[llvm] [DAG] SimplifyDemandedBits - ICMP_SLT(X, 0) - only sign mask of X is required (PR #164946)

[Craig Topper via llvm-commits]

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@@ -1754,24 +1754,31 @@ bool TargetLowering::SimplifyDemandedBits(
     SDValue Op0 = Op.getOperand(0);
     SDValue Op1 = Op.getOperand(1);
     ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
-    // If (1) we only need the sign-bit, (2) the setcc operands are the same
-    // width as the setcc result, and (3) the result of a setcc conforms to 0 or
-    // -1, we may be able to bypass the setcc.
-    if (DemandedBits.isSignMask() &&
-        Op0.getScalarValueSizeInBits() == BitWidth &&
-        getBooleanContents(Op0.getValueType()) ==
-            BooleanContent::ZeroOrNegativeOneBooleanContent) {
-      // If we're testing X < 0, then this compare isn't needed - just use X!
-      // FIXME: We're limiting to integer types here, but this should also work
-      // if we don't care about FP signed-zero. The use of SETLT with FP means
-      // that we don't care about NaNs.
-      if (CC == ISD::SETLT && Op1.getValueType().isInteger() &&
-          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode())))
-        return TLO.CombineTo(Op, Op0);
-
-      // TODO: Should we check for other forms of sign-bit comparisons?
-      // Examples: X <= -1, X >= 0
-    }
+    // If we're testing X < 0 or X >= 0 then we only need the sign mask of the
+    // previous result
+    // FIXME: We're limiting to integer types here, but this should also work
+    // if we don't care about FP signed-zero. The use of SETLT with FP means
+    // that we don't care about NaNs.
+    if ((CC == ISD::SETLT || CC == ISD::SETGE) &&
+        Op1.getValueType().isInteger() && isNullOrNullSplat(Op1)) {
+      KnownBits KnownOp0;
+      bool Changed = false;
+      if (SimplifyDemandedBits(
+              Op0, APInt::getSignMask(Op0.getScalarValueSizeInBits()),
+              DemandedElts, KnownOp0, TLO, Depth + 1))
+        Changed = true;
+      // If (1) we only need the sign-bit, (2) the setcc operands are the same
+      // width as the setcc result, and (3) the result of a setcc conforms to 0
+      // or -1, we may be able to bypass the setcc.
+      if (DemandedBits.isSignMask() &&
+          Op0.getScalarValueSizeInBits() == BitWidth &&
+          getBooleanContents(Op0.getValueType()) ==
+              BooleanContent::ZeroOrNegativeOneBooleanContent)
+        Changed |= TLO.CombineTo(Op, Op0);
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topperc wrote:

You can't removed a SETGE. You have to replace it with XOR.

https://github.com/llvm/llvm-project/pull/164946