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8/16/2004 - Interprocedural Sparse Conditional Constant Propagation

These are notes for the eventual implementation of PR415.


The -ipsccp pass should be a very powerful CCP implementation, based off of the
standard SCCP formulation, but extended to work interprocedurally.  It should
include the optimization for overdefined values obviously, as most things in
the program are not constants.  Given that, the algorithm should work something
like this:

1. Loop through the program, marking all address taken and external function
   arguments as overdefined.
2. Add all entry blocks to address taken and external functions to the "newly
   reachable BB list".
3. Loop over all globals, marking all address taken and external globals as
   overdefined.
4. Start worklist processing.
5. At the end, rewrite the program, and use -dae to eliminate dead args, mark
   any globals proven constant as const.

In particular, the algorithm should be able to handle CCP load instructions
from internal globals that are and are not marked as constant.  In particular,
the following global should be marked constant:

void test() {
  static int i = 0;
  if (i) --i;   // dead store
}

This will come up a lot when we start doing optimization of the
%llvm.global_ctors list.