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Suppress a harmless variable-time optimization by clang in _int_cmov

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Follow up on 52a03512c1d800603b5c923c1a28bdba12dadb30
This commit is contained in:
Tim Ruffing 2020-07-27 14:35:05 +02:00
parent 5b196338f0
commit 67a429f31f
1 changed files with 6 additions and 1 deletions
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7
src/util.h
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@ -197,10 +197,15 @@ static SECP256K1_INLINE void memczero(void *s, size_t len, int flag) {
/** If flag is true, set *r equal to *a; otherwise leave it. Constant-time. Both *r and *a must be initialized and non-negative.*/
static SECP256K1_INLINE void secp256k1_int_cmov(int *r, const int *a, int flag) {
unsigned int mask0, mask1, r_masked, a_masked;
/* Access flag with a volatile-qualified lvalue.
This prevents clang from figuring out (after inlining) that flag can
take only be 0 or 1, which leads to variable time code. */
volatile int vflag = flag;
/* Casting a negative int to unsigned and back to int is implementation defined behavior */
VERIFY_CHECK(*r >= 0 && *a >= 0);
mask0 = (unsigned int)flag + ~0u;
mask0 = (unsigned int)vflag + ~0u;
mask1 = ~mask0;
r_masked = ((unsigned int)*r & mask0);
a_masked = ((unsigned int)*a & mask1);