[DAG] optimize negation of bool

Use mask and negate for legalization of i1 source type with SIGN_EXTEND_INREG.
With the mask, this should be no worse than 2 shifts. The mask can be eliminated
in some cases, so that should be better than 2 shifts.

This change exposed some missing folds related to negation:
https://reviews.llvm.org/rL284239
https://reviews.llvm.org/rL284395

There may be others, so please let me know if you see any regressions.

Differential Revision: https://reviews.llvm.org/D25485

llvm-svn: 284611

1 files changed, 19 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 66d80089fcb..c468a0fabc1 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -2926,10 +2926,27 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     Results.push_back(Tmp1);
     break;
   case ISD::SIGN_EXTEND_INREG: {
-    // NOTE: we could fall back on load/store here too for targets without
-    // SAR.  However, it is doubtful that any exist.
     EVT ExtraVT = cast<VTSDNode>(Node->getOperand(1))->getVT();
     EVT VT = Node->getValueType(0);
+
+    // An in-register sign-extend of a boolean is a negation:
+    // 'true' (1) sign-extended is -1.
+    // 'false' (0) sign-extended is 0.
+    // However, we must mask the high bits of the source operand because the
+    // SIGN_EXTEND_INREG does not guarantee that the high bits are already zero.
+
+    // TODO: Do this for vectors too?
+    if (ExtraVT.getSizeInBits() == 1) {
+      SDValue One = DAG.getConstant(1, dl, VT);
+      SDValue And = DAG.getNode(ISD::AND, dl, VT, Node->getOperand(0), One);
+      SDValue Zero = DAG.getConstant(0, dl, VT);
+      SDValue Neg = DAG.getNode(ISD::SUB, dl, VT, Zero, And);
+      Results.push_back(Neg);
+      break;
+    }
+
+    // NOTE: we could fall back on load/store here too for targets without
+    // SRA.  However, it is doubtful that any exist.
     EVT ShiftAmountTy = TLI.getShiftAmountTy(VT, DAG.getDataLayout());
     if (VT.isVector())
       ShiftAmountTy = VT;