[SelectionDAG] Expand [US]CMP using arithmetic on boolean values

The previous expansion of [US]CMP was done using two selects and two
compares. It produced decent code, but on many platforms it is better
to implement [US]CMP nodes by performing the following operation:
  [us]cmp(x, y) = (x [us]> y) - (x [us]< y)
This patch adds this new expansion, as well as a hook in TargetLowering
to allow some targets to still use the select-based approach.

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -3390,6 +3390,10 @@ class TargetLoweringBase {
     return isOperationLegalOrCustom(Op, VT);
   }
 
+  /// Should we expand [US]CMP nodes using two selects and two compares, or by
+  /// doing arithmetic on boolean types
+  virtual bool shouldExpandCmpUsingSelects() const { return false; }
+
   /// Does this target support complex deinterleaving
   virtual bool isComplexDeinterleavingSupported() const { return false; }
 

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -10381,14 +10381,24 @@ SDValue TargetLowering::expandCMP(SDNode *Node, SelectionDAG &DAG) const {
 
   auto LTPredicate = (Opcode == ISD::UCMP ? ISD::SETULT : ISD::SETLT);
   auto GTPredicate = (Opcode == ISD::UCMP ? ISD::SETUGT : ISD::SETGT);
-
   SDValue IsLT = DAG.getSetCC(dl, BoolVT, LHS, RHS, LTPredicate);
   SDValue IsGT = DAG.getSetCC(dl, BoolVT, LHS, RHS, GTPredicate);
-  SDValue SelectZeroOrOne =
-      DAG.getSelect(dl, ResVT, IsGT, DAG.getConstant(1, dl, ResVT),
-                    DAG.getConstant(0, dl, ResVT));
-  return DAG.getSelect(dl, ResVT, IsLT, DAG.getConstant(-1, dl, ResVT),
-                       SelectZeroOrOne);
+
+  // We can't perform arithmetic on i1 values. Extending them would
+  // probably result in worse codegen, so let's just use two selects instead.
+  // Some targets are also just better off using selects rather than subtraction
+  // because one of the conditions can be merged with one of the selects
+  EVT BoolElVT = BoolVT.isVector() ? BoolVT.getVectorElementType() : BoolVT;
+  if (shouldExpandCmpUsingSelects() || !BoolElVT.knownBitsGT(MVT::i1)) {
+    SDValue SelectZeroOrOne =
+        DAG.getSelect(dl, ResVT, IsGT, DAG.getConstant(1, dl, ResVT),
+                      DAG.getConstant(0, dl, ResVT));
+    return DAG.getSelect(dl, ResVT, IsLT, DAG.getConstant(-1, dl, ResVT),
+                         SelectZeroOrOne);
+  }
+
+  return DAG.getSExtOrTrunc(DAG.getNode(ISD::SUB, dl, BoolVT, IsGT, IsLT), dl,
+                            ResVT);
 }
 
 SDValue TargetLowering::expandShlSat(SDNode *Node, SelectionDAG &DAG) const {

llvm/lib/Target/AArch64/AArch64ISelLowering.h

@@ -907,6 +907,8 @@ class AArch64TargetLowering : public TargetLowering {
 
   bool shouldConvertFpToSat(unsigned Op, EVT FPVT, EVT VT) const override;
 
+  bool shouldExpandCmpUsingSelects() const override { return true; }
+
   bool isComplexDeinterleavingSupported() const override;
   bool isComplexDeinterleavingOperationSupported(
       ComplexDeinterleavingOperation Operation, Type *Ty) const override;