[llvm] [ValueTracking] Add a special overflow check on multiplied by power-of-two values (PR #212594)

[Yingwei Zheng via llvm-commits]

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@@ -7542,6 +7542,23 @@ OverflowResult llvm::computeOverflowForSignedMul(const Value *LHS,
     if (LHSKnown.isNonNegative() || RHSKnown.isNonNegative())
       return OverflowResult::NeverOverflows;
   }
+
+  // Special case when we're multiplying a power-of-two value. In this case,
+  // we will never overflow as long as the shift amount is less than the number
+  // of sign bits.
+  if (isKnownToBeAPowerOfTwo(RHS, /*OrZero=*/false, SQ)) {
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dtcxzyw wrote:

Does the motivating case contain a variable RHS? Can we just handle the constant RHS case?


https://github.com/llvm/llvm-project/pull/212594