Simplified LLVM IR Modeling (SLIM)

The SLIM implementation takes a .ll file (which is the human readable form of LLVM-IR) as an input, processes it and populates the SLIM-IR data structures. There are various APIs available in order to manipulate the SLIM-IR, but this can be extended to include other APIs based on your requirements. It is easy to extend because the APIs in SLIM use the LLVM infrastructure and its documentation is easily available on the internet. Please raise a pull request if you have extended the implementation in any manner so that we can make the feature available for everyone.

Install the SLIM Library

To install the SLIM library, please follow these steps:

1. Create a build folder and change your working directory to this folder: mkdir -p build && cd build

2. Run the cmake command to generate the Makefile: cmake -S .. -B .

But if you want to use Memory SSA, then please run the cmake command by specifying the Memory SSA flag, the command is as follows: cmake -DMemorySSAFlag=ON -S .. -B .

3. Install the library by the command: sudo make install

The library will be installed at the location /usr/local/lib and the header files will be stored at the location /usr/local/include/slim. These locations can be used in the parent CMakeFile of your LLVM project. A sample CMakeLists.txt file is shown below:

cmake_minimum_required(VERSION 3.4)
set (CMAKE_CXX_STANDARD 14)
project(cs-ssa)

set(ENV{LLVM_DIR} /usr/lib/llvm-14/lib/cmake/llvm)

find_package(LLVM REQUIRED CONFIG)
add_definitions(${LLVM_DEFINITIONS})
include_directories(${LLVM_INCLUDE_DIRS})
link_directories(${LLVM_LIBRARY_DIRS})

include_directories(/usr/local/include/slim)

add_library(slim SHARED IMPORTED)

set_target_properties(slim PROPERTIES IMPORTED_LOCATION "/usr/local/lib/libslim.so")

llvm_map_components_to_libnames(llvm_libs ${LLVM_TARGETS_TO_BUILD} all)

add_executable(${PROJECT_NAME} main.cpp)

target_link_libraries(${PROJECT_NAME} slim ${llvm_libs})

Using SLIM

Before using SLIM, please ensure that your LLVM IR output (.ll) file is generated using the flags present in the below commands (otherwise the results may be incorrect):

lloutput() {
    clang-14 -fno-discard-value-names -emit-llvm -O0 -Xclang -disable-O0-optnone -g -S "$1" -o "${1%.*}.ll"
    opt-14 -instnamer -mem2reg -mergereturn -aa-pipeline='basic-aa' -S "${1%.*}.ll" > "${1%.*}.slim.ll"
}

You may also paste this at the end of ~/.bashrc file so that the output can be generated using the following command: lloutput <file-name>

Some basic information about the important flags in the above commands:

• -fno-discard-value-names is used to avoid the numeric (or temporary) naming of variables wherever possible and retain the original source program variable name. [1]
• -Xclang and -disable-O0-optnone flags are used to prevent clang from adding the optnone attribute to each function. The optnone attribute (when used with the optimization level -O0) prevents further optimizations like mem2reg, which is required by our implementation. [2]
• -O0 and -g flags are used to get the full debug information. This can be used to print the source program line number corresponding to an LLVM instruction, for example. [3]
• -instnamer flag is used to assign names to the LLVM IR instructions. [4]
• -mem2reg pass promotes the memory references to the SSA registers for the local variables whose addresses are not taken. After this pass, the global variables and the address-taken local variables will be treated as the memory variables which means that we will require the load/store instructions to fetch/update the value of these variables. [5]
• -mergereturn pass ensures that there is only one return statement in a function. If there are multiple exit points (returns) than it combines all of them using a PHI-instruction. [6]

The SLIM IR can be generated by first parsing the .ll file and then calling the constructor with the module as an argument. Once the IR object is created, all the instructions can be processed and various APIs can be used in order to fetch the required information. A sample main.cpp file is shown which shows how the object is created and how the IR can be dumped (for now we are considering only the stdout as the possible output destination, it can be changed using the 1> redirection but in future we will change the code to dump the IR into any file without the need of any redirection):

#include "llvm/IRReader/IRReader.h" // For parseIRFile

#include "slim/IR.h"

int main(int argc, char *argv[])
{
    if (argc < 2 || argc > 2)
    {
        llvm::errs() << "We expect exactly one argument - the name of the LLVM IR file!\n";
        exit(1);
    }

    llvm::SMDiagnostic smDiagnostic;

    std::unique_ptr<llvm::Module> module = parseIRFile(argv[1], smDiagnostic, context);

    slim::IR *transformIR = new slim::IR(module);

    transformIR->dumpIR();

    return 0;
}

The sample main.cpp file to use Memory SSA and to get the updated SLIM IR after non-adjacent load-store optimization is shown below:

#include "llvm/IRReader/IRReader.h" // For parseIRFile

#include "slim/IR.h"

int main(int argc, char *argv[])
{
    if (argc < 2 || argc > 2)
    {
        llvm::errs() << "We expect exactly one argument - the name of the LLVM IR file!\n";
        exit(1);
    }

    llvm::SMDiagnostic smDiagnostic;

    std::unique_ptr<llvm::Module> module = parseIRFile(argv[1], smDiagnostic, context);

    slim::IR *transformIR = new slim::IR(module);

    slim::IR *optimized_IR = transformIR->optimizeIR();

    optimized_IR->dumpIR();

    return 0;
}

Please feel free to raise a pull request or send a mail to pradhanaditya@cse.iitb.ac.in in case of any bug(s) or issue(s).

References:

1. https://stackoverflow.com/questions/50432967/how-to-save-the-variable-name-when-use-clang-to-generate-llvm-ir
2. https://stackoverflow.com/questions/46513801/llvm-opt-mem2reg-has-no-effect
3. https://llvm.org/docs/SourceLevelDebugging.html
4. https://stackoverflow.com/questions/68029581/how-to-assign-names-to-the-llvm-ir-instructions
5. https://llvm.org/docs/Passes.html#mem2reg-promote-memory-to-register
6. https://llvm.org/docs/Passes.html#mergereturn-unify-function-exit-nodes