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kern: simplify random bitmap selection to match latest Nintendo logic

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This commit is contained in:
Michael Scire 2021-10-25 17:00:59 -07:00
parent e780171c78
commit 80cae0cc2d
1 changed files with 9 additions and 28 deletions
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35
libraries/libmesosphere/include/mesosphere/kern_k_page_bitmap.hpp
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@ -50,37 +50,18 @@ namespace ams::kern {
size_t SelectRandomBit(u64 bitmap) {
u64 selected = 0;
u64 cur_num_bits = BITSIZEOF(bitmap) / 2;
u64 cur_mask = (1ull << cur_num_bits) - 1;
for (size_t cur_num_bits = BITSIZEOF(bitmap) / 2; cur_num_bits != 0; cur_num_bits /= 2) {
const u64 high = (bitmap >> cur_num_bits);
const u64 low = (bitmap & (~(UINT64_C(0xFFFFFFFFFFFFFFFF) << cur_num_bits)));
while (cur_num_bits) {
const u64 low = (bitmap >> 0) & cur_mask;
const u64 high = (bitmap >> cur_num_bits) & cur_mask;
bool choose_low;
if (high == 0) {
/* If only low val is set, choose low. */
choose_low = true;
} else if (low == 0) {
/* If only high val is set, choose high. */
choose_low = false;
} else {
/* If both are set, choose random. */
choose_low = this->GenerateRandomBit();
}
/* If we chose low, proceed with low. */
if (choose_low) {
bitmap = low;
selected += 0;
} else {
/* Choose high if we have high and (don't have low or select high randomly). */
if (high && (low == 0 || this->GenerateRandomBit())) {
bitmap = high;
selected += cur_num_bits;
} else {
bitmap = low;
selected += 0;
}
/* Proceed. */
cur_num_bits /= 2;
cur_mask >>= cur_num_bits;
}
return selected;