (0);
}
EOF
clang11 -O2 test.c && ./a.out
--HPS