Re: [Qemu-devel] [PATCH pic32 v3 02/16] pic32: use LCG algorithm for generated random index of TLBWR instruction

[Aurelien Jarno]

On 2015-07-05 23:14, Serge Vakulenko wrote:
> The LFSR algorithm, used for generating random TLB indexes for TLBWR 
> instruction,
> was inclined to produce a degenerate sequence in some cases.
> For example, for 16-entry TLB size and Wired=1, it gives: 15, 6, 7, 2,
> 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2, 7, 2...
> When replaced with LCG algorithm from ISO/IEC 9899 standard, the sequence
> looks much better, with about the same computational effort needed.
> 
> Signed-off-by: Serge Vakulenko <address@hidden>
> ---
>  hw/mips/cputimer.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/mips/cputimer.c b/hw/mips/cputimer.c
> index 75917cf..58707ed 100644
> --- a/hw/mips/cputimer.c
> +++ b/hw/mips/cputimer.c
> @@ -28,13 +28,16 @@
>  /* XXX: do not use a global */
>  uint32_t cpu_mips_get_random (CPUMIPSState *env)
>  {
> -    static uint32_t lfsr = 1;
> +    static uint32_t seed = 1;
>      static uint32_t prev_idx = 0;
>      uint32_t idx;
>      /* Don't return same value twice, so get another value */
>      do {
> -        lfsr = (lfsr >> 1) ^ (-(lfsr & 1u) & 0xd0000001u);
> -        idx = lfsr % (env->tlb->nb_tlb - env->CP0_Wired) + env->CP0_Wired;
> +        /* Use a simple algorithm of Linear Congruential Generator
> +         * from ISO/IEC 9899 standard. */
> +        seed = 1103515245 * seed + 12345;
> +        idx = (seed >> 16) % (env->tlb->nb_tlb - env->CP0_Wired) +
> +            env->CP0_Wired;
>      } while (idx == prev_idx);
>      prev_idx = idx;
>      return idx;

This is indeed better, it seems the generated values are also better in
the standard Linux case (16 entries, wired = 0).

Reviewed-by: Aurelien Jarno <address@hidden>

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net