[APInt] Allow GreatestCommonDivisor to take rvalue inputs efficiently. Use moves instead of copies in the loop.

[craig.topper]

Summary

GreatestComonDivisor currently makes a copy of both its inputs. Then in the loop we do one move and two copies, plus any allocation the urem call does.

This patch changes it to take its inputs by value so that we can do a move of any rvalue inputs instead of copying. Then in the loop we do 3 move assignments and no copies. This way the only possible allocations we have in the loop is from the urem call.

[dblaikie]

Looks fine (:

include/llvm/ADT/APInt.h

inline bool isShiftedMask(const APInt &APIVal) {
   return (APIVal != 0) && isMask((APIVal - 1) | APIVal);
 }
 
 /// \brief Compute GCD of two APInt values.
 ///
 /// This function returns the greatest common divisor of the two APInt values
 /// using Euclid's algorithm.
 ///
-/// \returns the greatest common divisor of Val1 and Val2
-APInt GreatestCommonDivisor(const APInt &Val1, const APInt &Val2);
+/// \returns the greatest common divisor of \param A and \param B.
+APInt GreatestCommonDivisor(APInt A, APInt B);
 
 /// \brief Converts the given APInt to a double value.
 ///
 /// Treats the APInt as an unsigned value for conversion purposes.
 inline double RoundAPIntToDouble(const APInt &APIVal) {
   return APIVal.roundToDouble();
 }
 

lib/Support/APInt.cpp

for ((Val = Val.lshr(1)); Val != 0; (Val = Val.lshr(1))) {
     Reversed |= (Val & One);
     --S;
   }
 
   Reversed <<= S;
   return Reversed;
 }
 
-APInt llvm::APIntOps::GreatestCommonDivisor(const APInt& API1,
-                                            const APInt& API2) {
-  APInt A = API1, B = API2;
+APInt llvm::APIntOps::GreatestCommonDivisor(APInt A, APInt B) {
   while (!!B) {
-    APInt T = B;
-    B = A.urem(B);
-    A = T;
+    APInt R = A.urem(B);
+    A = std::move(B);
+    B = std::move(R);
   }
   return A;
 }
 
 APInt llvm::APIntOps::RoundDoubleToAPInt(double Double, unsigned width) {
   union {
     double D;
     uint64_t I;