[libc++][math] Fix undue overflowing of std::hypot(x,y,z) (llvm#93350)

The 3-dimentionsional `std::hypot(x,y,z)` was sub-optimally implemented. This lead to possible over-/underflows in (intermediate) results which can be circumvented by this proposed change. The idea is to to scale the arguments (see linked issue for full discussion). Tests have been added for problematic over- and underflows. Closes llvm#92782
ldionne · Jul 23, 2024 · 9628777 · 9628777
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diff --git a/libcxx/include/__math/hypot.h b/libcxx/include/__math/hypot.h
@@ -15,10 +15,21 @@
 #include <__type_traits/is_same.h>
 #include <__type_traits/promote.h>
 
+#if _LIBCPP_STD_VER >= 17
+#  include <__algorithm/max.h>
+#  include <__math/abs.h>
+#  include <__math/roots.h>
+#  include <__utility/pair.h>
+#  include <limits>
+#endif
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 namespace __math {
@@ -41,8 +52,86 @@ inline _LIBCPP_HIDE_FROM_ABI typename __promote<_A1, _A2>::type hypot(_A1 __x, _
   return __math::hypot((__result_type)__x, (__result_type)__y);
 }
 
+#if _LIBCPP_STD_VER >= 17
+// Factors needed to determine if over-/underflow might happen for `std::hypot(x,y,z)`.
+// returns [overflow_threshold, overflow_scale]
+template <class _Real>
+_LIBCPP_HIDE_FROM_ABI std::pair<_Real, _Real> __hypot_factors() {
+  static_assert(std::numeric_limits<_Real>::is_iec559);
+
+  if constexpr (std::is_same_v<_Real, float>) {
+    static_assert(-125 == std::numeric_limits<_Real>::min_exponent);
+    static_assert(+128 == std::numeric_limits<_Real>::max_exponent);
+    return {0x1.0p+62f, 0x1.0p-70f};
+  } else if constexpr (std::is_same_v<_Real, double>) {
+    static_assert(-1021 == std::numeric_limits<_Real>::min_exponent);
+    static_assert(+1024 == std::numeric_limits<_Real>::max_exponent);
+    return {0x1.0p+510, 0x1.0p-600};
+  } else { // long double
+    static_assert(std::is_same_v<_Real, long double>);
+
+    // preprocessor guard necessary, otherwise literals (e.g. `0x1.0p+8'190l`) throw warnings even when shielded by `if
+    // constexpr`
+#  if __DBL_MAX_EXP__ == __LDBL_MAX_EXP__
+    static_assert(sizeof(_Real) == sizeof(double));
+    return static_cast<std::pair<_Real, _Real>>(__math::__hypot_factors<double>());
+#  else
+    static_assert(sizeof(_Real) > sizeof(double));
+    static_assert(-16381 == std::numeric_limits<_Real>::min_exponent);
+    static_assert(+16384 == std::numeric_limits<_Real>::max_exponent);
+    return {0x1.0p+8190l, 0x1.0p-9000l};
+#  endif
+  }
+}
+
+// Computes the three-dimensional hypotenuse: `std::hypot(x,y,z)`.
+// The naive implementation might over-/underflow which is why this implementation is more involved:
+//    If the square of an argument might run into issues, we scale the arguments appropriately.
+// See https://github.com/llvm/llvm-project/issues/92782 for a detailed discussion and summary.
+template <class _Real>
+_LIBCPP_HIDE_FROM_ABI _Real __hypot(_Real __x, _Real __y, _Real __z) {
+  const _Real __max_abs = std::max(__math::fabs(__x), std::max(__math::fabs(__y), __math::fabs(__z)));
+  const auto [__overflow_threshold, __overflow_scale] = __math::__hypot_factors<_Real>();
+  _Real __scale;
+  if (__max_abs > __overflow_threshold) { // x*x + y*y + z*z might overflow
+    __scale = __overflow_scale;
+    __x *= __scale;
+    __y *= __scale;
+    __z *= __scale;
+  } else if (__max_abs < 1 / __overflow_threshold) { // x*x + y*y + z*z might underflow
+    __scale = 1 / __overflow_scale;
+    __x *= __scale;
+    __y *= __scale;
+    __z *= __scale;
+  } else
+    __scale = 1;
+  return __math::sqrt(__x * __x + __y * __y + __z * __z) / __scale;
+}
+
+inline _LIBCPP_HIDE_FROM_ABI float hypot(float __x, float __y, float __z) { return __math::__hypot(__x, __y, __z); }
+
+inline _LIBCPP_HIDE_FROM_ABI double hypot(double __x, double __y, double __z) { return __math::__hypot(__x, __y, __z); }
+
+inline _LIBCPP_HIDE_FROM_ABI long double hypot(long double __x, long double __y, long double __z) {
+  return __math::__hypot(__x, __y, __z);
+}
+
+template <class _A1,
+          class _A2,
+          class _A3,
+          std::enable_if_t< is_arithmetic_v<_A1> && is_arithmetic_v<_A2> && is_arithmetic_v<_A3>, int> = 0 >
+_LIBCPP_HIDE_FROM_ABI typename __promote<_A1, _A2, _A3>::type hypot(_A1 __x, _A2 __y, _A3 __z) _NOEXCEPT {
+  using __result_type = typename __promote<_A1, _A2, _A3>::type;
+  static_assert(!(
+      std::is_same_v<_A1, __result_type> && std::is_same_v<_A2, __result_type> && std::is_same_v<_A3, __result_type>));
+  return __math::__hypot(
+      static_cast<__result_type>(__x), static_cast<__result_type>(__y), static_cast<__result_type>(__z));
+}
+#endif
+
 } // namespace __math
 
