Parts of std::numeric_limits<_BitInt(N)> broken

[QrczakMK]

std::numeric_limits<_BitInt(N)>::digits and std::numeric_limits<unsigned _BitInt(N)>::digits use sizeof to infer the range, and thus yield incorrect results if there are padding bits.

This causes std::numeric_limits<_BitInt(N)>::min() and max() to fail the build if there are padding bits, because they try to constexpr-compute a value which does not fit in the type.

Demo: https://godbolt.org/z/4bbb6YaPa

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Marcin Kowalczyk (QrczakMK)

`std::numeric_limits<_BitInt(N)>::digits` and `std::numeric_limits<unsigned _BitInt(N)>::digits` use `sizeof` to infer the range, and thus yield incorrect results if there are padding bits.

This causes std::numeric_limits<_BitInt(N)>::min() and max() to fail the build if there are padding bits, because they try to constexpr-compute a value which does not fit in the type.

Demo: https://godbolt.org/z/4bbb6YaPa

[frederick-vs-ja]

I think we can fix this issue like this. But I'm not sure whether maintainers want such fix.

Outdated

diff --git a/libcxx/include/limits b/libcxx/include/limits
index 1c1a0b29b718..f122671196c2 100644
--- a/libcxx/include/limits
+++ b/libcxx/include/limits
@@ -178,6 +178,22 @@ protected:
   static _LIBCPP_CONSTEXPR const float_round_style round_style = round_toward_zero;
 };
 
+template <class _Tp, bool _IsSigned>
+struct __libcpp_compute_integer_digits {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(sizeof(_Tp) * __CHAR_BIT__ - _IsSigned);
+};
+
+#  ifdef _LIBCPP_COMPILER_CLANG_BASED
+template <unsigned int _Digits, bool _IsSigned>
+struct __libcpp_compute_integer_digits<_BitInt(_Digits), _IsSigned> {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(_Digits) - 1;
+};
+template <unsigned int _Digits, bool _IsSigned>
+struct __libcpp_compute_integer_digits<unsigned _BitInt(_Digits), _IsSigned> {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(_Digits);
+};
+#  endif
+
 template <class _Tp, int __digits, bool _IsSigned>
 struct __libcpp_compute_min {
   static _LIBCPP_CONSTEXPR const _Tp value = _Tp(_Tp(1) << __digits);
@@ -196,7 +212,7 @@ protected:
   static _LIBCPP_CONSTEXPR const bool is_specialized = true;
 
   static _LIBCPP_CONSTEXPR const bool is_signed   = type(-1) < type(0);
-  static _LIBCPP_CONSTEXPR const int digits       = static_cast<int>(sizeof(type) * __CHAR_BIT__ - is_signed);
+  static _LIBCPP_CONSTEXPR const int digits       = __libcpp_compute_integer_digits<type, is_signed>::value;
   static _LIBCPP_CONSTEXPR const int digits10     = digits * 3 / 10;
   static _LIBCPP_CONSTEXPR const int max_digits10 = 0;
   static _LIBCPP_CONSTEXPR const type __min       = __libcpp_compute_min<type, digits, is_signed>::value;

diff --git a/libcxx/include/limits b/libcxx/include/limits
index 1c1a0b29b718..f3f17afebb22 100644
--- a/libcxx/include/limits
+++ b/libcxx/include/limits
@@ -188,6 +188,22 @@ struct __libcpp_compute_min<_Tp, __digits, false> {
   static _LIBCPP_CONSTEXPR const _Tp value = _Tp(0);
 };
 
+template <class _Tp, bool _IsSigned>
+struct __libcpp_compute_integer_digits {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(sizeof(_Tp) * __CHAR_BIT__ - _IsSigned);
+};
+
+#  ifdef _LIBCPP_COMPILER_CLANG_BASED
+template <unsigned int _Digits>
+struct __libcpp_compute_integer_digits<_BitInt(_Digits), true> {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(_Digits) - 1;
+};
+template <unsigned int _Digits>
+struct __libcpp_compute_integer_digits<unsigned _BitInt(_Digits), false> {
+  static _LIBCPP_CONSTEXPR const int value = static_cast<int>(_Digits);
+};
+#  endif
+
 template <class _Tp>
 class __libcpp_numeric_limits<_Tp, true> {
 protected:
@@ -195,9 +211,11 @@ protected:
 
   static _LIBCPP_CONSTEXPR const bool is_specialized = true;
 
-  static _LIBCPP_CONSTEXPR const bool is_signed   = type(-1) < type(0);
-  static _LIBCPP_CONSTEXPR const int digits       = static_cast<int>(sizeof(type) * __CHAR_BIT__ - is_signed);
-  static _LIBCPP_CONSTEXPR const int digits10     = digits * 3 / 10;
+  static _LIBCPP_CONSTEXPR const bool is_signed = type(-1) < type(0);
+  static _LIBCPP_CONSTEXPR const int digits     = __libcpp_compute_integer_digits<type, is_signed>::value;
+  static _LIBCPP_CONSTEXPR const int digits10   = // works for digits in [1, 8388608]
+      static_cast<int>((digits * 330985980542LL) >> 40) - (digits == 5456722 || digits == 5781869 || digits == 6107016);
   static _LIBCPP_CONSTEXPR const int max_digits10 = 0;
   static _LIBCPP_CONSTEXPR const type __min       = __libcpp_compute_min<type, digits, is_signed>::value;
   static _LIBCPP_CONSTEXPR const type __max       = is_signed ? type(type(~0) ^ __min) : type(~0);

[QrczakMK]

Furthermore, the computation of digits10 as digits * 3 / 10 is inaccurate for larger bit widths. For example std::numeric_limits<unsigned _BitInt(256)>::digits10 is 76, should be 77.

I believe digits10 = static_cast<int>(digits * /*log10(2)=*/0.3010299956639812) is accurate for all supported bit widths.

[philnik777]

To fix this we'd have to add a specialization for _BitInt, or address #108402 (which I would prefer).