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cxxrtl: Remove redundant divmod #4061

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merged 2 commits into from
Dec 9, 2023
Merged

cxxrtl: Remove redundant divmod #4061

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f3aed05
cxxrtl: Remove redundant divmod
merryhime Dec 9, 2023
3873ab7
cxxrtl: Extract divmod algorithm into value
merryhime Dec 9, 2023
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143 changes: 38 additions & 105 deletions backends/cxxrtl/runtime/cxxrtl/cxxrtl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -519,14 +519,6 @@ struct value : public expr_base<value<Bits>> {
return count;
}

size_t chunks_used() const {
for (size_t n = chunks; n > 0; n--) {
if (data[n - 1] != 0)
return n;
}
return 0;
}

template<bool Invert, bool CarryIn>
std::pair<value<Bits>, bool /*CarryOut*/> alu(const value<Bits> &other) const {
value<Bits> result;
Expand Down Expand Up @@ -585,82 +577,35 @@ struct value : public expr_base<value<Bits>> {
return result;
}

// parallel to BigUnsigned::divideWithRemainder; quotient is stored in q,
// *this is left with the remainder. See that function for commentary describing
// how/why this works.
void divideWithRemainder(const value<Bits> &b, value<Bits> &q) {
assert(this != &q);

if (this == &b || &q == &b) {
value<Bits> tmpB(b);
divideWithRemainder(tmpB, q);
return;
}

q = value<Bits> {0u};

size_t blen = b.chunks_used();
if (blen == 0) {
return;
}

size_t len = chunks_used();
if (len < blen) {
return;
}

size_t i, j, k;
size_t i2;
chunk_t temp;
bool borrowIn, borrowOut;

size_t origLen = len;
len++;
chunk::type blk[len];
std::copy(data, data + origLen, blk);
blk[origLen] = 0;
chunk::type subtractBuf[len];
std::fill(subtractBuf, subtractBuf + len, 0);

size_t qlen = origLen - blen + 1;

i = qlen;
while (i > 0) {
i--;
i2 = chunk::bits;
while (i2 > 0) {
i2--;
for (j = 0, k = i, borrowIn = false; j <= blen; j++, k++) {
temp = blk[k] - getShiftedBlock(b, j, i2);
borrowOut = (temp > blk[k]);
if (borrowIn) {
borrowOut |= (temp == 0);
temp--;
}
subtractBuf[k] = temp;
borrowIn = borrowOut;
}
for (; k < origLen && borrowIn; k++) {
borrowIn = (blk[k] == 0);
subtractBuf[k] = blk[k] - 1;
}
if (!borrowIn) {
q.data[i] |= (chunk::type(1) << i2);
while (k > i) {
k--;
blk[k] = subtractBuf[k];
}
}
std::pair<value<Bits>, value<Bits>> udivmod(value<Bits> divisor) const {
value<Bits> quotient;
value<Bits> dividend = *this;
if (dividend.ucmp(divisor))
return {/*quotient=*/value<Bits>{0u}, /*remainder=*/dividend};
uint32_t divisor_shift = dividend.ctlz() - divisor.ctlz();
divisor = divisor.shl(value<Bits>{divisor_shift});
for (size_t step = 0; step <= divisor_shift; step++) {
quotient = quotient.shl(value<Bits>{1u});
if (!dividend.ucmp(divisor)) {
dividend = dividend.sub(divisor);
quotient.set_bit(0, true);
}
divisor = divisor.shr(value<Bits>{1u});
}

std::copy(blk, blk + origLen, data);
return {quotient, /*remainder=*/dividend};
}

