initial step at adding a dag-to-dag isel for X86 backend.  Patch contributed
by Evan Cheng!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24371 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86TargetMachine.cpp b/lib/Target/X86/X86TargetMachine.cpp
index 4862a23..1d87152 100644
--- a/lib/Target/X86/X86TargetMachine.cpp
+++ b/lib/Target/X86/X86TargetMachine.cpp
@@ -47,6 +47,9 @@
                 cl::location(X86ScalarSSE),
                 cl::init(false));
 
+  cl::opt<bool> EnableX86DAGDAG("enable-x86-dag-isel", cl::Hidden,
+                      cl::desc("Enable DAG-to-DAG isel for X86"));
+  
   // FIXME: This should eventually be handled with target triples and
   // subtarget support!
   cl::opt<X86VectorEnum, true>
@@ -124,7 +127,10 @@
   PM.add(createUnreachableBlockEliminationPass());
 
   // Install an instruction selector.
-  PM.add(createX86ISelPattern(*this));
+  if (EnableX86DAGDAG)
+    PM.add(createX86ISelDag(*this));
+  else
+    PM.add(createX86ISelPattern(*this));
 
   // Run optional SSA-based machine code optimizations next...
   if (!NoSSAPeephole)
@@ -192,7 +198,10 @@
   PM.add(createUnreachableBlockEliminationPass());
 
   // Install an instruction selector.
-  PM.add(createX86ISelPattern(TM));
+  if (EnableX86DAGDAG)
+    PM.add(createX86ISelDag(TM));
+  else
+    PM.add(createX86ISelPattern(TM));
 
   // Run optional SSA-based machine code optimizations next...
   if (!NoSSAPeephole)