[llvm] [SelectionDAG] Add computeKnownBits support for ISD::STEP_VECTOR (PR #80452)

[Luke Lau via llvm-commits]

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@@ -3110,6 +3110,19 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
     }
     break;
   }
+  case ISD::STEP_VECTOR: {
+    const APInt &Step = Op.getConstantOperandAPInt(0);
+
+    if (Step.isPowerOf2())
+      Known.Zero.setLowBits(Step.logBase2());
+
+    const Function &F = getMachineFunction().getFunction();
+    const APInt MaxNumElts = getVScaleRange(&F, BitWidth).getUnsignedMax() *
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lukel97 wrote:

Yeah, I did a quick check that * does indeed perform wrapping. But forgot to actually bail if it overflows.

https://github.com/llvm/llvm-project/pull/80452