[LLVMdev] LLVM instrumentation overhead
nipun2512 at gmail.com
Fri Dec 9 11:32:25 PST 2011
Thanks for the detailed answer, this gives me a good starting point to
I was also wondering if you could give an idea (in terms of %ge) the
overhead one can expect with such an instrumentation. I want something
really lightweight and simple which can possible be applied to
production systems, so overhead is a concern.
On 12/09/2011 02:21 PM, John Criswell wrote:
> On 12/7/11 4:51 PM, Nipun Arora wrote:
>> I need to write a transform pass which instruments the target program to
>> output the name of each function executed, and the rdtsc counter along
>> with it.
> Doing this in LLVM is really straightforward. You simply iterate
> through all the functions in a module and add instructions to their
> entry basic blocks to do whatever it is that you want to do.
> I believe you already know how to find all the functions and their
> entry blocks. Review the Programmer's Guide and the doxygen docs on
> llvm::Module and llvm::Function if there's something you don't
> The only other question is how to insert instructions. For that, you
> can take one of two approaches. First, you can use the IRBuilder
> class (http://llvm.org/doxygen/classllvm_1_1IRBuilder.html). Second,
> you can simply use the appropriate constructor/new methods of the
> Instruction classes to create and insert the instructions that you
> want. I believe IRBuilder is now the preferred way to do things as
> its API changes less often.
> For the instrumentation that you want to do, the easiest thing to do
> would be to insert a call in every function to a function that you
> implement in a run-time library that does whatever the instrumentation
> should do. This makes the compiler transform very simple.
>> Can anyone give me an idea of how to go about it?(I've worked around
>> with LLVM pass framework and opt to do static analysis, but would like
>> to do a lightweight instrumentation). Also can anyone give an
>> approximate idea of the overhead for such instrumentation?
> To make things faster, you could compile your run-time library as a
> static library linked in using clang's/libLTO's link-time
> optimization. Your run-time library can then be inter-procedurally
> inlined with the program that you are instrumenting.
> -- John T.
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