[PATCH] Add ladder graph creation script

[Daniel Berlin]

Hi chandlerc,

creates ladder graph source code, used to drive N^2 behavior from things
like dominance frontiers, etc.

http://reviews.llvm.org/D8879

Files:
  utils/create_ladder_graph.py

Index: utils/create_ladder_graph.py
===================================================================
--- /dev/null
+++ utils/create_ladder_graph.py
@@ -0,0 +1,43 @@
+#!/usr/bin/env python
+"""A ladder graph creation program.
+
+This is a python program that creates c source code that will generate
+CFGs that are ladder graphs.  Ladder graphs are generally the worst case
+for a lot of dominance related algorithms (Dominance frontiers, etc),
+and often generate N^2 or worse behavior.
+
+One good use of this program is to test whether your linear time algorithm is
+really behaving linearly.
+"""
+
+import argparse
+def main():
+  parser = argparse.ArgumentParser(description=__doc__)
+  parser.add_argument('rungs', type=int,
+                      help="Number of ladder rungs. Must be a multiple of 2")
+  args = parser.parse_args()
+  if (args.rungs % 2) != 0:
+    print "Rungs must be a multiple of 2"
+    return
+  print "int ladder(int *foo, int *bar, int x) {"
+  rung1 = xrange(0, args.rungs, 2)
+  rung2 = xrange(1, args.rungs, 2)
+  for i in rung1:
+    print "rung1%d:" % i
+    print "*foo = x++;"
+    if i != rung1[-1]:
+      print "if (*bar) goto rung1%d;" % (i+2)
+      print "else goto rung2%d;" % (i+1)
+    else:
+      print "goto rung2%d;" % (i+1)
+  for i in rung2:
+    print "rung2%d:" % i
+    print "*foo = x++;"
+    if i != rung2[-1]:
+      print "goto rung2%d;" % (i+2)
+    else:
+      print "return *foo;"
+  print "}"
+
+if __name__ == '__main__':
+  main()

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