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opt_expr: Fix #4590

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If all the (non-select) inputs of a `$_MUX{4,8,16}_` are undefined, replace it, just like we do for `$mux` and `$_MUX_`.
Add `tests/opt/opt_expr_mux_undef.ys` to verify this.

This doesn't do any const folding on the wide muxes, or shrinking to less wide muxes.  It only handles the case where all inputs are 'x and the mux can be completely removed.
This commit is contained in:
Krystine Sherwin 2025-04-04 12:25:31 +13:00
parent 63b3ce0c77
commit 406b400458
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14
passes/opt/opt_expr.cc
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@ -1573,6 +1573,20 @@ skip_identity:
}
}
if (mux_undef && cell->type.in(ID($_MUX4_), ID($_MUX8_), ID($_MUX16_))) {
int num_inputs = 4;
if (cell->type == ID($_MUX8_)) num_inputs = 8;
if (cell->type == ID($_MUX16_)) num_inputs = 16;
int undef_inputs = 0;
for (auto &conn : cell->connections())
if (!conn.first.in(ID::S, ID::T, ID::U, ID::V, ID::Y))
undef_inputs += conn.second.is_fully_undef();
if (undef_inputs == num_inputs) {
replace_cell(assign_map, module, cell, "mux_undef", ID::Y, cell->getPort(ID::A));
goto next_cell;
}
}
#define FOLD_1ARG_CELL(_t) \
if (cell->type == ID($##_t)) { \
RTLIL::SigSpec a = cell->getPort(ID::A); \