[libcxx-commits] [libcxx] [libc++] Fix sub-overflow in std::gcd implementation (PR #117984)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: None (serge-sans-paille)

<details>
<summary>Changes</summary>

Fix #<!-- -->117249

---
Full diff: https://github.com/llvm/llvm-project/pull/117984.diff


1 Files Affected:

- (modified) libcxx/include/__numeric/gcd_lcm.h (+8-13) 


``````````diff
diff --git a/libcxx/include/__numeric/gcd_lcm.h b/libcxx/include/__numeric/gcd_lcm.h
index 9be6cf8516b131..b85ae4e41bb2e7 100644
--- a/libcxx/include/__numeric/gcd_lcm.h
+++ b/libcxx/include/__numeric/gcd_lcm.h
@@ -55,7 +55,7 @@ template <class _Tp>
 constexpr _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
   static_assert(!is_signed<_Tp>::value, "");
 
-  // From: https://lemire.me/blog/2013/12/26/fastest-way-to-compute-the-greatest-common-divisor
+  // 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)
@@ -76,21 +76,16 @@ constexpr _LIBCPP_HIDDEN _Tp __gcd(_Tp __a, _Tp __b) {
   if (__a == 0)
     return __b;
 
-  int __az    = std::__countr_zero(__a);
-  int __bz    = std::__countr_zero(__b);
-  int __shift = std::min(__az, __bz);
-  __a >>= __az;
-  __b >>= __bz;
+  int __shift = std::__countr_zero(__a | __b);
+  __a >>= std::__countr_zero(__a);
   do {
-    _Tp __diff = __a - __b;
-    if (__a > __b) {
-      __a = __b;
-      __b = __diff;
+    _Tp __t = __b >> std::__countr_zero(__b);
+    if (__a > __t) {
+      __b = __a - __t;
+      __a = __t;
     } else {
-      __b = __b - __a;
+      __b = __t - __a;
     }
-    if (__diff != 0)
-      __b >>= std::__countr_zero(__diff);
   } while (__b != 0);
   return __a << __shift;
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/117984