Placement ECO and legalization

Placement ECO and legalization. When an ECO inserts new cells, the placer first drops them at their ideal position, where timing and wirelength want them. Those spots normally land on top of cells that are already placed, and may not sit on the placement site grid, so the layout is illegal. Legalization repairs it: every cell has to end up inside a standard-cell row, aligned to the site grid, and not overlapping any other cell.

The catch is displacement. Moving a cell away from its ideal spot erodes the timing and wirelength the placer chose, so a legalizer moves each cell as little as possible: it snaps to the nearest legal gap, not just any gap. On a packed die the nearest gap is often not in the cell's own row, so a legalizer will hop to an adjacent row, and displacement counts that vertical move too.

The placer has already dropped four ECO cells onto a full die at their ideal spots, and every one lands on occupied sites, so all four start illegal (shown red, with a dashed marker at each cell's ideal). Pick a cell, then click a site to move its left edge into a free gap wide enough for it, with no overlap. Some cells have no close gap in their own row, so the smallest move is into a neighbouring row. Keep the total displacement at or under budget, then Submit placement.