Fix ReinterpretAsInt causing undefind behavior + segfaulting on release mode

mjpc/utilities.cc

@@ -15,6 +15,7 @@
 #include "mjpc/utilities.h"
 
 #include <algorithm>
+#include <bit>
 #include <cerrno>
 #include <cmath>
 #include <cstdint>
@@ -115,12 +116,16 @@ void Clamp(double* x, const double* bounds, int n) {
   }
 }
 
-int ReinterpretAsInt(double value) {
-  return *std::launder(reinterpret_cast<const int*>(&value));
+int64_t ReinterpretAsInt(double value) {
+  static_assert(sizeof(double) == sizeof(int64_t),
+                 "double and int64_t must have same size");
+  return std::bit_cast<int64_t>(value);
 }
 
 double ReinterpretAsDouble(int64_t value) {
-  return *std::launder(reinterpret_cast<const double*>(&value));
+  static_assert(sizeof(double) == sizeof(int64_t),
+                 "double and int64_t must have same size");
+  return std::bit_cast<double>(value);
 }
 
 absl::flat_hash_map<std::string, std::vector<std::string>>
@@ -225,7 +230,7 @@ int ParameterIndex(const mjModel* model, std::string_view name) {
 double DefaultResidualSelection(const mjModel* m, int numeric_index) {
   // list selections are stored as ints, but numeric values are doubles.
   int64_t value = m->numeric_data[m->numeric_adr[numeric_index]];
-  return *std::launder(reinterpret_cast<const double*>(&value));
+  return ReinterpretAsDouble(value);
 }
 
 int CostTermByName(const mjModel* m, const std::string& name) {

mjpc/utilities.h

@@ -66,8 +66,8 @@ T GetNumberOrDefault(T default_value, const mjModel* m, std::string_view name) {
   return GetNumber<T>(m, name).value_or(default_value);
 }
 
-// reinterpret double as int
-int ReinterpretAsInt(double value);
+// reinterpret double as int64_t
+int64_t ReinterpretAsInt(double value);
 
 // reinterpret int64_t as double
 double ReinterpretAsDouble(int64_t value);