Floorplan
Read the situation and the numbers it starts from, then pick the change you would make. Every option was run, so the answer comes from the measurement rather than from anyone's judgement, and after you choose you see what all of them did. Progress is stored in this browser.
All 2 situations in this kind, with their tables
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How much of the keep-out is worth keeping?Leave the halo alone: -0.13 · Halve it: -0.12 · Cut it to almost nothing: -0.15
Halving the halo is worth having. It frees 5400 square micrometres of floorplan, shortens the wire by 4 percent, and takes the total negative slack from -47.93 to -37.27 nanoseconds, because cells that were pushed away from the macros can now sit where their connections are. Cutting it to nothing undoes all of that and more: the wire goes back up, the worst slack becomes the worst of the three, and the total negative slack nearly doubles against the halved run. With no room at the macro edge, every pin on those macros has to be reached through a strip that is now full of cells, and the ground bounce under them goes from 1.38 percent to 2.75. The knob has an optimum and it is not at either end, which is the only reliable thing anyone can tell you about a halo.
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Which shape costs the least wire?Keep it square: 3605 · Make it twice as tall as it is wide: 3840 · Make it twice as wide as it is tall: 3754
Square wins, by 4 percent over wide and 7 percent over tall. Among rectangles of the same area a square has the shortest average distance between two points in it, and stretching the block lengthens the nets that run along its long side. The two stretched shapes do not cost the same, and the voltage drop moves in opposite directions, 0.32 percent in the tall block and 0.52 in the wide one, a reminder that a block is not symmetric: its routing layers alternate direction and its supply straps run one way. Timing barely notices either shape.