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

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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 <serge.vakulenko@gmail.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
This commit is contained in:
Serge Vakulenko 2015-07-05 23:14:50 -07:00 committed by Leon Alrae
parent b307446e04
commit ceb0ee147d
1 changed files with 6 additions and 3 deletions
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9
hw/mips/cputimer.c
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@ -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;