Alexis - 1 year ago 56

C Question

So I have the following code:

`double which_min(const int i, const int j, const int nx,`

const double step, const double local_cost,

double *D)

{

double tuple[3];

// DIAG, LEFT, UP

tuple[0] = (D[d2s(i-1, j-1, nx)] == NOT_VISITED) ? DBL_MAX : D[d2s(i-1, j-1, nx)] + step * local_cost;

tuple[1] = (D[d2s(i, j-1, nx)] == NOT_VISITED) ? DBL_MAX : D[d2s(i, j-1, nx)] + local_cost;

tuple[2] = (D[d2s(i-1, j, nx)] == NOT_VISITED) ? DBL_MAX : D[d2s(i-1, j, nx)] + local_cost;

/*

if (i == 83 && j == 124) printf("less? %d\n", tuple[1] < tuple[0]);

if (i == 83 && j == 124) printf("equal? %d\n", tuple[1] == tuple[0]);

if (i == 83 && j == 124) printf("greater? %d\n", tuple[1] > tuple[0]);

*/

int min = (tuple[0] <= tuple[1]) ? 0 : 1;

min = (tuple[min] <= tuple[2]) ? min : 2;

if (i == 83 && j == 124) printf("min = %d\n", min + 1);

return ((double) min + 1.0);

}

For the problematic case I'm working with, when

`step = 1`

`i = 83`

`j = 124`

`min = 2.`

However, if I uncomment the other

`if`

`min`

`less? 1`

equal? 1

greater? 0

min = 1

I know comparing doubles can be finicky, and I read this answer that mentions moving from 80-bit register to 64-bit memory, so I'm guessing the

`printf`

- It still feels counter intuitive that is both less AND equal to
`tuple[1]`

, how is that possible?*`tuple[0]`

- Can I change this so that it gives the same result consistently?

And by the way, this only happens in a 32-bit system, the 64-bit version doesn't give me any problems.

EDIT: I guess that the first

`printf`

`==`

`1`

EDIT2: Indeed, changing the order of

`if`

`if (i == 83 && j == 124)`

{

printf("equal? %d\n", tuple[1] == tuple[0]);

printf("less? %d\n", tuple[1] < tuple[0]);

printf("greater? %d\n", tuple[1] > tuple[0]);

}

Results in

`equal? 0`

less? 0

greater? 0

min = 1

And the values I get with

`printf(%a)`

`tuple[0] = 0x1.2594a8056d275p+5`

tuple[1] = 0x1.2594a8056d275p+5

tuple[2] = 0x1.fffffffffffffp+1023

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Answer Source

Indeed the results stored in `tuple`

were being read with more precision than expected, causing some of the comparisons to yield contradicting results.

Adding the `-ffloat-store`

flag to the compiler remedies the situation, but as mentioned in the comments, it might not be ideal, and apparently it isn't portable.

Declaring `tuple`

as `volatile`

seems to do the trick, but it had to be done as

```
volatile double *tuple = (double *)malloc(3 * sizeof(double));
```

And then

```
free((double *)tuple);
```

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