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isKnownToBeAPowerOfTwo: (X & Y) + Y is a power of 2 or zero if y is also.

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This is useful if something that looks like (x & (1 << y)) ? 64 : 32 is
the divisor in a modulo operation.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182200 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2013-05-18 19:30:37 +00:00
parent 688b5103eb
commit cb9d4667b7
2 changed files with 25 additions and 0 deletions
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lib/Analysis/ValueTracking.cpp
+11
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@@ -855,6 +855,17 @@ bool llvm::isKnownToBeAPowerOfTwo(Value *V, bool OrZero, unsigned Depth) {
return false;
}
// Adding a power of two to the same power of two is a power of two or zero.
if (OrZero && match(V, m_Add(m_Value(X), m_Value(Y)))) {
if (match(X, m_And(m_Value(), m_Specific(Y)))) {
if (isKnownToBeAPowerOfTwo(Y, /*OrZero*/true, Depth))
return true;
} else if (match(Y, m_And(m_Value(), m_Specific(X)))) {
if (isKnownToBeAPowerOfTwo(X, /*OrZero*/true, Depth))
return true;
}
}
// An exact divide or right shift can only shift off zero bits, so the result
// is a power of two only if the first operand is a power of two and not
// copying a sign bit (sdiv int_min, 2).