Added Harald Pretl's Python script that can convert the sky130.lib.spice
file to extract a single corner and output a valid library file with
only the one corner;  this greatly speeds up ngspice, which does not
handle the library file parsing gracefully.  Thanks to Harald for the
contribution!

runtime/spice_model_red.py

diff --git a/runtime/spice_model_red.py b/runtime/spice_model_red.py
new file mode 100644
index 0000000..e9c235a
--- /dev/null
+++ b/runtime/spice_model_red.py
@@ -0,0 +1,119 @@
+#!/usr/bin/env python3
+#
+# This script traverses SPICE model files (e.g. from SKY130) and
+# extracts only the wanted model section, removes all comments and
+# empty lines, and resolves all includes so that a flat model file
+# results. This should speed up ngspice starts.
+#
+# (c) 2021 Harald Pretl, Johannes Kepler University Linz
+
+import sys,re,os
+
+def process_file(file_in_name, top_file):
+    global is_warning
+    try:
+        f_in = open(file_in_name, 'r')
+    except FileNotFoundError:
+        print('Warning! File ' + file_in_name + ' not found.')
+        is_warning = True
+        return;
+
+    # process_file can be called recursively, so that nested include
+    # files can be traversed
+
+    # write_active indicates whether we are in the right model section; in
+    # include files, it is always true
+
+    if top_file == True:
+        write_active = False
+    else:
+        write_active = True
+    
+    for line in f_in:
+        line_trim = (line.lower()).strip()
+
+        if top_file == True:
+            # we assume that .lib statements are only used in the main file
+            if '.lib' in line_trim:
+                if model_section in line_trim: 
+                    write_active = True
+                else:
+                    write_active = False
+        
+            if '.endl' == line_trim:
+                write_active = False
+                f_out.write(line)
+
+        if len(line_trim) > 0: # write no empty lines
+            if (line_trim[0] != '*'): # write no comments
+                if (write_active == True):
+                    if '.include' in line_trim:
+                        # need to save and restore working dir so that nested 
+                        # includes work
+                        current_wd = os.getcwd()
+                        newfile = re.findall(r'"(.*?)(?<!\\)"', line_trim)
+                        print('Reading ',newfile[0])
+
+                        # enter new working dir
+                        new_wd = os.path.dirname(newfile[0]) 
+                        if len(new_wd) > 0:
+                            try:
+                                os.chdir(new_wd)
+                            except OSError:
+                                print('Warning: Could not enter directory ' + new_wd)
+                                is_warning = True
+
+                        # traverse into new include file
+                        new_file_name = os.path.basename(newfile[0]) 
+                        process_file(new_file_name, False) 
+                        
+                        # restore old working dir after return
+                        os.chdir(current_wd)
+                    else:
+                        f_out.write(line)
+
+    f_in.close()
+    return;
+
+# main routine
+
+if len(sys.argv) == 3:
+    model_section = sys.argv[2]
+else:
+    model_section = 'tt'
+
+if (len(sys.argv) == 2) or (len(sys.argv) == 3):
+    infile_name = sys.argv[1]
+    outfile_name = infile_name + '.' + model_section + '.red'
+
+    try:
+        f_out = open(outfile_name, 'w')
+    except OSError:
+        print('Error: Cannot write file ' + outfile_name + '.')
+        sys.exit(2)
+    
+    is_warning = False
+    process_file(infile_name, True)
+    f_out.close()
+    
+    print()
+    print('Model file ' + outfile_name + ' written.')
+    if is_warning == True:
+        print('There have been warnings! Please check output log.')
+        sys.exit(1)
+    else:
+        sys.exit(0) 
+else:
+    print()
+    print('spice_model_red.py    SPICE model file reducer')
+    print('                      (c) 2021 Harald Pretl, JKU')
+    print()
+    print('Usage: spice_model_red <inputfile> [corner] (default corner = tt)')
+    print()
+    print('Return codes for script automation:')
+    print('  0 = all OK')
+    print('  1 = warnings')
+    print('  2 = errors')
+    print('  3 = call of script w/o parameters (= showing this message)')
+    print()
+    sys.exit(3)