Three ECO edits

Most ECO patches combine three edits. Spare cells add logic, rewiring moves connections, and resizing swaps a cell for an equivalent one.

All three keep the rest of the netlist untouched. The table below fixes each edit with one example.

Isolating the cone in Yosys

The script below builds a two-output block with disjoint cones. In this disjoint toy the global per-type counts coincide with per-cone membership, and a general cone check selects by fanin from each output.

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read_verilog <<EOV
module eco_cone(input wire a, b, c, output wire y1, y2);
wire n1 = a & b;
assign y1 = n1 | c;
assign y2 = a ^ b;
endmodule
EOV
hierarchy -check -top eco_cone
proc
flatten
opt
techmap
opt_clean
check
select -assert-count 1 t:$_AND_
select -assert-count 1 t:$_OR_
select -assert-count 1 t:$_XOR_
select -assert-count 2 t:$_AND_ t:$_OR_
select -assert-none t:$_MUX_
log PASS: y1 cone holds 2 cells and y2 cone holds 1 cell with no mux cells

In this disjoint toy the y1 cone holds the AND and OR cells, 2 in total. The y2 cone holds the XOR cell, 1 in total, and no MUX cell appears anywhere.

Sizing the spare pool

Spare cells are placed before the change is known. The pool must cover the planned edits with margin.

EXAMPLE parameters (not tool defaults): 200 spare cells per block
first ECO edit consumes 3 cells: 200 - 3 = 197 remain
second ECO edit consumes 2 cells: 197 - 2 = 195 remain
used after both edits = 3 + 2 = 5 of 200 = 5 / 200 = 2.5%
rule used here: stop patching and re-run when the pool cannot cover the next edit

Common mistakes

Related topics

• Hierarchy and dont_touch

• Debugging QoR