congestion related command

1.

set_congestion_options
-max_util valueMT7505: 0.85
设置最大的std cell placement密度，防止placement密度太大导致congestion

2.
placer_max_cell_density_threshold
set placer_max_cell_density_threshold valueMT7505: 0.8
设一个整体design的std cell大概的密度值，来防止placement过于松散，也防止wire太长导致的timing边长
manual:
For placement that is not set up for congestion removal, you can control how densely cells
can be packed by using the placer_max_cell_density_threshold variable. You can set this variable to a value between 1 and the overall average utilization of your design.Choosing a value near one allows cells to clump together more densely. A value of one allows no gaps between cells. You should choose a high value for designs with low utilization to improve timing and a low value for congested designs to avoid cell clumping. By default, this variable is set to -1 to disable this feature.

3.set_app_var placer_reduce_high_density_regions true (MTK未用到)
用来优化channels & around macros这些容易出现congestion的high_density_regions 的std cell density。该variable的使用会增长place_opt, place_opt_feasibility, create_placement, refine_placement, psynopt -congestion, or clock_opt 这些 placement-based command 的 running time (比如 coarse placement runtime 会double)