Add a note that we should match rlwnm better

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30508 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/README.txt

@@ -6,6 +6,40 @@ TODO:
 
 ===-------------------------------------------------------------------------===
 
+We only produce the rlwnm instruction for rotate instructions.  We should
+at least match stuff like:
+
+unsigned rot_and(unsigned X, int Y) {
+  unsigned T = (X << Y) | (X >> (32-Y));
+  T &= 127;
+  return T;
+}
+
+_foo3:
+        rlwnm r2, r3, r4, 0, 31
+        rlwinm r3, r2, 0, 25, 31
+        blr
+
+... which is the basic pattern that should be written in the instr.  It may
+also be useful for stuff like:
+
+long long foo2(long long X, int C) {
+  return X << (C&~32);
+}
+
+which currently produces:
+
+_foo2:
+        rlwinm r2, r5, 0, 27, 25
+        subfic r5, r2, 32
+        slw r3, r3, r2
+        srw r5, r4, r5
+        or r3, r3, r5
+        slw r4, r4, r2
+        blr
+
+===-------------------------------------------------------------------------===
+
 Support 'update' load/store instructions.  These are cracked on the G5, but are
 still a codesize win.