Instrumenting C/C++ programs

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.
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…)
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.

Please suggest me how to go about it .
thanks
himanshu

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 () 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. 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). 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 () as well as the Programmer’s Guide (). Doxygen is also valuable (). 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 ( and ). It’s about as simple as an instrumentation pass gets. – John T.

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=markup and 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

Hi, Himanshu. I once wrote an LLVM IR-based memory profiling pass. Basically, I followed the code for EdgeProfiling. The source code is enclosed here, which worked with LLVM 2.8. Hope it is helpful.

MemoryProfiling.cpp—the instrumentation pass, which inserts profiling function calls into the original program
MemoryProfiling.c—the profiling library containing the profiling calls
llvm-memory-profiling.patch—the other modifications
notes.txt—some information collected when I was working on this profiling pass

Xiaoming

notes.txt (2.58 KB)

llvm-memory-profiling.patch (3.43 KB)

MemoryProfiling.c (2.16 KB)

MemoryProfiling.cpp (4.53 KB)

Hey Xiaoming, Thanks for those source codes. Can you please explain what this llvm-memory-profiling.patch does ?

Thanks,
Himanshu

The patch file is a diff file generated by “svn diff”. It is about other modifications such as adding some lines to make files to make the added .c and .cpp files compiled. You may apply the patch file to LLVM source code by using a “patch” command or make the changes manually by reading the patch file.

Xiaoming

Hey guys,

I have seen your interesting conversion. I am new for LLVM as well as clang compiler. I am going to use it now. it is quite difficult when someone starts it from scratch. But it is enjoyable and it is possible to familiarize with the environment by short time. Am gonna to use clang + llvm to generate AST from source code by using clang and to do high level transformation. Then I want to generate LLVM IR. Do you have any idea how to achieve this? i want some examples how clang converts source codes to AST and generating LLVM IR. Thanks in advance.

Hello,

What you’re describing is basically the parsing and codegen portions of clang. I’d suggest reading through the code there and posting to clang-dev if you have any specific questions. The clang devs are more likely to see your questions there as not all of them also hang out here.

Regards,
Jim

Hello guys,

I am trying to know how clang generates AST from source code. But it is not clear for me how clang generates it. Can you tell me which class or function of clang is building/generating AST? I want to see the specific function or algorithm of clang which generates AST. Thanks in advance for your idea.

As Jim already indicated, the LLVM development list is the wrong forum for Clang questions. You want the Clang development list. I have added that mailing list, and am BCC-ing the LLVM ones so we don’t bother them further.

To answer your question, the root function behind parsing an AST is here: http://clang.llvm.org/doxygen/namespaceclang.html#af5bbdd5c2bf254fc38f96c5dc24dba81

Understand that “parsing an AST” is a huge part of what Clang does. Most of the code in Clang is devoted to this task. Reading a single function won’t tell you how it works, you’ll need to dig into the code.