Due to project requirements, i need to modify the RXW attribute of some functions. Such as modify a function attribute to WX. I tried to separate the functions in different sections, but what problem is that i don't know how to modify those attribute in LLVM IR. Is there any good way?
From my understanding here. You don't modify a function as RWX, you mark the memory page containing that function to be RWX.
You choices would be:
- Post process after the file has been linked
- Use LLVM IR to add external function calls to system API which will mark the memory page as RWX
Related
Is it possible to apply LLVM transformation pass to a specific basic block, instead of the whole IR?
I know how to apply a pass to the whole IR:
$ opt –S –instcombine test.ll –o out.ll
But there might be several basic blocks inside test.ll and I want to apply –instcombine to just one of them.
Generally, no. Some LLVM passes are written to work on whole modules, others on whole functions. Some are also safe to use for single basic blocks (more by chance than by design), but LLVM's pass interface deals with only the design unit (functions in case of function passes, modules in case of module passes). That is, function passes are given a function by the pass manager, and nothing else.
I am trying to create a temporary file on a Linux system, but interfacing through C++ (so that the Linux commands are run through the C++ program).
To do so, I am using mktemp, which produces a temporary file.
I would need to later refer back to this file.
However, the filename is randomly generated and I am wondering if there is an easy way to access the filename.
The big honking comment in mktemp(3)'s manual page explicitly tells you to use mkstemp(3) instead of mktemp(3), and explains the good reason why it is so.
If you actually read the manual page for mkstemp(3) it clearly explains that the library function modifies the character buffer that's passed to it as a parameter to reflect the actual name of the created temporary file.
So to determine the name of the temporary file, simply refer to the character buffer you passed to this library function.
I'm working on a library which I'd like certain introspection features to be available. Let's say I'm compiling with clang, so I have access to libtooling or whatever.
What I'd like specifically is for someone to be able to view the LLVM IR of an already-compiled function as part of the program. I know that, when compiling, I can use -emit-llvm to get the IR. But that saves it to a file. What I'd like is for the LLVM IR to be embedded in and retrievable from the program itself -- e.g. my_function_object.llvm_ir()
Is such a thing possible? Thanks!
You're basically trying to have reflection to your program. Reflection requires the existence of metadata in your binary. This doesn't exist out of the box in LLVM, as far as I know.
To achieve an effect like this, you could create a global key-value dictionary in your program, exposed via an exported function - something like IRInstruction* retrieve_llvm_ir_stream(char* name).
This dictionary would map some kind of identifier (for example, the exported name) of a given function to an in-memory array that represents the IR stream of that function (each instruction represented as a custom IRInstruction struct, for example). The types and functions of the representation format (like the custom IRInstruction struct) will have to be included in your source.
At the step of the IR generation, this dictionary will be empty. Immediately after the IR generation step, you'll need to add a custom build step: open the IR file and populate the dictionary with the data - for each exported function of your program, inject its name as a key to the dictionary and its IR stream as a value. The IR stream would be generated from the definitions of your functions, as read by your custom build tool (which would leverage the LLVM API to read the generated IR and convert it to your format).
Then, proceed to the assembler and linker as before.
Is there any document on the list of Analysis and Transform passes available for use in the AnalysisUsage::addRequired<> and Pass::geAnalysis<> functions?
I can get a list of passes in http://llvm.org/docs/Passes.html, but it only shows the command line names for the passes. How can I know the underlying pass classes?
Not really, no. Just look at the source. The header files in include/llvm/Analysis/ and include/llvm/Transforms/ will tell you everything you need to know.
Moreover, grepping over the source for getAnalysis< will tell you which passes are used as analyses inside the LLVM source code.
I want to create a pass that will insert a function (that is in some c file) after every load instruction.I know how to insert a function with no arguments.But a function that also have argument. I am not able to insert that.How should i do that?
Thanks in advance.
Use the CallInst::Create static method. It takes an ArrayRef of arguments. By grepping for this method call in the source of LLVM & Clang you can find many examples for using it.