[JSON] Work around excess-precision issue when comparing T_Integer numbers.

[mgorny]

I can confirm that this fixes the issue for me.

llvm/trunk/include/llvm/Support/JSON.h

enum ValueType : char {
     T_Object,
     T_Array,
   };
   // All members mutable, see moveFrom().
   mutable ValueType Type;
   mutable llvm::AlignedCharArrayUnion<bool, double, int64_t, llvm::StringRef,
                                       std::string, json::Array, json::Object>
       Union;
+  friend bool operator==(const Value &, const Value &);
 };
 
 bool operator==(const Value &, const Value &);
 inline bool operator!=(const Value &L, const Value &R) { return !(L == R); }
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const Value &);
 
 /// ObjectKey is a used to capture keys in Object. Like Value but:
 ///   - only strings are allowed

llvm/trunk/lib/Support/JSON.cpp

bool operator==(const Value &L, const Value &R) {
   if (L.kind() != R.kind())
     return false;
   switch (L.kind()) {
   case Value::Null:
     return *L.getAsNull() == *R.getAsNull();
   case Value::Boolean:
     return *L.getAsBoolean() == *R.getAsBoolean();
   case Value::Number:
+    // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=323
+    // The same integer must convert to the same double, per the standard.
+    // However we see 64-vs-80-bit precision comparisons with gcc-7 -O3 -m32.
+    // So we avoid floating point promotion for exact comparisons.
+    if (L.Type == Value::T_Integer || R.Type == Value::T_Integer)
+      return L.getAsInteger() == R.getAsInteger();
     return *L.getAsNumber() == *R.getAsNumber();
   case Value::String:
     return *L.getAsString() == *R.getAsString();
   case Value::Array:
     return *L.getAsArray() == *R.getAsArray();
   case Value::Object:
     return *L.getAsObject() == *R.getAsObject();
   }