From bc46765ddea425005a7262d03e2b2defd2106a8a Mon Sep 17 00:00:00 2001
From: Giovanni Cerretani <g.cerretani@caen.it>
Date: Thu, 17 Apr 2025 11:50:51 +0200
Subject: [PATCH 1/3] Avoid useless call to sqrt, log and _XLgamma on
 poisson_distrubution in case of small mean, since those values are never
 used.

---
 stl/inc/random | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/stl/inc/random b/stl/inc/random
index fa401b630cd..c538d9d3510 100644
--- a/stl/inc/random
+++ b/stl/inc/random
@@ -2782,10 +2782,13 @@ public:
         void _Init(_Ty1 _Mean0) noexcept { // set internal state
             _STL_ASSERT(0.0 < _Mean0, "invalid mean argument for poisson_distribution");
             _Mean = _Mean0;
+            if (_Mean0 < 12.0) {
+                _Small._Init(_Mean0);
+                return;
+            }
             _Sqrt = _CSTD sqrt(2.0 * _Mean0);
             _Logm = _CSTD log(_Mean0);
             _Gx1  = _Mean0 * _Logm - _XLgamma(_Mean0 + 1.0);
-            _Small._Init(_Mean0);
         }
 
         _Ty1 _Mean;

From 1bbe8398e1027a02c86ca969bde112bc1322c887 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Thu, 17 Apr 2025 14:49:04 -0700
Subject: [PATCH 2/3] Extract `_Small_threshold`.

---
 stl/inc/random | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/stl/inc/random b/stl/inc/random
index c538d9d3510..8fe60a755be 100644
--- a/stl/inc/random
+++ b/stl/inc/random
@@ -2779,10 +2779,12 @@ public:
             return _Mean;
         }
 
+        static constexpr double _Small_threshold = 12.0;
+
         void _Init(_Ty1 _Mean0) noexcept { // set internal state
             _STL_ASSERT(0.0 < _Mean0, "invalid mean argument for poisson_distribution");
             _Mean = _Mean0;
-            if (_Mean0 < 12.0) {
+            if (_Mean0 < _Small_threshold) {
                 _Small._Init(_Mean0);
                 return;
             }
@@ -2871,7 +2873,7 @@ public:
 private:
     template <class _Engine>
     result_type _Eval(_Engine& _Eng, const param_type& _Par0) const {
-        if (_Par0._Mean < 12.0) {
+        if (_Par0._Mean < param_type::_Small_threshold) {
             return _Par0._Small(_Eng);
         }
 

From 283f4da0f975c97feb030c00e0ff31cffdb3a78c Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@microsoft.com>
Date: Thu, 17 Apr 2025 14:53:18 -0700
Subject: [PATCH 3/3] Add data member initializers: zero is a strict subset of
 garbage.

---
 stl/inc/random | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/stl/inc/random b/stl/inc/random
index 8fe60a755be..c4fdde3f0de 100644
--- a/stl/inc/random
+++ b/stl/inc/random
@@ -2740,7 +2740,7 @@ public:
     }
 
 private:
-    _Ty1 _Gx0;
+    _Ty1 _Gx0 = 0.0;
 };
 
 _EXPORT_STD template <class _Ty = int>
@@ -2793,10 +2793,10 @@ public:
             _Gx1  = _Mean0 * _Logm - _XLgamma(_Mean0 + 1.0);
         }
 
-        _Ty1 _Mean;
-        _Ty1 _Sqrt;
-        _Ty1 _Logm;
-        _Ty1 _Gx1;
+        _Ty1 _Mean = 0.0;
+        _Ty1 _Sqrt = 0.0;
+        _Ty1 _Logm = 0.0;
+        _Ty1 _Gx1  = 0.0;
 
         _Small_poisson_distribution<_Ty> _Small;
     };