fixup! [libc++] Fix sub-overflow in std::gcd implementation

Author: serge-sans-paille

libcxx/include/__numeric/gcd_lcm.h

@@ -55,7 +55,8 @@ template <class _Tp>
 constexpr _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
   static_assert(!is_signed<_Tp>::value, "");
 
-  // From: https://lemire.me/blog/2024/04/13/greatest-common-divisor-the-extended-euclidean-algorithm-and-speed/
+  // Using Binary GCD algorithm https://en.wikipedia.org/wiki/Binary_GCD_algorithm, based on an implementation
+  // from https://lemire.me/blog/2024/04/13/greatest-common-divisor-the-extended-euclidean-algorithm-and-speed/
   //
   // If power of two divides both numbers, we can push it out.
   // - gcd( 2^x * a, 2^x * b) = 2^x * gcd(a, b)

libcxx/test/std/numerics/numeric.ops/numeric.ops.gcd/gcd.pass.cpp

@@ -27,17 +27,7 @@ constexpr struct {
   int x;
   int y;
   int expect;
-} Cases[] = {
-    {0, 0, 0},
-    {1, 0, 1},
-    {0, 1, 1},
-    {1, 1, 1},
-    {2, 3, 1},
-    {2, 4, 2},
-    {36, 17, 1},
-    {36, 18, 18}
-};
-
+} Cases[] = {{0, 0, 0}, {1, 0, 1}, {0, 1, 1}, {1, 1, 1}, {2, 3, 1}, {2, 4, 2}, {11, 9, 1}, {36, 17, 1}, {36, 18, 18}};
 
 template <typename Input1, typename Input2, typename Output>
 constexpr bool test0(int in1, int in2, int out)