[APFloat] Remove dead store in the move constructor of IEEEFloat

[vgao]

Summary

In the move constructor, field semantics is first set to &semBogus, which is a static constant.
It is then set to rhs.semantics, killing the first store. This dead store is not removed in -O3.

freeSignificand() is called between the two stores, and it does read from the field semantics. However with semantics set to &semBogus, this operation always evaluates to nothing.

This diff adds a helper function (moveFields) that copies the fields without calling freeSignificand().
By doing this we save a store (and potentially a call to freeSignicand()) in IEEEFloat(IEEEFloat &&), and do not lose any performance in operator =(IEEEFloat &&), since moveFields is inlined in both callsites.

[vgao]

fix path

llvm/trunk/include/llvm/ADT/APFloat.h

   void copySignificand(const IEEEFloat &);
   void freeSignificand();
 
+  void moveFields(IEEEFloat &&);
+
   /// Note: this must be the first data member.
   /// The semantics that this value obeys.
   const fltSemantics *semantics;

llvm/trunk/lib/Support/APFloat.cpp

   return *this;
 }
 
-IEEEFloat &IEEEFloat::operator=(IEEEFloat &&rhs) {
-  freeSignificand();
-
+void IEEEFloat::moveFields(IEEEFloat &&rhs) {
   semantics = rhs.semantics;
   significand = rhs.significand;
   exponent = rhs.exponent;

   sign = rhs.sign;
 
   rhs.semantics = &semBogus;
+}
+
+IEEEFloat &IEEEFloat::operator=(IEEEFloat &&rhs) {
+  freeSignificand();
+  moveFields(std::move(rhs));
   return *this;
 }
 

   assign(rhs);
 }
 
-IEEEFloat::IEEEFloat(IEEEFloat &&rhs) : semantics(&semBogus) {
-  *this = std::move(rhs);
-}
+IEEEFloat::IEEEFloat(IEEEFloat &&rhs) { moveFields(std::move(rhs)); }
 
 IEEEFloat::~IEEEFloat() { freeSignificand(); }