add the 'lucas' optimization

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25830 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/README.txt

@@ -402,3 +402,28 @@ Implement TargetConstantVec, and set up PPC to custom lower ConstantVec into
 TargetConstantVec's if it's one of the many forms that are algorithmically
 computable using the spiffy altivec instructions.
 
+===-------------------------------------------------------------------------===
+
+Compile this:
+
+double %test(double %X) {
+        %Y = cast double %X to long
+        %Z = cast long %Y to double
+        ret double %Z
+}
+
+to this:
+
+_test:
+        fctidz f0, f1
+        stfd f0, -8(r1)
+        lwz r2, -4(r1)
+        lwz r3, -8(r1)
+        stw r2, -12(r1)
+        stw r3, -16(r1)
+        lfd f0, -16(r1)
+        fcfid f1, f0
+        blr
+
+without the lwz/stw's.
+