[LLVMdev] Getting started with LLVM Passes

[Chris Lattner]

... here is some generally useful information I should have cc'd to 
llvmdev in the first place ...

-Chris

-- 
http://nondot.org/sabre/
http://llvm.cs.uiuc.edu/

---------- Forwarded message ----------
Date: Sun, 10 Jul 2005 21:41:55 -0500 (CDT)
From: Chris Lattner <sabre at nondot.org>
To: Sean Peisert <peisert at gmail.com>
Subject: Re: [LLVMdev] Getting started with LLVM Passes

On Fri, 8 Jul 2005, Sean Peisert wrote:

> Hi Chris,
> 
>>> I'm now off on to addressing my own code transformations.  The ones
>>> that I may begin with are the '=' operator, as well as sbrk and mmap
>>> syscalls.  It isn't completely obvious to me which pass class to use
>>> for these.  Would you recommend the FunctionPass classes for both of
>>> these?
>> 
>> I don't understand what you would like your transform to do, can you
>> explain it a bit more?
> 
> Absolutely.  I'd eventually like to implement a large number of
> transforms.  For starters, I'd like to implement a transform such that
> the compiler inserts code into any binary (including libraries) which
> forces the binary to keep track of the time, size, and location of any
> write to memory by way of an operation which ultimately results in an
> assignment in the IR or AST, as well as the size and location of all
> memory allocations by way of the sbrk and mmap syscalls.

Ok, this shouldn't be hard.  Take a look at the existing LLVM instrumentation 
passes in lib/Transforms/Instrumentation.  These add various bits of 
instrumentation (duh) code to track things like the # of times a BB executes. 
It would be simple to track other things.

> You suggested in the past that instrumenting the LLVM store might be
> the proper way to do this, but I've not yet seen reference to
> what/where that precisely might be.  Could you possibly point me in
> the right direction for this?

Sure, check out the code in that dir for examples of generic instrumentation 
stuff.

> How might one get at the LLVM stores,

Look in the programmers manual for how to traverse the IR.  Basically do this:

   for each instruction:
     if (isa<StoreInst>(I)) {
       ... add instrumentation ...
     }

> or do transforms in the IR or syscall level?

syscalls look just like normal calls, I'm not sure what you mean by the IR 
level.  Another good example would be this simple pass:

http://illuvium.com/cgi-bin/cvsweb.cgi/llvm-poolalloc/lib/PoolAllocate/AccessTrace.cpp?rev=1.3

It basically loops over all of the loads in the program, filters out ones it 
doesn't want, then adds a call immediately before the load to a library 
function.  My guess is that this is exactly the sort of thing you want to do.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.cs.uiuc.edu/