Optimize ZERO_EXTEND and SIGN_EXTEND in both SelectionDAG Builder and type

legalization stage. With those two optimizations, fewer signed/zero extension
instructions can be inserted, and then we can expose more opportunities to
Machine CSE pass in back-end.

llvm-svn: 216066

1 files changed, 26 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index e264cd9ae4a..1ca496bc3cf 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -757,6 +757,28 @@ SDValue RegsForValue::getCopyFromRegs(SelectionDAG &DAG,
   return DAG.getNode(ISD::MERGE_VALUES, dl, DAG.getVTList(ValueVTs), Values);
 }
 
+static ISD::NodeType getPreferredExtendForValue(const Value *V) {
+  // For the users of the source value being used for compare instruction, if
+  // the number of signed predicate is greater than unsigned predicate, we
+  // prefer to use SIGN_EXTEND.
+  //
+  // With this optimization, we would be able to reduce some redundant sign or
+  // zero extension instruction, and eventually more machine CSE opportunities
+  // can be exposed.
+  ISD::NodeType ExtendKind = ISD::ANY_EXTEND;
+  unsigned int NumOfSigned = 0, NumOfUnsigned = 0;
+  for (const User *U : V->users()) {
+    if (const CmpInst *CI = dyn_cast<CmpInst>(U)) {
+      NumOfSigned += CI->isSigned();
+      NumOfUnsigned += CI->isUnsigned();
+    }
+  }
+  if (NumOfSigned > NumOfUnsigned)
+    ExtendKind = ISD::SIGN_EXTEND;
+
+  return ExtendKind;
+}
+
 /// getCopyToRegs - Emit a series of CopyToReg nodes that copies the
 /// specified value into the registers specified by this object.  This uses
 /// Chain/Flag as the input and updates them for the output Chain/Flag.
@@ -765,6 +787,7 @@ void RegsForValue::getCopyToRegs(SDValue Val, SelectionDAG &DAG, SDLoc dl,
                                  SDValue &Chain, SDValue *Flag,
                                  const Value *V) const {
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  ISD::NodeType ExtendKind = getPreferredExtendForValue(V);
 
   // Get the list of the values's legal parts.
   unsigned NumRegs = Regs.size();
@@ -773,8 +796,9 @@ void RegsForValue::getCopyToRegs(SDValue Val, SelectionDAG &DAG, SDLoc dl,
     EVT ValueVT = ValueVTs[Value];
     unsigned NumParts = TLI.getNumRegisters(*DAG.getContext(), ValueVT);
     MVT RegisterVT = RegVTs[Value];
-    ISD::NodeType ExtendKind =
-      TLI.isZExtFree(Val, RegisterVT)? ISD::ZERO_EXTEND: ISD::ANY_EXTEND;
+
+    if (ExtendKind == ISD::ANY_EXTEND && TLI.isZExtFree(Val, RegisterVT))
+      ExtendKind = ISD::ZERO_EXTEND;
 
     getCopyToParts(DAG, dl, Val.getValue(Val.getResNo() + Value),
                    &Parts[Part], NumParts, RegisterVT, V, ExtendKind);