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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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11
lib/Analysis/ValueTracking.cpp
View File

@ -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).

14
test/Transforms/InstCombine/rem.ll
View File

@ -149,3 +149,17 @@ define i64 @test15(i32 %x, i32 %y) {
%urem = urem i64 %zext1, %zext0
ret i64 %urem
}
define i32 @test16(i32 %x, i32 %y) {
; CHECK: @test16
; CHECK-NEXT: [[SHR:%.*]] = lshr i32 %y, 11
; CHECK-NEXT: [[AND:%.*]] = and i32 [[SHR]], 4
; CHECK-NEXT: [[OR:%.*]] = or i32 [[AND]], 3
; CHECK-NEXT: [[REM:%.*]] = and i32 [[OR]], %x
; CHECK-NEXT: ret i32 [[REM]]
%shr = lshr i32 %y, 11
%and = and i32 %shr, 4
%add = add i32 %and, 4
%rem = urem i32 %x, %add
ret i32 %rem
}