 _LIBCPP_END_NAMESPACE_STD
+_LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___MATH_HYPOT_H
diff --git a/libcxx/include/cmath b/libcxx/include/cmath
@@ -313,6 +313,7 @@ constexpr long double lerp(long double a, long double b, long double t) noexcept
 */
 
 #include <__config>
+#include <__math/hypot.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_arithmetic.h>
 #include <__type_traits/is_constant_evaluated.h>
@@ -553,30 +554,6 @@ using ::scalbnl _LIBCPP_USING_IF_EXISTS;
 using ::tgammal _LIBCPP_USING_IF_EXISTS;
 using ::truncl _LIBCPP_USING_IF_EXISTS;
 
-#if _LIBCPP_STD_VER >= 17
-inline _LIBCPP_HIDE_FROM_ABI float hypot(float __x, float __y, float __z) {
-  return sqrt(__x * __x + __y * __y + __z * __z);
-}
-inline _LIBCPP_HIDE_FROM_ABI double hypot(double __x, double __y, double __z) {
-  return sqrt(__x * __x + __y * __y + __z * __z);
-}
-inline _LIBCPP_HIDE_FROM_ABI long double hypot(long double __x, long double __y, long double __z) {
-  return sqrt(__x * __x + __y * __y + __z * __z);
-}
-
-template <class _A1, class _A2, class _A3>
-inline _LIBCPP_HIDE_FROM_ABI
-typename enable_if_t< is_arithmetic<_A1>::value && is_arithmetic<_A2>::value && is_arithmetic<_A3>::value,
-                      __promote<_A1, _A2, _A3> >::type
-hypot(_A1 __lcpp_x, _A2 __lcpp_y, _A3 __lcpp_z) _NOEXCEPT {
-  typedef typename __promote<_A1, _A2, _A3>::type __result_type;
-  static_assert(
-      !(is_same<_A1, __result_type>::value && is_same<_A2, __result_type>::value && is_same<_A3, __result_type>::value),
-      "");
-  return std::hypot((__result_type)__lcpp_x, (__result_type)__lcpp_y, (__result_type)__lcpp_z);
-}
-#endif
-
 template <class _A1, __enable_if_t<is_floating_point<_A1>::value, int> = 0>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool __constexpr_isnan(_A1 __lcpp_x) _NOEXCEPT {
 #if __has_builtin(__builtin_isnan)

diff --git a/libcxx/test/libcxx/transitive_includes/cxx17.csv b/libcxx/test/libcxx/transitive_includes/cxx17.csv
@@ -130,6 +130,9 @@ chrono type_traits
 chrono vector
 chrono version
 cinttypes cstdint
+cmath cstddef
+cmath cstdint
+cmath initializer_list
 cmath limits
 cmath type_traits
 cmath version

