Add code that checks for noop copies, which triggers when either:

1. a target doesn't know how to fold load/stores into copies, or
2. the spiller rewrites the input to a copy to the same register as the dest
   instead of to the reloaded reg.

This will be moved/improved in the near future, but allows elimination of
some ancient x86 hacks.  This eliminates 92 copies from SMG2000 on X86 and
163 copies from 252.eon.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25922 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/VirtRegMap.cpp

@@ -38,6 +38,7 @@ namespace {
   Statistic<> NumLoads ("spiller", "Number of loads added");
   Statistic<> NumReused("spiller", "Number of values reused");
   Statistic<> NumDSE   ("spiller", "Number of dead stores elided");
+  Statistic<> NumDCE   ("spiller", "Number of copies elided");
 
   enum SpillerName { simple, local };
 
@@ -658,6 +659,18 @@ void LocalSpiller::RewriteMBB(MachineBasicBlock &MBB, const VirtRegMap &VRM) {
         }
       }
     }
+     
+    // Okay, the instruction has been completely processed, input and output 
+    // registers have been added.  As a final sanity check, make sure this is
+    // not a noop-copy.  If it is, nuke it.
+    {
+      unsigned Src, Dst;
+      if (TII->isMoveInstr(MI, Src, Dst) && Src == Dst) {
+        ++NumDCE;
+        DEBUG(std::cerr << "Removing now-noop copy: " << MI);
+        MBB.erase(&MI);
+      }
+    }
   ProcessNextInst:
     MII = NextMII;
   }