[llvm] [DemandedBits] Support non-constant shift amounts (PR #148880)

[Yingwei Zheng via llvm-commits]

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@@ -76,6 +76,19 @@ void DemandedBits::determineLiveOperandBits(
           computeKnownBits(V2, Known2, DL, &AC, UserI, &DT);
         }
       };
+  auto GetShiftedRange = [&](uint64_t Min, uint64_t Max, bool ShiftLeft) {
+    auto ShiftF = [ShiftLeft](const APInt &Mask, unsigned ShiftAmnt) {
+      return ShiftLeft ? Mask.shl(ShiftAmnt) : Mask.lshr(ShiftAmnt);
+    };
+    AB = APInt::getZero(BitWidth);
+    uint64_t LoopRange = Max - Min;
+    APInt Mask = AOut;
+    for (unsigned ShiftAmnt = 1; ShiftAmnt <= LoopRange; ShiftAmnt <<= 1) {
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dtcxzyw wrote:

> KnownBits::shl/lshr/ashr does an intersection; we don't do an intersection, we do a union.

`~Intersection.Zeros` is a union.


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