diff --git a/libcxx/test/libcxx/transitive_includes/cxx20.csv b/libcxx/test/libcxx/transitive_includes/cxx20.csv
@@ -135,6 +135,9 @@ chrono type_traits
 chrono vector
 chrono version
 cinttypes cstdint
+cmath cstddef
+cmath cstdint
+cmath initializer_list
 cmath limits
 cmath type_traits
 cmath version

diff --git a/libcxx/test/libcxx/transitive_includes/cxx23.csv b/libcxx/test/libcxx/transitive_includes/cxx23.csv
@@ -83,6 +83,9 @@ chrono string_view
 chrono vector
 chrono version
 cinttypes cstdint
+cmath cstddef
+cmath cstdint
+cmath initializer_list
 cmath limits
 cmath version
 codecvt cctype

diff --git a/libcxx/test/libcxx/transitive_includes/cxx26.csv b/libcxx/test/libcxx/transitive_includes/cxx26.csv
@@ -83,6 +83,9 @@ chrono string_view
 chrono vector
 chrono version
 cinttypes cstdint
+cmath cstddef
+cmath cstdint
+cmath initializer_list
 cmath limits
 cmath version
 codecvt cctype

diff --git a/libcxx/test/std/numerics/c.math/cmath.pass.cpp b/libcxx/test/std/numerics/c.math/cmath.pass.cpp
@@ -12,14 +12,17 @@
 
 // <cmath>
 
+#include <array>
 #include <cmath>
 #include <limits>
 #include <type_traits>
 #include <cassert>
 
+#include "fp_compare.h"
 #include "test_macros.h"
 #include "hexfloat.h"
 #include "truncate_fp.h"
+#include "type_algorithms.h"
 
 // convertible to int/float/double/etc
 template <class T, int N=0>
@@ -1113,6 +1116,56 @@ void test_fmin()
     assert(std::fmin(1,0) == 0);
 }
 
+#if TEST_STD_VER >= 17
+struct TestHypot3 {
+  template <class Real>
+  void operator()() const {
+    const auto check = [](Real elem, Real abs_tol) {
+      assert(std::isfinite(std::hypot(elem, Real(0), Real(0))));
+      assert(fptest_close(std::hypot(elem, Real(0), Real(0)), elem, abs_tol));
+      assert(std::isfinite(std::hypot(elem, elem, Real(0))));
+      assert(fptest_close(std::hypot(elem, elem, Real(0)), std::sqrt(Real(2)) * elem, abs_tol));
+      assert(std::isfinite(std::hypot(elem, elem, elem)));
+      assert(fptest_close(std::hypot(elem, elem, elem), std::sqrt(Real(3)) * elem, abs_tol));
+    };
+
+    { // check for overflow
+      const auto [elem, abs_tol] = []() -> std::array<Real, 2> {
+        if constexpr (std::is_same_v<Real, float>)
+          return {1e20f, 1e16f};
+        else if constexpr (std::is_same_v<Real, double>)
+          return {1e300, 1e287};
+        else { // long double
+#  if __DBL_MAX_EXP__ == __LDBL_MAX_EXP__
+          return {1e300l, 1e287l}; // 64-bit
+#  else
+          return {1e4000l, 1e3985l}; // 80- or 128-bit
+#  endif
+        }
+      }();
+      check(elem, abs_tol);
+    }
+
+    { // check for underflow
+      const auto [elem, abs_tol] = []() -> std::array<Real, 2> {
+        if constexpr (std::is_same_v<Real, float>)
+          return {1e-20f, 1e-24f};
+        else if constexpr (std::is_same_v<Real, double>)
+          return {1e-287, 1e-300};
+        else { // long double
+#  if __DBL_MAX_EXP__ == __LDBL_MAX_EXP__
+          return {1e-287l, 1e-300l}; // 64-bit
+#  else
+          return {1e-3985l, 1e-4000l}; // 80- or 128-bit
+#  endif
+        }
+      }();
+      check(elem, abs_tol);
+    }
+  }
+};
+#endif
+
 void test_hypot()
 {
     static_assert((std::is_same<decltype(std::hypot((float)0, (float)0)), float>::value), "");
@@ -1135,25 +1188,31 @@ void test_hypot()
     static_assert((std::is_same<decltype(hypot(Ambiguous(), Ambiguous())), Ambiguous>::value), "");
     assert(std::hypot(3,4) == 5);
 
