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

[Panagiotis Karouzakis 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) {
+      APInt Shifted = ShiftF(Mask, ShiftAmnt);
+      Mask |= Shifted;
+    }
----------------
karouzakisp wrote:

> It looks like an overestimation. See the following case:
> 
> ```
> AOut = 00000001 Min = 0 Max = 4 ShiftLeft = true
> Expected Mask = 00011111
> 
> Mask = 00000001
> ShiftAmnt = 1 Mask = 00000011
> ShiftAmnt = 2 Mask = 00001111
> ShiftAmnt = 4 Mask = 11111111
> ```
@dtcxzyw Could you please review? Thanks


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