Skip to content

Avoid UB in MSB computation in feistel permutation. #87

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

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

Avoid UB in MSB computation in feistel permutation. #87

Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
29 changes: 20 additions & 9 deletions kagen/tools/random_permutation.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -79,19 +79,30 @@ inline uint8_t most_significant_bit_set(Data bytes) {
asm("bsr %1,%0" : "=r"(msb) : "r"(bytes));
return asserting_cast<uint8_t>(msb);
}*/
template <typename Data>
template <
typename Data, typename Enable = std::enable_if_t<
std::is_same_v<Data, unsigned int> || std::is_same_v<Data, unsigned long>
|| std::is_same_v<Data, unsigned long long>>>
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);
constexpr std::size_t width = std::numeric_limits<Data>::digits;

if constexpr (arg_width == std::numeric_limits<unsigned int>::digits) {
log2 -= __builtin_clz(arg);
} else {
static_assert(arg_width == std::numeric_limits<unsigned long>::digits, "unsupported data type width");
log2 -= __builtin_clzl(arg);

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

return log2 - 1;
std::size_t leading_zeros = 0;

if constexpr (std::is_same_v<Data, unsigned int>) {
leading_zeros = __builtin_clz(arg);
} else if constexpr (std::is_same_v<Data, unsigned long>) {
leading_zeros = __builtin_clzl(arg);
} else {
static_assert(std::is_same_v<Data, unsigned long long>);
leading_zeros = __builtin_clzll(arg);
}
return asserting_cast<std::uint8_t>(width - leading_zeros - 1);
}

#define UNUSED(expr) (void)(expr)
Expand Down