Now try with something that emulates a bit more reality (cold branch), in
making that front branch be random, instead of scorching hot towards a
single hint.
[main_test.c]
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
extern int testovf(size_t a, size_t b);
extern size_t base(void);
size_t num_values = 100000;
size_t qrand(void)
{
return ((size_t) rand() << 4) ^ rand();
}
int main(void)
{
size_t i, j, c;
size_t *vals = calloc(num_values, sizeof(size_t));
for (c = 0; c < num_values; c++) {
do {
vals[c] = qrand();
} while (!vals[c]);
}
c = 0;
for (i = 0; i < 4000; i++) {
for (j = base(); j < num_values; j++) {
c += testovf(vals[j - 1], vals[j]);
}
}
printf("c = %zd\n", c);
return 0;
}
[test_div.c]
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#define SIZE_MAX (~(size_t) 0)
int testovf(size_t a, size_t b)
{
if (a > 0 && SIZE_MAX / a < b)
return 0;
return 1;
}
size_t base(void)
{
return 0;
}
real 0m3.438s
user 0m3.430s
sys 0m0.004s
[test_obsd.c]
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#define SIZE_MAX (~(size_t) 0)
#define MUL_NO_OVERFLOW (1UL << (sizeof(size_t) * 4))
int testovf(size_t a, size_t b)
{
if ((a >= MUL_NO_OVERFLOW || b >= MUL_NO_OVERFLOW) &&
a > 0 && SIZE_MAX / a < b) {
return 1;
}
return 0;
}
size_t base(void)
{
return 0;
}
real 0m3.875s
user 0m3.871s
sys 0m0.000s
[test_int128.c]
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#define SIZE_MAX (~(size_t) 0)
int testovf(size_t a, size_t b)
{
unsigned __int128 p = (unsigned __int128) a * (unsigned __int128) b;
if (__builtin_expect(p > SIZE_MAX, 0))
return 1;
return 0;
}
size_t base(void)
{
return 0;
}
real 0m0.555s
user 0m0.550s
sys 0m0.004s
On Tue, Dec 1, 2015 at 11:18 AM, Dan Cross <[email protected]> wrote:
> On Tue, Dec 1, 2015 at 11:54 AM, 'Davide Libenzi' via Akaros <
> [email protected]> wrote:
>
>> Eh? I didn't *defend* it, I just explained it. That code was a direct
>>> import from OpenBSD.... I didn't write it, and given what it's doing, I saw
>>> no reason to change it. :-)
>>>
>>
>> You are arguing for simplicity, and yet you want to leave complex and
>> useless optimizations in place? ☺
>>
>
> Oh, and also: https://www.youtube.com/watch?v=nXaxk27zwlk <-- Well worth
> the watch.
>
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