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[APInt] Fix nearestLogBase2 to return correct answers for very large APInt and APInt with a bitwidth of 1.

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I also improved the comments, added some more tests, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199610 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Michael Gottesman 2014-01-19 20:33:48 +00:00
parent e7413972a4
commit 7320de5ce2
2 changed files with 34 additions and 7 deletions
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30
include/llvm/ADT/APInt.h
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@ -1505,16 +1505,32 @@ public:
}
/// \returns the nearest log base 2 of this APInt. Ties round up.
///
/// NOTE: When we have a BitWidth of 1, we define:
///
/// log2(0) = UINT32_MAX
/// log2(1) = 0
///
/// to get around any mathematical concerns resulting from
/// referencing 2 in a space where 2 does no exist.
unsigned nearestLogBase2() const {
// This is implemented by taking the normal log 2 of a number and adding 1
// to it if MSB - 1 is set.
// Special case when we have a bitwidth of 1. If VAL is 1, then we
// get 0. If VAL is 0, we get UINT64_MAX which gets truncated to
// UINT32_MAX.
if (BitWidth == 1)
return VAL - 1;
// We follow the model from logBase2 that logBase2(0) == UINT32_MAX. This
// works since if we have 0, MSB will be 0. Then we subtract one yielding
// UINT32_MAX. Finally extractBit of MSB - 1 will be UINT32_MAX implying
// that we get BitWidth - 1.
// Handle the zero case.
if (!getBoolValue())
return UINT32_MAX;
// The non-zero case is handled by computing:
//
// nearestLogBase2(x) = logBase2(x) + x[logBase2(x)-1].
//
// where x[i] is referring to the value of the ith bit of x.
unsigned lg = logBase2();
return lg + unsigned((*this)[std::min(lg - 1, BitWidth - 1)]);
return lg + unsigned((*this)[lg - 1]);
}
/// \returns the log base 2 of this APInt if its an exact power of two, -1

11
unittests/ADT/APIntTest.cpp
View File

@ -665,6 +665,17 @@ TEST(APIntTest, nearestLogBase2) {
uint64_t I6[4] = {0x0, 0x0, 0x0, 0x18};
APInt A6(integerPartWidth*4, ArrayRef<integerPart>(I6, 4));
EXPECT_EQ(A6.nearestLogBase2(), A6.ceilLogBase2());
// Test BitWidth == 1 special cases.
APInt A7(1, 1);
EXPECT_EQ(A7.nearestLogBase2(), 0ULL);
APInt A8(1, 0);
EXPECT_EQ(A8.nearestLogBase2(), UINT32_MAX);
// Test the zero case when we have a bit width large enough such
// that the bit width is larger than UINT32_MAX-1.
APInt A9(UINT32_MAX, 0);
EXPECT_EQ(A9.nearestLogBase2(), UINT32_MAX);
}
}