[PATCH] Implement variable-sized alloca instrumentation.

[Maxim Ostapenko]

Handling nontrivial cases may be quite tricky. Perhaps we can implement something like linked list of "bad " allocas, storing the address and size of next/previous alloca in the left redzone and marking the last/first one with some magic value. Then, before each ret instruction, we can iterate over this list and unpoison these allocas.

http://reviews.llvm.org/D6055