Add nowrap u64 multiplication function

Follows #30920

Signed-off-by: José M. Guisado <guigom@riseup.net>

src/lib/intmath/muldiv.c

@@ -69,6 +69,18 @@ gcd64(uint64_t a, uint64_t b)
   return a;
 }
 
+uint64_t
+tor_mul_u64_nowrap(uint64_t a, uint64_t b)
+{
+  if (a == 0 || b == 0) {
+    return 0;
+  } else if (PREDICT_UNLIKELY(UINT64_MAX / a < b)) {
+    return UINT64_MAX;
+  } else {
+    return a*b;
+  }
+}
+
 /* Given a fraction *<b>numer</b> / *<b>denom</b>, simplify it.
  * Requires that the denominator is greater than 0. */
 void

src/lib/intmath/muldiv.h

@@ -18,6 +18,8 @@ unsigned round_to_next_multiple_of(unsigned number, unsigned divisor);
 uint32_t round_uint32_to_next_multiple_of(uint32_t number, uint32_t divisor);
 uint64_t round_uint64_to_next_multiple_of(uint64_t number, uint64_t divisor);
 
+uint64_t tor_mul_u64_nowrap(uint64_t a, uint64_t b);
+
 void simplify_fraction64(uint64_t *numer, uint64_t *denom);
 
 /* Compute the CEIL of <b>a</b> divided by <b>b</b>, for nonnegative <b>a</b>

src/test/test_util.c

@@ -33,6 +33,7 @@
 #include "lib/process/env.h"
 #include "lib/process/pidfile.h"
 #include "lib/intmath/weakrng.h"
+#include "lib/intmath/muldiv.h"
 #include "lib/thread/numcpus.h"
 #include "lib/math/fp.h"
 #include "lib/math/laplace.h"
@@ -5973,6 +5974,14 @@ test_util_nowrap_math(void *arg)
   tt_u64_op(UINT32_MAX, OP_EQ, tor_add_u32_nowrap(2, UINT32_MAX-1));
   tt_u64_op(UINT32_MAX, OP_EQ, tor_add_u32_nowrap(UINT32_MAX, UINT32_MAX));
 
+  tt_u64_op(0, OP_EQ, tor_mul_u64_nowrap(0, 0));
+  tt_u64_op(1, OP_EQ, tor_mul_u64_nowrap(1, 1));
+  tt_u64_op(2, OP_EQ, tor_mul_u64_nowrap(2, 1));
+  tt_u64_op(4, OP_EQ, tor_mul_u64_nowrap(2, 2));
+  tt_u64_op(UINT64_MAX, OP_EQ, tor_mul_u64_nowrap(UINT64_MAX, 1));
+  tt_u64_op(UINT64_MAX, OP_EQ, tor_mul_u64_nowrap(2, UINT64_MAX));
+  tt_u64_op(UINT64_MAX, OP_EQ, tor_mul_u64_nowrap(UINT64_MAX, UINT64_MAX));
+
  done:
   ;
 }