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ecp5/pack: split carry chains by re-driving CIN from S0 

When splitting a carry chain the original COUT -> carry -> CIN
was preserved. This was not routable if the carry chain is split.
Instead now the COUT is wired to a feed out CC2C whose `S0` port
is the start of the new carry chain.
This commit is contained in:
Rowan Goemans 2026-03-08 06:04:51 +01:00 committed by myrtle
parent e9b7da5a0f
commit 60f02a200d
1 changed files with 53 additions and 36 deletions
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89
ecp5/pack.cc
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@ -629,7 +629,8 @@ class Ecp5Packer
}
// Create a feed out and loop through from the carry chain
CellInfo *make_carry_feed_out(NetInfo *carry, boost::optional<PortRef> chain_next = boost::optional<PortRef>())
CellInfo *make_carry_feed_out(NetInfo *carry, boost::optional<PortRef> chain_next = boost::optional<PortRef>(),
bool break_chain = false)
{
std::unique_ptr<CellInfo> feedout = create_ecp5_cell(ctx, id_CCU2C);
@ -638,26 +639,39 @@ class Ecp5Packer
feedout->params[id_INJECT1_0] = std::string("NO");
feedout->params[id_INJECT1_1] = std::string("NO");
PortRef carry_drv = carry->driver;
carry->driver.cell = nullptr;
feedout->connectPort(id_S0, carry);
NetInfo *new_cin = ctx->createNet(ctx->id(feedout->name.str(ctx) + "$CIN"));
new_cin->driver = carry_drv;
carry_drv.cell->ports.at(carry_drv.port).net = new_cin;
feedout->connectPort(id_CIN, new_cin);
if (chain_next) {
// Loop back into LUT4_1 for feedthrough
feedout->connectPort(id_A1, carry);
if (break_chain) {
// Break: use S0 as a fabric-routable carry signal to drive next CIN.
feedout->connectPort(id_CIN, carry);
if (chain_next->cell && chain_next->cell->ports.at(chain_next->port).user_idx)
carry->users.remove(chain_next->cell->ports.at(chain_next->port).user_idx);
NetInfo *new_cout = ctx->createNet(ctx->id(feedout->name.str(ctx) + "$COUT"));
feedout->connectPort(id_COUT, new_cout);
NetInfo *new_net = ctx->createNet(ctx->id(feedout->name.str(ctx) + "$S0"));
feedout->connectPort(id_S0, new_net);
chain_next->cell->ports[chain_next->port].net = nullptr;
chain_next->cell->connectPort(chain_next->port, new_cout);
chain_next->cell->connectPort(chain_next->port, new_net);
} else {
PortRef carry_drv = carry->driver;
carry->driver.cell = nullptr;
feedout->connectPort(id_S0, carry);
NetInfo *new_cin = ctx->createNet(ctx->id(feedout->name.str(ctx) + "$CIN"));
new_cin->driver = carry_drv;
carry_drv.cell->ports.at(carry_drv.port).net = new_cin;
feedout->connectPort(id_CIN, new_cin);
if (chain_next) {
// Loop back into LUT4_1 for feedthrough
feedout->connectPort(id_A1, carry);
if (chain_next->cell && chain_next->cell->ports.at(chain_next->port).user_idx)
carry->users.remove(chain_next->cell->ports.at(chain_next->port).user_idx);
NetInfo *new_cout = ctx->createNet(ctx->id(feedout->name.str(ctx) + "$COUT"));
feedout->connectPort(id_COUT, new_cout);
chain_next->cell->ports[chain_next->port].net = nullptr;
chain_next->cell->connectPort(chain_next->port, new_cout);
}
}
CellInfo *feedout_ptr = feedout.get();
@ -689,28 +703,31 @@ class Ecp5Packer
}
}
chains.back().cells.push_back(cell);
bool at_end = (curr_cell == carryc.cells.end() - 1);
bool split_chain = int(chains.back().cells.size()) > max_length;
if (split_chain) {
CellInfo *passout = make_carry_feed_out(cell->ports.at(id_COUT).net);
chains.back().cells.back() = passout;
start_of_chain = true;
} else {
NetInfo *carry_net = cell->ports.at(id_COUT).net;
bool at_end = (curr_cell == carryc.cells.end() - 1);
if (carry_net != nullptr && (carry_net->users.entries() > 1 || at_end)) {
boost::optional<PortRef> nextport;
if (!at_end) {
auto next_cell = *(curr_cell + 1);
PortRef nextpr;
nextpr.cell = next_cell;
nextpr.port = id_CIN;
nextport = nextpr;
}
CellInfo *passout = make_carry_feed_out(cell->ports.at(id_COUT).net, nextport);
chains.back().cells.push_back(passout);
}
++curr_cell;
NetInfo *carry_net = cell->ports.at(id_COUT).net;
boost::optional<PortRef> nextport;
if (!at_end) {
auto next_cell = *(curr_cell + 1);
PortRef nextpr;
nextpr.cell = next_cell;
nextpr.port = id_CIN;
nextport = nextpr;
}
if (carry_net != nullptr) {
const bool need_out = split_chain || carry_net->users.entries() > 1 || at_end;
if (need_out) {
const bool do_break = split_chain && !at_end;
CellInfo *out = make_carry_feed_out(carry_net, nextport, do_break);
if (split_chain && at_end)
chains.back().cells.back() = out;
else
chains.back().cells.push_back(out);
}
}
if (split_chain)
start_of_chain = true;
++curr_cell;
}
return chains;
}