-#if TEST_STD_VER > 14
-    static_assert((std::is_same<decltype(std::hypot((float)0, (float)0, (float)0)), float>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (bool)0, (float)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (unsigned short)0, (double)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (int)0, (long double)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (double)0, (long)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (long double)0, (unsigned long)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (int)0, (long long)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (int)0, (unsigned long long)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (double)0, (double)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (long double)0, (long double)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (float)0, (double)0)), double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (float)0, (long double)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((float)0, (double)0, (long double)0)), long double>::value), "");
-    static_assert((std::is_same<decltype(std::hypot((int)0, (int)0, (int)0)), double>::value), "");
-    static_assert((std::is_same<decltype(hypot(Ambiguous(), Ambiguous(), Ambiguous())), Ambiguous>::value), "");
+#if TEST_STD_VER >= 17
+    // clang-format off
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (float)0,          (float)0)),              float>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (bool)0,           (float)0)),              double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (unsigned short)0, (double)0)),             double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (int)0,            (long double)0)),        long double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (double)0,         (long)0)),               double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (long double)0,    (unsigned long)0)),      long double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (int)0,            (long long)0)),          double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (int)0,            (unsigned long long)0)), double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (double)0,         (double)0)),             double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (long double)0,    (long double)0)),        long double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (float)0,          (double)0)),             double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (float)0,          (long double)0)),        long double>));
+    static_assert((std::is_same_v<decltype(std::hypot((float)0, (double)0,         (long double)0)),        long double>));
+    static_assert((std::is_same_v<decltype(std::hypot((int)0,   (int)0,            (int)0)),                double>));
+    static_assert((std::is_same_v<decltype(hypot(Ambiguous(), Ambiguous(), Ambiguous())), Ambiguous>));
+    // clang-format on
 
     assert(std::hypot(2,3,6) == 7);
     assert(std::hypot(1,4,8) == 9);
+
+    // Check for undue over-/underflows of intermediate results.
+    // See discussion at https://github.com/llvm/llvm-project/issues/92782.
+    types::for_each(types::floating_point_types(), TestHypot3());
 #endif
 }
 

diff --git a/libcxx/test/support/fp_compare.h b/libcxx/test/support/fp_compare.h
@@ -9,39 +9,34 @@
 #ifndef SUPPORT_FP_COMPARE_H
 #define SUPPORT_FP_COMPARE_H
 
-#include <cmath>      // for std::abs
-#include <algorithm>  // for std::max
+#include <cmath>     // for std::abs
+#include <algorithm> // for std::max
 #include <cassert>
+#include <__config>
 
 // See https://www.boost.org/doc/libs/1_70_0/libs/test/doc/html/boost_test/testing_tools/extended_comparison/floating_point/floating_points_comparison_theory.html
 
-template<typename T>
-bool fptest_close(T val, T expected, T eps)
-{
-    constexpr T zero = T(0);
-    assert(eps >= zero);
+template <typename T>
+bool fptest_close(T val, T expected, T eps) {
+  _LIBCPP_CONSTEXPR T zero = T(0);
+  assert(eps >= zero);
 
-    // Handle the zero cases
-    if (eps      == zero) return val == expected;
-    if (val      == zero) return std::abs(expected) <= eps;
-    if (expected == zero) return std::abs(val)      <= eps;
+  // Handle the zero cases
+  if (eps == zero)
+    return val == expected;
+  if (val == zero)
+    return std::abs(expected) <= eps;
+  if (expected == zero)
+    return std::abs(val) <= eps;
 
-    return std::abs(val - expected) < eps
-        && std::abs(val - expected)/std::abs(val) < eps;
+  return std::abs(val - expected) < eps && std::abs(val - expected) / std::abs(val) < eps;
 }
 
-template<typename T>
-bool fptest_close_pct(T val, T expected, T percent)
-{
-    constexpr T zero = T(0);
-    assert(percent >= zero);
-
-    // Handle the zero cases
-    if (percent == zero) return val == expected;
-    T eps = (percent / T(100)) * std::max(std::abs(val), std::abs(expected));
-
-    return fptest_close(val, expected, eps);
+template <typename T>
+bool fptest_close_pct(T val, T expected, T percent) {
+  assert(percent >= T(0));
+  T eps = (percent / T(100)) * std::max(std::abs(val), std::abs(expected));
+  return fptest_close(val, expected, eps);
 }
 
-
 #endif // SUPPORT_FP_COMPARE_H