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

[Yingwei Zheng via llvm-commits]

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@@ -76,6 +76,15 @@ void DemandedBits::determineLiveOperandBits(
           computeKnownBits(V2, Known2, DL, &AC, UserI, &DT);
         }
       };
+  auto GetShiftedRange = [&](unsigned Min, unsigned Max, bool ShiftLeft) {
+    using ShiftFn = APInt (APInt::*)(unsigned) const;
+    auto Shift = ShiftLeft ? static_cast<ShiftFn>(&APInt::shl)
+                           : static_cast<ShiftFn>(&APInt::lshr);
+    AB = APInt::getZero(BitWidth);
+    for (unsigned ShiftAmount = Min; ShiftAmount <= Max; ++ShiftAmount) {
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dtcxzyw wrote:

We should convert it into an O(LlogL) algorithm with the [Exponentiation by squaring](https://en.wikipedia.org/wiki/Exponentiation_by_squaring) trick. In the current implementation, we may need 128 shifts + 128 ors for i128.

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