[PATCH 2/3] libstdc++: Fix up std::generate_canonical for 32-bit arches

[Jonathan Wakely]

Make use of __detail::_Select_uint_least_t<d>::type for
std::generate_canonical, so that we have a 128-bit integer type even on
32-bit targets.

libstdc++-v3/ChangeLog:

	* include/bits/random.tcc (__generate_canonical_pow2): Elide
	variable into poopcount call. Cast floating-point literal to
	_RealT.
	(__generate_canonical_any): Use direct-initialization for _UInt
	variables. Cast floating-point literal to _RealT.
	(generate_canonical): Remove unused typedef. Use
	_Select_uint_least_t to select the unsigned integer type to use.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
---

Tested x86_64-linux
Also checked -m32 and -D_GLIBCXX_USE_OLD_GENERATE_CANONICAL

 libstdc++-v3/include/bits/random.tcc | 51 +++++++---------------------
 1 file changed, 12 insertions(+), 39 deletions(-)

diff --git a/libstdc++-v3/include/bits/random.tcc b/libstdc++-v3/include/bits/random.tcc
index 68eae419b721..6d9ce7d6a623 100644
--- a/libstdc++-v3/include/bits/random.tcc
+++ b/libstdc++-v3/include/bits/random.tcc
@@ -3522,10 +3522,9 @@ namespace __detail
       // r = 2;  // Redundant, we only support radix 2.
       using _Rng = decltype(_Urbg::max());
       const _Rng __rng_range_less_1 = _Urbg::max() - _Urbg::min();
-      const _UInt __uint_range_less_1 = ~_UInt(0);
       // R = _UInt(__rng_range_less_1) + 1;  // May wrap to 0.
       const auto __log2_R = __builtin_popcountg(__rng_range_less_1);
-      const auto __log2_uint_max = __builtin_popcountg(__uint_range_less_1);
+      const auto __log2_uint_max = __builtin_popcountg(~_UInt(0));
       // rd = _UInt(1) << __d;  // Could overflow, UB.
       const unsigned __k = (__d + __log2_R - 1) / __log2_R;
       const unsigned __log2_Rk_max = __k * __log2_R;
@@ -3533,7 +3532,7 @@ namespace __detail
 	__log2_uint_max < __log2_Rk_max ? __log2_uint_max : __log2_Rk_max;
       static_assert(__log2_Rk >= __d);
       // Rk = _UInt(1) << __log2_Rk;  // Likely overflows, UB.
-      constexpr _RealT __Rk = _RealT(_UInt(1) << (__log2_Rk - 1)) * 2.0;
+      constexpr _RealT __Rk = _RealT(_UInt(1) << (__log2_Rk - 1)) * _RealT(2.0);
 #if defined(_GLIBCXX_GENERATE_CANONICAL_STRICT)
       const unsigned __log2_x = __log2_Rk - __d; // # of spare entropy bits.
 #else
@@ -3586,25 +3585,25 @@ namespace __detail
     _RealT
     __generate_canonical_any(_Urbg& __urng)
     {
-      static_assert(__d < __builtin_popcountg(~_UInt(0)));
       // Names below are chosen to match the description in the Standard.
       // Parameter d is the actual target number of bits.
-      const _UInt __r = 2;
+      const _UInt __r{2};
       const _UInt __R = _UInt(_Urbg::max() - _Urbg::min()) + 1;
       const _UInt __rd = _UInt(1) << __d;
       const unsigned __k = __gen_can_rng_calls_needed(__R, __rd);
       const _UInt __Rk = __gen_can_pow(__R, __k);
-      const _UInt __x =  __Rk/__rd;
+      const _UInt __x =  __Rk / __rd;
 
       while (true)
 	{
-	  _UInt __Ri = 1, __sum = __urng() - _Urbg::min();
+	  _UInt __Ri{1};
+	  _UInt __sum{__urng() - _Urbg::min()};
 	  for (int __i = __k - 1; __i > 0; --__i)
 	    {
 	      __Ri *= __R;
-	      __sum += (__urng() - _Urbg::min()) * __Ri;
+	      __sum += _UInt{__urng() - _Urbg::min()} * __Ri;
 	    }
-	  const _RealT __ret = _RealT(__sum / __x) / __rd;
+	  const _RealT __ret = _RealT(__sum / __x) / _RealT(__rd);
 	  if (__ret < _RealT(1.0))
 	    return __ret;
 	}
@@ -3660,7 +3659,6 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	"float16_t type is not supported, consider using bfloat16_t");
 #endif
 
-      using _Rng = decltype(_Urbg::max());
       const unsigned __d_max = std::numeric_limits<_RealT>::digits;
       const unsigned __d = __digits > __d_max ? __d_max : __digits;
 
@@ -3668,38 +3666,13 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
       // is enough bits. If not, we need more.
       if constexpr (__is_power_of_2_less_1(_Urbg::max() - _Urbg::min()))
 	{
-	  if constexpr (__d <= 32)
-	    return __generate_canonical_pow2<_RealT, uint32_t, __d>(__urng);
-	  else if constexpr (__d <= 64)
-	    return __generate_canonical_pow2<_RealT, uint64_t, __d>(__urng);
-	  else
-	    {
-#if defined(__SIZEOF_INT128__)
-	      // Accommodate double double or float128.
-	      return __extension__ __generate_canonical_pow2<
-		_RealT, unsigned __int128, __d>(__urng);
-#else
-	      static_assert(false,
-		"float precision >64 requires __int128 support");
-#endif
-	    }
+	  using type = typename __detail::_Select_uint_least_t<__d>::type;
+	  return __generate_canonical_pow2<_RealT, type, __d>(__urng);
 	}
       else // Need up to 2x bits.
 	{
-	  if constexpr (__d <= 32)
-	    return __generate_canonical_any<_RealT, uint64_t, __d>(__urng);
-	  else
-	    {
-#if defined(__SIZEOF_INT128__)
-	      static_assert(__d <= 64,
-		"irregular RNG with float precision >64 is not supported");
-	      return __extension__ __generate_canonical_any<
-		_RealT, unsigned __int128, __d>(__urng);
-#else
-	      static_assert(false, "irregular RNG with float precision"
-		 " >32 requires __int128 support");
-#endif
-	    }
+	  using type = typename __detail::_Select_uint_least_t<__d * 2>::type;
+	  return __generate_canonical_any<_RealT, type, __d>(__urng);
 	}
     }
 _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
-- 
2.52.0