Update readme

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26924 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/README.txt

@@ -549,3 +549,37 @@ We need to codegen -0.0 vector efficiently (no constant pool load).
 When -ffast-math is on, we can use 0.0.
 
 ===-------------------------------------------------------------------------===
+
+float foo(float X) { return (int)(X); }
+
+Currently produces
+
+_foo:
+        lis r2, ha16(LCPI1_0)
+        lis r3, 17200
+        fctiwz f0, f1
+        stfd f0, -8(r1)
+        lwz r4, -4(r1)
+        xoris r4, r4, 32768
+        stw r4, -12(r1)
+        stw r3, -16(r1)
+        lfs f0, lo16(LCPI1_0)(r2)
+        lfd f1, -16(r1)
+        fsub f0, f1, f0
+        frsp f1, f0
+        blr
+
+When we have ppc64 working properly, it could produce the nicer code:
+
+_foo:
+        fctiwz f0, f1
+        stfd f0, -8(r1)
+        lwz r4, -4(r1)
+        extsh r4, r4
+        std r4, -16(r1)
+        lfd f1, -16(r1)
+        fcfid f0, f0
+        frsp f0, f0
+        blr
+
+Note: this would speed up SingleSource/Misc/pi by about 30%