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Implement countLeadingOnes() and getMinSignedBits(). This helps to minimize 

the bit width of negative numbers by computing the minimum bit width for a
negative value. E.g. 0x1800000000000000 could be just 0x8000000000000000

llvm-svn: 34695
This commit is contained in:
Reid Spencer 2007-02-27 21:59:26 +00:00
parent 6f0969ecb9
commit 31acef50ad
2 changed files with 48 additions and 2 deletions
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15
llvm/include/llvm/ADT/APInt.h
View File

@ -446,6 +446,12 @@ public:
return BitWidth - countLeadingZeros();
}
inline uint32_t getMinSignedBits() const {
if (isNegative())
return BitWidth - countLeadingOnes() + 1;
return getActiveBits();
}
/// This method attempts to return the value of this APInt as a zero extended
/// uint64_t. The bitwidth must be <= 64 or the value must fit within a
/// uint64_t. Otherwise an assertion will result.
@ -587,9 +593,16 @@ public:
/// @returns getNumWords() * APINT_BITS_PER_WORD if the value is zero.
/// @returns the number of zeros from the most significant bit to the first
/// one bits.
/// @brief Count the number of trailing one bits.
/// @brief Count the number of leading one bits.
uint32_t countLeadingZeros() const;
/// countLeadingOnes - This function counts the number of contiguous 1 bits
/// in the high order bits. The count stops when the first 0 bit is reached.
/// @returns 0 if the high order bit is not set
/// @returns the number of 1 bits from the most significant to the least
/// @brief Count the number of leading one bits.
uint32_t countLeadingOnes() const;
/// countTrailingZeros - This function is an APInt version of the
/// countTrailingZoers_{32,64} functions in MathExtras.h. It counts
/// the number of zeros from the least significant bit to the first one bit.

35
llvm/lib/Support/APInt.cpp
View File

@ -702,6 +702,38 @@ uint32_t APInt::countLeadingZeros() const {
return Count;
}
static uint32_t countLeadingOnes_64(uint64_t V, uint32_t skip) {
uint32_t Count = 0;
if (skip)
V <<= skip;
while (V && (V & (1ULL << 63))) {
Count++;
V <<= 1;
}
return Count;
}
uint32_t APInt::countLeadingOnes() const {
if (isSingleWord())
return countLeadingOnes_64(VAL, APINT_BITS_PER_WORD - BitWidth);
uint32_t highWordBits = BitWidth % APINT_BITS_PER_WORD;
uint32_t shift = (highWordBits == 0 ? 0 : APINT_BITS_PER_WORD - highWordBits);
int i = getNumWords() - 1;
uint32_t Count = countLeadingOnes_64(pVal[i], shift);
if (Count == highWordBits) {
for (i--; i >= 0; --i) {
if (pVal[i] == -1ULL)
Count += APINT_BITS_PER_WORD;
else {
Count += countLeadingOnes_64(pVal[i], 0);
break;
}
}
}
return Count;
}
uint32_t APInt::countTrailingZeros() const {
if (isSingleWord())
return CountTrailingZeros_64(VAL);
@ -1701,6 +1733,7 @@ void APInt::dump() const
else for (unsigned i = getNumWords(); i > 0; i--) {
cerr << pVal[i-1] << " ";
}
cerr << " (" << this->toString(10) << ")\n" << std::setbase(10);
cerr << " U(" << this->toString(10) << ") S(" << this->toStringSigned(10)
<< ")\n" << std::setbase(10);
}
#endif