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Avoid UB in MSB computation in feistel permutation. #87

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Merged
niklas-uhl merged 3 commits into from
Dec 4, 2025
Merged

Avoid UB in MSB computation in feistel permutation. #87

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08d222f
Avoid UB in MSB computation in feistel permutation.
niklas-uhl Dec 4, 2025
ec7c0af
Make msb function less easy to misuse.
niklas-uhl Dec 4, 2025
f0a0882
fixup! Make msb function less easy to misuse.
niklas-uhl Dec 4, 2025
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5 changes: 5 additions & 0 deletions kagen/tools/random_permutation.h
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Original file line number Diff line number Diff line change
Expand Up @@ -84,6 +84,11 @@ std::uint8_t most_significant_bit_set(const Data arg) {
constexpr std::size_t arg_width = std::numeric_limits<Data>::digits;
auto log2 = static_cast<Data>(arg_width);

// __builtin_clz(l) is undefined for 0
if (arg == 0) {
return 0;
}

if constexpr (arg_width == std::numeric_limits<unsigned int>::digits) {
log2 -= __builtin_clz(arg);
} else {
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