[LLVMdev] Instrumenting C/C++ programs

[Himanshu Shekhar]

Hey John,
Thank you for the detailed reply.
I tried to figure out myself which IR should I use for my purpose ( Clang's
Abstract Syntax Tree (AST) or LLVM's SSA Intermediate Representation (IR).
), but couldn't really figure out which one to use.
Here is what I m trying to do.
Given any C/C++ program (like the one given below), I am trying to insert
calls to some function, before and after *every instruction that
reads/writes to/from memory*. For example consider the below C++  program (
Account.cpp)
/***********************************************************/

#include <stdio.h>

class Account {
  int balance;

public:
  Account(int b)
 {
    balance = b;
  }
  ~Account(){ }

  int read() {
    int r;
    r = balance;
    return r;
  }

  void deposit(int n) {
      balance = balance + n;
  }

  void withdraw(int n) {
      int r = read();
      balance = r - n;
  }
};

int main (){
  Account* a = new Account(10);
  a->deposit(1);
  a->withdraw(2);
  delete a;
}

/***********************************************************/
So after the instrumentation my program should look like :

/***********************************************************/

#include <stdio.h>

class Account {
  int balance;

public:
  Account(int b)
 {
    balance = b;
  }
  ~Account(){ }

  int read() {
    int r;
    foo();
    r = balance;
    foo();
    return r;
  }

  void deposit(int n) {
      foo();
      balance = balance + n;
      foo();
  }

  void withdraw(int n) {
      foo();
      int r = read();
      foo();
      foo();
      balance = r - n;
      foo();
  }
};

int main (){
  Account* a = new Account(10);
  a->deposit(1);
  a->withdraw(2);
  delete a;
}

/***********************************************************/
where *foo() *may be any function like get the current system time or
increment a counter .. so on. I understand that to insert function like
above I will have to first get the IR and then run an instrumentation pass
on the IR which will insert such calls into the IR, but I don't really know
how to achieve it. Please suggest me with examples how to go about it.
Also I understand that once I compile the program into the IR, it would be
really difficult to get 1:1 mapping between my original program and the
instrumented IR. So, is it possible to reflect the changes made in the IR (
because of instrumentation ) into the original program.

In order to get started with LLVM pass and how to make one on my own, I
looked at an example of a pass that adds run-time checks to LLVM IR loads
and stores, the SAFECode's load/store instrumentation pass (
http://llvm.org/viewvc/llvm-project/safecode/trunk/include/safecode/LoadStoreChecks.h?view=markupand
http://llvm.org/viewvc/llvm-project/safecode/trunk/lib/InsertPoolChecks/LoadStoreChecks.cpp?view=markup).
But I couldn't figure out how to run this pass. Please give me steps how to
run this pass on some program say the above Account.cpp.

Thanks,
Himanshu




On Fri, Sep 23, 2011 at 11:13 PM, John Criswell <criswell at illinois.edu>wrote:

>  On 9/23/11 12:24 PM, Himanshu Shekhar wrote:
>
> I just  read that LLVM project could be used to do static analysis on C/C++
> codes using the analyzer Clang which the front end of LLVM. I wanted to know
> if it is possible to extract all the accesses to memory(variables, local as
> well as global) in the source code using LLVM.
>
>
> When doing analysis with Clang and LLVM, you first must make a choice about
> which IR to use: Clang's Abstract Syntax Tree (AST) or LLVM's SSA
> Intermediate Representation (IR).  Clang takes source code and converts it
> into an AST; it later takes the AST and converts it to LLVM IR.  LLVM then
> performs mid-level compiler analysis and optimization on code in LLVM IR
> form and then translates from LLVM IR to native code.
>
> Clang ASTs will give you much higher level information than LLVM IR.  On
> the other hand, LLVM IR is probably easier to work with and is programming
> language agnostic.
>
> You might want to read about the LLVM Language Reference Manual (
> http://llvm.org/docs/LangRef.html) to get a feel of whether it is suitable
> for your analysis.  There may be a similar document for Clang, but I'm not
> familiar with it since I haven't worked with Clang ASTs myself.
>
>
> Is there any inbuilt library present in LLVM which I could use to extract
> this information. If not please suggest me how to write functions to do the
> same.(existing source code, reference, tutorial, example...)
>
>
> It is easy to write an LLVM pass that plugs into the opt tool that searches
> for explicit accesses to memory.  The LLVM load and store instructions
> access memory (similar to how loads and stores are used to access memory in
> a RISC instruction set).  That said, it is not clear whether this is what
> you want to do.  Some source-level variables are translated into one or more
> SSA virtual registers, so you'll never see a load or store to them (as they
> may never exist in memory but only in registers).  Additionally, some loads
> and stores to memory are not visible at the LLVM IR level.  For example,
> loads and stores to stack spill slots are not visible at the LLVM IR level
> because they're only created during code generation (and technically,
> they're generated in a third IR called Machine Instructions that is used
> specifically for code generation).
>
>
>
> Of what i studied is, I need to first convert the source code into LLVM IR
> and then make an instrumenting pass which would go over this bitcode file
> and insert calls to do the analysis, but don't know exactly how to do it.
>
>
> The first thing you need to do is figure out which representation of the
> program (Clang ASTs, LLVM IR, LLVM's code generation IR) is the best for
> solving your particular problem.  If you want, you can provide more details
> on what you're trying to do; people on the list can then provide feedback on
> which representation is most suitable for what you want to do.
>
> If you decide to work with LLVM IR, I then recommend reading the "How to
> Write an LLVM Pass" document (http://llvm.org/docs/WritingAnLLVMPass.html)
> as well as the Programmer's Guide (
> http://llvm.org/docs/ProgrammersManual.html).  Doxygen is also valuable (
> http://llvm.org/doxygen/).
>
> For an example of a pass that adds run-time checks to LLVM IR loads and
> stores, look at SAFECode's load/store instrumentation pass (
> http://llvm.org/viewvc/llvm-project/safecode/trunk/include/safecode/LoadStoreChecks.h?view=markupand
> http://llvm.org/viewvc/llvm-project/safecode/trunk/lib/InsertPoolChecks/LoadStoreChecks.cpp?view=markup).
> It's about as simple as an instrumentation pass gets.
>
> -- John T.
>
>
> Please suggest me how to go about it .
> thanks
> himanshu
> --
>
>
>
>
> _______________________________________________
> LLVM Developers mailing listLLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110928/b7cfbf71/attachment.html>