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* Use 16x16 bit ==> 32 bit multiplications. This gives a speedup of 5 on

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the MSP430.
This commit is contained in:
bg- 2007-07-03 13:26:00 +00:00
parent eb7f7ba65b
commit 5b52db6708
2 changed files with 32 additions and 9 deletions
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35
core/lib/rand.c
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@ -28,7 +28,7 @@
*
* This file is part of the Contiki operating system.
*
* @(#)$Id: rand.c,v 1.6 2007/06/05 10:02:53 bg- Exp $
* @(#)$Id: rand.c,v 1.7 2007/07/03 13:26:00 bg- Exp $
*/
#include <stdlib.h>
@ -61,16 +61,39 @@ CTASSERT(RAND_MAX == 0x7fff);
* When possible, keep rand_state across reboots.
*/
#ifdef __GNUC__
unsigned long rand_state __attribute__((section(".noinit")));
int32_t rand_state __attribute__((section(".noinit")));
#else
unsigned long rand_state = 123459876ul;
int32_t rand_state = 1;
#endif
int
rand()
rand(void)
{
rand_state = (16807*rand_state) % 2147483647ul;
return (rand_state ^ (rand_state >> 16)) & RAND_MAX;
uint32_t hi, lo;
/*
* Perform two 16x16 bits multiplication with 32 bit results.
*/
lo = 16807ul * (uint16_t)(rand_state);
hi = 16807ul * (uint16_t)(rand_state >> 16);
lo += (hi & 0x7fff) << 16;
/*
* lo += hi >> 15; But faster using 16 bit registers.
*/
hi <<= 1;
lo += (uint16_t)(hi >> 16);
if ((int32_t)lo <= 0) /* Deal with rand_state == 0. */
lo -= 0x7fffffff;
rand_state = lo;
if (sizeof(int) == sizeof(int16_t))
return (lo ^ (lo >> 16)) & RAND_MAX; /* Not ANSI C! */
else
return lo & RAND_MAX;
}
void