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xilinx_dffopt: Keep order of LUT inputs.

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See rationale at https://github.com/YosysHQ/yosys/pull/1557#discussion_r359196549
This commit is contained in:
Marcin Kościelnicki 2019-12-19 08:49:21 +01:00
parent f52c6efd9d
commit 561ae1c5c4
1 changed files with 30 additions and 16 deletions
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46
techlibs/xilinx/xilinx_dffopt.cc
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@ -27,8 +27,31 @@ typedef std::pair<Const, std::vector<SigBit>> LutData;
// Compute a LUT implementing (select ^ select_inv) ? alt_data : data. Returns true if successful.
bool merge_lut(LutData &result, const LutData &data, const LutData select, bool select_inv, SigBit alt_data, int max_lut_size) {
// First, gather input signals.
// First, gather input signals -- insert new signals at the beginning
// of the vector, so they don't disturb the likely-critical D LUT input
// timings.
result.second = data.second;
// D lut inputs initially start at 0.
int idx_data = 0;
// Now add the control input LUT inputs.
std::vector<int> idx_sel;
for (auto bit : select.second) {
int idx = -1;
for (int i = 0; i < GetSize(result.second); i++)
if (result.second[i] == bit)
idx = i;
if (idx == -1) {
idx = 0;
// Insert new signal at the beginning and bump all indices.
result.second.insert(result.second.begin(), bit);
idx_data++;
for (int &sidx : idx_sel)
sidx++;
}
idx_sel.push_back(idx);
}
// Insert the Q signal, if any, to the slowest input -- it will have
// no problem meeting timing.
int idx_alt = -1;
if (alt_data.wire) {
// Check if we already have it.
@ -37,22 +60,13 @@ bool merge_lut(LutData &result, const LutData &data, const LutData select, bool
idx_alt = i;
// If not, add it.
if (idx_alt == -1) {
idx_alt = GetSize(result.second);
result.second.push_back(alt_data);
idx_alt = 0;
result.second.insert(result.second.begin(), alt_data);
idx_data++;
for (int &sidx : idx_sel)
sidx++;
}
}
std::vector<int> idx_sel;
for (auto bit : select.second) {
int idx = -1;
for (int i = 0; i < GetSize(result.second); i++)
if (result.second[i] == bit)
idx = i;
if (idx == -1) {
idx = GetSize(result.second);
result.second.push_back(bit);
}
idx_sel.push_back(idx);
}
// If LUT would be too large, bail.
if (GetSize(result.second) > max_lut_size)
@ -75,7 +89,7 @@ bool merge_lut(LutData &result, const LutData &data, const LutData select, bool
new_bit = alt_data.data == State::S1;
} else {
// Use original LUT.
int lut_idx = i & ((1 << GetSize(data.second)) - 1);
int lut_idx = i >> idx_data & ((1 << GetSize(data.second)) - 1);
new_bit = data.first.bits[lut_idx] == State::S1;
}
result.first.bits[i] = new_bit ? State::S1 : State::S0;