Allow cell flattening in magic extraction

[d-m-bailey]

Prompt

Currently, magic extracts each layout cell separately. This can lead to confusing netlists with unnecessary hierarchies when using parameterized layout cells from GDS. For example, nmos_m10_w7_000_sli_dli_da_p in the openram macros.

Proposal

Allow user specified flattening of cells when reading GDS.
For example, executing these preprocessing commands before GDS input.

gds flatglob {*nmos_m*} 
gds flatglob {*pmos_m*}
gds flatglob {sky130*__*\[A-Z\]*} 
gds flatglob {*_example_*}
gds flatglob {*\$\$*}
gds flatglob {*_CDNS_*}
gds flatten yes

This could be accomplished by modifying run_magic_spice_export of scripts/tcl_scripts/magic.tcl as follows
Before:

if { \[info exist ::env(MAGIC_EXT_USE_GDS)\] && \$::env(MAGIC_EXT_USE_GDS) } {
        gds read \$::env(CURRENT_GDS)
} else {

After:

if { \[info exist ::env(MAGIC_EXT_USE_GDS)\] && \$::env(MAGIC_EXT_USE_GDS) } {
       if { \[info exist ::env(MAGIC_GDS_PREPROCESSING_FILE)\] } {
          source \$::env(MAGIC_GDS_PREPROCESSING_FILE)
       }
        gds read \$::env(CURRENT_GDS)
} else {

The preprocessing commands would be stored in the file specified by MAGIC_GDS_PREPROCESSING_FILE.

Alternatives

There might be a way to store all the preprocessing commands in a variable and eval it.

[donn]

Ah, god, no eval. This codebase is a security nightmare as is.

But this is a reasonable ask.

[d-m-bailey]

How about a list of cells to flatten?
Example:

{*nmos_m*} 
{*pmos_m*}

Then programmatically loop through the list adding gds flatten.

if { \[info exist ::env(MAGIC_EXT_USE_GDS)\] && \$::env(MAGIC_EXT_USE_GDS) } {
       if { \[info exist ::env(MAGIC_GDS_FLATTEN_CELLS)\] } {
          foreach cell \$::env(MAGIC_GDS_FLATTEN_CELLS) {
             gds flatten \$cell
          }
          gds flatten yes
       }
        gds read \$::env(CURRENT_GDS)
} else {