[llvm] [LLVM] Improve the DemandedBits Analysis (PR #148880)

[Craig Topper via llvm-commits]

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@@ -197,6 +209,19 @@ void DemandedBits::determineLiveOperandBits(
         // (they must be zero).
         if (cast<LShrOperator>(UserI)->isExact())
           AB |= APInt::getLowBitsSet(BitWidth, ShiftAmt);
+      } else {
+        ComputeKnownBits(BitWidth, UserI->getOperand(1), nullptr);
+        unsigned Min = Known.getMinValue().getLimitedValue(BitWidth - 1);
+        unsigned Max = Known.getMaxValue().getLimitedValue(BitWidth - 1);
+        // Suppose AOut == 0b0000 0011
+        // [min, max] = [1, 3]
+        // shift by 1 we get 0b0000 0110
+        // shift by 2 we get 0b0000 1100
+        // shift by 3 we get 0b0001 1000
+        // we take the or here because need to cover all the above possibilities
+        AB = (AOut.shl(Min) | AOut.shl(Max));
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topperc wrote:

Doesn't this need to be the OR of all possible shift amounts between Min and Max? Not just the end points.

https://github.com/llvm/llvm-project/pull/148880