[LLVMdev] (Possibly buggy?) doFinalization method behavior of FunctionPass

[Cristianno Martins]

Hello there,

I'm writing some LLVM passes, and just ran into an interesting situation:
now, I don't know if I misunderstood the way doFinalization is supposed to
work, but I hope someone could help =)

One of the transformations I wrote needed to replace some instructions
within the code, so I needed to clean up the code after the process was
completed. The pass basically swapped some function calls (from the
standard C library) with my own implementation of those functions. Changing
the code in this way, though, creates some dead code (like those dead
prototypes that are not being used anymore).

I, then, implemented the "clean up" strategy overriding doFinalization.
Unfortunately, any modifications done to the module in this method appears
to be ignored by LLVM. I even dumped the module directly from within the
method, and could see that the modifications were applied to that reference
of the module, but the .bc file opt wrote into does not retain these
changes.

Now, bear with me here: I know that other passes like DCE could be used to
clean the bytecode, but some of the code I implemented in doFinalization
actually needed to run only once, and necessarily after the pass has
finished: this is where I check to see if there is some extra situation I
need to address, optimize some of the replaced instructions, and verify if
any of the functions that I want to remove had their addresses taken by any
instruction.

Also, doFinalization has a bool return type, but it doesn't appear to have
any different behavior if I return either value =/ (I assumed the general
idea would be "return true if the module was modified in any way", like
runOnFunction, but I couldn't find anything to support that anywhere).

Thus, am I wrong about how to use doFinalization? If so, is there any way
to guarantee running some code only once and only when a pass already
finished its job?

Thanks in advance,

Oh, and before I forget, this is the version of the opt I'm running:

LLVM (http://llvm.org/):
  LLVM version 3.7.0svn
  DEBUG build with assertions.
  Built May  4 2015 (00:18:21).
  Default target: x86_64-apple-darwin14.3.0
  Host CPU: sandybridge

--
Cristianno Martins
PhD Student of Computer Science
University of Campinas
cmartins at ic.unicamp.br
<cristiannomartins at hotmail.com>
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