Commit ceb0ee14 authored by Serge Vakulenko's avatar Serge Vakulenko Committed by Leon Alrae
Browse files

pic32: use LCG algorithm for generated random index of TLBWR instruction 



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: default avatarSerge Vakulenko <serge.vakulenko@gmail.com>
Reviewed-by: default avatarAurelien Jarno <aurelien@aurel32.net>
Reviewed-by: default avatarLeon Alrae <leon.alrae@imgtec.com>
Signed-off-by: default avatarLeon Alrae <leon.alrae@imgtec.com>
parent b307446e

hw/mips/cputimer.c

+6 −3
Original line number Diff line number Diff line
@@ -30,13 +30,16 @@ 
/* 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;