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Add a new function tcExtract for extracting a bignum from an

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arbitrary range of bits embedded in the middle of another bignum.
This kind of operation is desirable in many cases of software
floating point, e.g. converting bignum integers to floating point
numbers of fixed precision (you want to extract the precision most
significant bits).

Elsewhere, add an assertion, and exit the shift functions early if
the shift count is zero.

llvm-svn: 42745
This commit is contained in:
Neil Booth 2007-10-08 13:47:12 +00:00
parent 739a5252cb
commit f003f9bf1a
2 changed files with 83 additions and 34 deletions
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7
include/llvm/ADT/APInt.h
View File

@ -1069,6 +1069,13 @@ public:
/// Extract the given bit of a bignum; returns 0 or 1. Zero-based.
static int tcExtractBit(const integerPart *, unsigned int bit);
/// Copy the bit vector of width srcBITS from SRC, starting at bit
/// srcLSB, to DST, of dstCOUNT parts, such that the bit srcLSB
/// becomes the least significant bit of DST. All high bits above
/// srcBITS in DST are zero-filled.
static void tcExtract(integerPart *, unsigned int dstCount, const integerPart *,
unsigned int srcBits, unsigned int srcLSB);
/// Set the given bit of a bignum. Zero-based.
static void tcSetBit(integerPart *, unsigned int bit);

110
lib/Support/APInt.cpp
View File

@ -2101,6 +2101,8 @@ APInt::tcSet(integerPart *dst, integerPart part, unsigned int parts)
{
unsigned int i;
assert (parts > 0);
dst[0] = part;
for(i = 1; i < parts; i++)
dst[i] = 0;
@ -2182,6 +2184,42 @@ APInt::tcMSB(const integerPart *parts, unsigned int n)
return -1U;
}
/* Copy the bit vector of width srcBITS from SRC, starting at bit
srcLSB, to DST, of dstCOUNT parts, such that the bit srcLSB becomes
the least significant bit of DST. All high bits above srcBITS in
DST are zero-filled. */
void
APInt::tcExtract(integerPart *dst, unsigned int dstCount, const integerPart *src,
unsigned int srcBits, unsigned int srcLSB)
{
unsigned int firstSrcPart, dstParts, shift, n;
dstParts = (srcBits + integerPartWidth - 1) / integerPartWidth;
assert (dstParts <= dstCount);
firstSrcPart = srcLSB / integerPartWidth;
tcAssign (dst, src + firstSrcPart, dstParts);
shift = srcLSB % integerPartWidth;
tcShiftRight (dst, dstParts, shift);
/* We now have (dstParts * integerPartWidth - shift) bits from SRC
in DST. If this is less that srcBits, append the rest, else
clear the high bits. */
n = dstParts * integerPartWidth - shift;
if (n < srcBits) {
integerPart mask = lowBitMask (srcBits - n);
dst[dstParts - 1] |= ((src[firstSrcPart + dstParts] & mask)
<< n % integerPartWidth);
} else if (n > srcBits) {
dst[dstParts - 1] &= lowBitMask (srcBits % integerPartWidth);
}
/* Clear high parts. */
while (dstParts < dstCount)
dst[dstParts++] = 0;
}
/* DST += RHS + C where C is zero or one. Returns the carry flag. */
integerPart
APInt::tcAdd(integerPart *dst, const integerPart *rhs,
@ -2451,31 +2489,33 @@ APInt::tcDivide(integerPart *lhs, const integerPart *rhs,
void
APInt::tcShiftLeft(integerPart *dst, unsigned int parts, unsigned int count)
{
unsigned int jump, shift;
if (count) {
unsigned int jump, shift;
/* Jump is the inter-part jump; shift is is intra-part shift. */
jump = count / integerPartWidth;
shift = count % integerPartWidth;
/* Jump is the inter-part jump; shift is is intra-part shift. */
jump = count / integerPartWidth;
shift = count % integerPartWidth;
while (parts > jump) {
integerPart part;
while (parts > jump) {
integerPart part;
parts--;
parts--;
/* dst[i] comes from the two parts src[i - jump] and, if we have
an intra-part shift, src[i - jump - 1]. */
part = dst[parts - jump];
if (shift) {
part <<= shift;
/* dst[i] comes from the two parts src[i - jump] and, if we have
an intra-part shift, src[i - jump - 1]. */
part = dst[parts - jump];
if (shift) {
part <<= shift;
if (parts >= jump + 1)
part |= dst[parts - jump - 1] >> (integerPartWidth - shift);
}
dst[parts] = part;
}
dst[parts] = part;
}
while (parts > 0)
dst[--parts] = 0;
while (parts > 0)
dst[--parts] = 0;
}
}
/* Shift a bignum right COUNT bits in-place. Shifted in bits are
@ -2483,29 +2523,31 @@ APInt::tcShiftLeft(integerPart *dst, unsigned int parts, unsigned int count)
void
APInt::tcShiftRight(integerPart *dst, unsigned int parts, unsigned int count)
{
unsigned int i, jump, shift;
if (count) {
unsigned int i, jump, shift;
/* Jump is the inter-part jump; shift is is intra-part shift. */
jump = count / integerPartWidth;
shift = count % integerPartWidth;
/* Jump is the inter-part jump; shift is is intra-part shift. */
jump = count / integerPartWidth;
shift = count % integerPartWidth;
/* Perform the shift. This leaves the most significant COUNT bits
of the result at zero. */
for(i = 0; i < parts; i++) {
integerPart part;
/* Perform the shift. This leaves the most significant COUNT bits
of the result at zero. */
for(i = 0; i < parts; i++) {
integerPart part;
if (i + jump >= parts) {
part = 0;
} else {
part = dst[i + jump];
if (shift) {
part >>= shift;
if (i + jump + 1 < parts)
part |= dst[i + jump + 1] << (integerPartWidth - shift);
if (i + jump >= parts) {
part = 0;
} else {
part = dst[i + jump];
if (shift) {
part >>= shift;
if (i + jump + 1 < parts)
part |= dst[i + jump + 1] << (integerPartWidth - shift);
}
}
}
dst[i] = part;
dst[i] = part;
}
}
}