Skip to content

Commit

Permalink
Add secp256k1_ctz{32,64}_var functions
Browse files Browse the repository at this point in the history
These functions count the number of trailing zeroes in non-zero integers.
  • Loading branch information
sipa committed Mar 8, 2021
1 parent 4c3ba88 commit de0a643
Show file tree
Hide file tree
Showing 2 changed files with 86 additions and 0 deletions.
21 changes: 21 additions & 0 deletions src/tests.c
Original file line number Diff line number Diff line change
Expand Up @@ -416,6 +416,25 @@ void run_scratch_tests(void) {
secp256k1_context_destroy(none);
}

void run_ctz_tests(void) {
static const uint32_t b32[] = {1, 0xffffffff, 0x5e56968f, 0xe0d63129};
static const uint64_t b64[] = {1, 0xffffffffffffffff, 0xbcd02462139b3fc3, 0x98b5f80c769693ef};
int shift;
unsigned i;
for (i = 0; i < sizeof(b32) / sizeof(b32[0]); ++i) {
for (shift = 0; shift < 32; ++shift) {
CHECK(secp256k1_ctz32_var_debruijn(b32[i] << shift) == shift);
CHECK(secp256k1_ctz32_var(b32[i] << shift) == shift);
}
}
for (i = 0; i < sizeof(b64) / sizeof(b64[0]); ++i) {
for (shift = 0; shift < 64; ++shift) {
CHECK(secp256k1_ctz64_var_debruijn(b64[i] << shift) == shift);
CHECK(secp256k1_ctz64_var(b64[i] << shift) == shift);
}
}
}

/***** HASH TESTS *****/

void run_sha256_tests(void) {
Expand Down Expand Up @@ -5606,6 +5625,8 @@ int main(int argc, char **argv) {
run_rand_bits();
run_rand_int();

run_ctz_tests();

run_sha256_tests();
run_hmac_sha256_tests();
run_rfc6979_hmac_sha256_tests();
Expand Down
65 changes: 65 additions & 0 deletions src/util.h
Original file line number Diff line number Diff line change
Expand Up @@ -276,4 +276,69 @@ SECP256K1_GNUC_EXT typedef __int128 int128_t;
# endif
#endif

#ifndef __has_builtin
#define __has_builtin(x) 0
#endif

/* Determine the number of trailing zero bits in a (non-zero) 32-bit x.
* This function is only intended to be used as fallback for
* secp256k1_ctz32_var, but permits it to be tested separately. */
static SECP256K1_INLINE int secp256k1_ctz32_var_debruijn(uint32_t x) {
static const uint8_t debruijn[32] = {
0x00, 0x01, 0x02, 0x18, 0x03, 0x13, 0x06, 0x19, 0x16, 0x04, 0x14, 0x0A,
0x10, 0x07, 0x0C, 0x1A, 0x1F, 0x17, 0x12, 0x05, 0x15, 0x09, 0x0F, 0x0B,
0x1E, 0x11, 0x08, 0x0E, 0x1D, 0x0D, 0x1C, 0x1B
};
return debruijn[((x & -x) * 0x04D7651F) >> 27];
}

/* Determine the number of trailing zero bits in a (non-zero) 64-bit x.
* This function is only intended to be used as fallback for
* secp256k1_ctz64_var, but permits it to be tested separately. */
static SECP256K1_INLINE int secp256k1_ctz64_var_debruijn(uint64_t x) {
static const uint8_t debruijn[64] = {
0, 1, 2, 53, 3, 7, 54, 27, 4, 38, 41, 8, 34, 55, 48, 28,
62, 5, 39, 46, 44, 42, 22, 9, 24, 35, 59, 56, 49, 18, 29, 11,
63, 52, 6, 26, 37, 40, 33, 47, 61, 45, 43, 21, 23, 58, 17, 10,
51, 25, 36, 32, 60, 20, 57, 16, 50, 31, 19, 15, 30, 14, 13, 12
};
return debruijn[((x & -x) * 0x022FDD63CC95386D) >> 58];
}

/* Determine the number of trailing zero bits in a (non-zero) 32-bit x. */
static SECP256K1_INLINE int secp256k1_ctz32_var(uint32_t x) {
VERIFY_CHECK(x != 0);
#if (__has_builtin(__builtin_ctz) || SECP256K1_GNUC_PREREQ(3,4))
/* If the unsigned type is sufficient to represent the largest uint32_t, consider __builtin_ctz. */
if (((unsigned)UINT32_MAX) == UINT32_MAX) {
return __builtin_ctz(x);
}
#endif
#if (__has_builtin(__builtin_ctzl) || SECP256K1_GNUC_PREREQ(3,4))
/* Otherwise consider __builtin_ctzl (the unsigned long type is always at least 32 bits). */
return __builtin_ctzl(x);
#else
/* If no suitable CTZ builtin is available, use a (variable time) software emulation. */
return secp256k1_ctz32_var_debruijn(x);
#endif
}

/* Determine the number of trailing zero bits in a (non-zero) 64-bit x. */
static SECP256K1_INLINE int secp256k1_ctz64_var(uint64_t x) {
VERIFY_CHECK(x != 0);
#if (__has_builtin(__builtin_ctzl) || SECP256K1_GNUC_PREREQ(3,4))
/* If the unsigned long type is sufficient to represent the largest uint64_t, consider __builtin_ctzl. */
if (((unsigned long)UINT64_MAX) == UINT64_MAX) {
return __builtin_ctzl(x);
}
#endif
#if (__has_builtin(__builtin_ctzll) || SECP256K1_GNUC_PREREQ(3,4))
/* Otherwise consider __builtin_ctzll (the unsigned long long type is always at least 64 bits). */
return __builtin_ctzll(x);
#else
/* If no suitable CTZ builtin is available, use a (variable time) software emulation. */
return secp256k1_ctz64_var_debruijn(x);
#endif
}

#endif /* SECP256K1_UTIL_H */

0 comments on commit de0a643

Please sign in to comment.