[llvm] [KnownBits] Implement knownbits lshr/ashr with exact flag (PR #84254)

[Jay Foad via llvm-commits]

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@@ -367,6 +367,18 @@ KnownBits KnownBits::lshr(const KnownBits &LHS, const KnownBits &RHS,
   // Find the common bits from all possible shifts.
   APInt MaxValue = RHS.getMaxValue();
   unsigned MaxShiftAmount = getMaxShiftAmount(MaxValue, BitWidth);
+
+  // If exact, bound MaxShiftAmount to first known 1 in LHS.
+  if (Exact) {
+    unsigned FirstOne = LHS.countMaxTrailingZeros();
+    if (FirstOne < MinShiftAmount) {
+      // Always poison. Return zero because we don't like returning conflict.
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jayfoad wrote:

Not your fault, but I really don't like that we work so hard everywhere to avoid returning conflict. We should embrace conflict!

https://github.com/llvm/llvm-project/pull/84254