[llvm] [ValueTracking] Compute known bits of or/and/xor recurrences from start and step (PR #222334)

[Ömer Sinan Ağacan via llvm-commits]

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@@ -1919,8 +1920,17 @@ static void computeKnownBitsFromOperator(const Operator *I,
         RecQ.CxtI = LatchTerm;
         computeKnownBits(Step, DemandedElts, KnownStep, RecQ, Depth + 1);
 
-        Known.Zero.setLowBits(std::min(KnownStart.countMinTrailingZeros(),
-                                       KnownStep.countMinTrailingZeros()));
+        if (Opcode == Instruction::Or || Opcode == Instruction::Xor) {
+          Known.Zero |= KnownStart.Zero & KnownStep.Zero;
+          if (Opcode == Instruction::Or)
+            Known.One |= KnownStart.One;
+        } else if (Opcode == Instruction::And) {
+          Known.Zero |= KnownStart.Zero;
+          Known.One |= KnownStart.One & KnownStep.One;
+        } else {
+          Known.Zero.setLowBits(std::min(KnownStart.countMinTrailingZeros(),
+                                         KnownStep.countMinTrailingZeros()));
+        }
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osa1 wrote:

I think the code becomes easier to read and understand if you move `and`, `or`, `xor` cases to their own `case` blocks instead of having them with `add`, `sub` etc. and then special case here. I wasn't aware of this PR and implemented the `and` and `or` handling that way in https://github.com/llvm/llvm-project/pull/225775 if you want to compare.

https://github.com/llvm/llvm-project/pull/222334