static chunk::type getShiftedBlock(const value<Bits> &num, size_t x, size_t y) {
chunk::type part1 = (x == 0 || y == 0) ? 0 : (num.data[x - 1] >> (chunk::bits - y));
chunk::type part2 = (x == num.chunks) ? 0 : (num.data[x] << y);
return part1 | part2;
std::pair<value<Bits>, value<Bits>> sdivmod(const value<Bits> &other) const {
value<Bits + 1> quotient;
value<Bits + 1> remainder;
value<Bits + 1> dividend = sext<Bits + 1>();
value<Bits + 1> divisor = other.template sext<Bits + 1>();
if (dividend.is_neg()) dividend = dividend.neg();
if (divisor.is_neg()) divisor = divisor.neg();
std::tie(quotient, remainder) = dividend.udivmod(divisor);
if (dividend.is_neg() != divisor.is_neg()) quotient = quotient.neg();
if (dividend.is_neg()) remainder = remainder.neg();
return {quotient.template trunc<Bits>(), remainder.template trunc<Bits>()};
}
};

Expand Down Expand Up @@ -849,9 +794,9 @@ std::ostream &operator<<(std::ostream &os, const value_formatted<Bits> &vf)
if (val.is_zero())
buf += '0';
while (!val.is_zero()) {
value<Bits> quotient;
val.divideWithRemainder(value<Bits>{10u}, quotient);
buf += '0' + val.template trunc<(Bits > 4 ? 4 : Bits)>().val().template get<uint8_t>();
value<Bits> quotient, remainder;
std::tie(quotient, remainder) = val.udivmod(value<Bits>{10u});
buf += '0' + remainder.template trunc<(Bits > 4 ? 4 : Bits)>().val().template get<uint8_t>();
val = quotient;
}
if (negative || vf.plus)
Expand Down Expand Up @@ -1735,35 +1680,23 @@ CXXRTL_ALWAYS_INLINE
std::pair<value<BitsY>, value<BitsY>> divmod_uu(const value<BitsA> &a, const value<BitsB> &b) {
constexpr size_t Bits = max(BitsY, max(BitsA, BitsB));
value<Bits> quotient;
value<Bits> remainder;
value<Bits> dividend = a.template zext<Bits>();
value<Bits> divisor = b.template zext<Bits>();
if (dividend.ucmp(divisor))
return {/*quotient=*/value<BitsY> { 0u }, /*remainder=*/dividend.template trunc<BitsY>()};
uint32_t divisor_shift = dividend.ctlz() - divisor.ctlz();
divisor = divisor.shl(value<32> { divisor_shift });
for (size_t step = 0; step <= divisor_shift; step++) {
quotient = quotient.shl(value<1> { 1u });
if (!dividend.ucmp(divisor)) {
dividend = dividend.sub(divisor);
quotient.set_bit(0, true);
}
divisor = divisor.shr(value<1> { 1u });
}
return {quotient.template trunc<BitsY>(), /*remainder=*/dividend.template trunc<BitsY>()};
std::tie(quotient, remainder) = dividend.udivmod(divisor);
return {quotient.template trunc<BitsY>(), remainder.template trunc<BitsY>()};
}

template<size_t BitsY, size_t BitsA, size_t BitsB>
CXXRTL_ALWAYS_INLINE
std::pair<value<BitsY>, value<BitsY>> divmod_ss(const value<BitsA> &a, const value<BitsB> &b) {
value<BitsA + 1> ua = a.template sext<BitsA + 1>();
value<BitsB + 1> ub = b.template sext<BitsB + 1>();
if (ua.is_neg()) ua = ua.neg();
if (ub.is_neg()) ub = ub.neg();
value<BitsY> y, r;
std::tie(y, r) = divmod_uu<BitsY>(ua, ub);
if (a.is_neg() != b.is_neg()) y = y.neg();
if (a.is_neg()) r = r.neg();
return {y, r};
constexpr size_t Bits = max(BitsY, max(BitsA, BitsB));
value<Bits> quotient;
value<Bits> remainder;
value<Bits> dividend = a.template sext<Bits>();
value<Bits> divisor = b.template sext<Bits>();
std::tie(quotient, remainder) = dividend.sdivmod(divisor);
return {quotient.template trunc<BitsY>(), remainder.template trunc<BitsY>()};
}

template<size_t BitsY, size_t BitsA, size_t BitsB>
Expand Down
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