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authorSerge Vakulenko <serge.vakulenko@gmail.com>2015-07-05 23:14:50 -0700
committerLeon Alrae <leon.alrae@imgtec.com>2015-09-18 09:20:48 +0100
commitceb0ee147df35adc7b705da1c84a4624c9cabb21 (patch)
treea87800759b1e4d48231346889f882d3afbb8401c /hw/mips/cputimer.c
parentb307446e04232b3a87e9da04886895a8e5a4a407 (diff)
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: 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>
Diffstat (limited to 'hw/mips/cputimer.c')
-rw-r--r--hw/mips/cputimer.c9
1 files changed, 6 insertions, 3 deletions
diff --git a/hw/mips/cputimer.c b/hw/mips/cputimer.c
index 577c9aeab8..1603600118 100644
--- a/hw/mips/cputimer.c
+++ b/hw/mips/cputimer.c
@@ -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;