2 files changed, 41 insertions, 2 deletions
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
index 3812823..9b9a6a2 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
@@ -2013,6 +2013,43 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
     FirstAnswer = std::min(Src1NumSignBits, Src2NumSignBits) - 1;
     break;
   }
+  case TargetOpcode::G_ADD: {
+    Register Src2 = MI.getOperand(2).getReg();
+    unsigned Src2NumSignBits =
+        computeNumSignBits(Src2, DemandedElts, Depth + 1);
+    if (Src2NumSignBits <= 2)
+      return 1; // Early out.
+
+    Register Src1 = MI.getOperand(1).getReg();
+    unsigned Src1NumSignBits =
+        computeNumSignBits(Src1, DemandedElts, Depth + 1);
+    if (Src1NumSignBits == 1)
+      return 1; // Early Out.
+
+    // Special case decrementing a value (ADD X, -1):
+    KnownBits Known2 = getKnownBits(Src2, DemandedElts, Depth);
+    if (Known2.isAllOnes()) {
+      KnownBits Known1 = getKnownBits(Src1, DemandedElts, Depth);
+      // If the input is known to be 0 or 1, the output is 0/-1, which is all
+      // sign bits set.
+      if ((Known1.Zero | 1).isAllOnes())
+        return TyBits;
+
+      // If we are subtracting one from a positive number, there is no carry
+      // out of the result.
+      if (Known1.isNonNegative()) {
+        FirstAnswer = Src1NumSignBits;
+        break;
+      }
+
+      // Otherwise, we treat this like an ADD.
+    }
+
+    // Add can have at most one carry bit.  Thus we know that the output
+    // is, at worst, one more bit than the inputs.
+    FirstAnswer = std::min(Src1NumSignBits, Src2NumSignBits) - 1;
+    break;
+  }
   case TargetOpcode::G_FCMP:
   case TargetOpcode::G_ICMP: {
     bool IsFP = Opcode == TargetOpcode::G_FCMP;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index c9aeef7..876066e9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5063,8 +5063,7 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     break;
   case ISD::ADD:
   case ISD::ADDC:
-    // Add can have at most one carry bit.  Thus we know that the output
-    // is, at worst, one more bit than the inputs.
+    // TODO: Move Operand 1 check before Operand 0 check
     Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
     if (Tmp == 1) return 1; // Early out.
 
@@ -5088,6 +5087,9 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
 
     Tmp2 = ComputeNumSignBits(Op.getOperand(1), DemandedElts, Depth + 1);
     if (Tmp2 == 1) return 1; // Early out.
+
+    // Add can have at most one carry bit.  Thus we know that the output
+    // is, at worst, one more bit than the inputs.
     return std::min(Tmp, Tmp2) - 1;
   case ISD::SUB:
     Tmp2 = ComputeNumSignBits(Op.getOperand(1), DemandedElts, Depth + 1);