[llvm-dev] How To Emit an LLVM IR or Object Code From A Source Code Inside A Tool

[Seyyid Hikmet Çelik via llvm-dev]

Hi everyone,
I am kind of a newbie here, working on llvm as a undergraduate student and
I am struggling about some situations. Yet I can write meaningless passes,
working tools by copying or trying to porting from llvm source and know how
to build them. I am searching on the web, using https://llvm.org/doxygen to
understand some functions, classes and how I can integrate these kind of
things in my experimental codes but I think that I can't go any further now
that's why I am writing this e-mail and probably more comes for different
questions.
I am trying to write a tool which will get target triple as a command
option and a C or C++ code, will emit and change LLVM IR code (maybe adding
a pass or meaningless operation for now :) ) inside, will emit and change
object code from changed LLVM IR, and lastly will give output as an
executable file.
Example tool in summary as I thought:
Usage (I can add some command line options later):

mytool --triple riscv32 example.c


Sequence of this complete tool:
Run Tool --> Get Target Triple --> Emit LLVM IR code from source code -->
Change LLVM IR code --> Emit Object Code --> Change Object Code --> Give
Executable File Output

For this, firstly I thought implement to get file and target triple (If I
am mistaken or incomplete you can warn):

#include "llvm/Support/CommandLine.h"
static cl::list<std::string>
InputFilenames(cl::Positional, cl::desc("<input files>"),cl::ZeroOrMore);
extern cl::opt<std::string> TripleName;

cl::opt<std::string>
llvm::TripleName("triple", cl::desc("target triple"));


Suppose that I did more coding and get source code file in main function,
after that how can I emit LLVM IR or object code inside and apply on some
operations?

I am asking this because for example we can use a pass in command line as:

opt -load libMyPass.so -mypass example.ll -o output.ll

or in clang with -Xclang -load -Xclang option.

or this is more proper example:

clang -mem2reg example.ll -o output.ll


And we can also use these passes in our codes (in other passes or tools):

static std::unique_ptr<legacy::FunctionPassManager> TheFPM;
TheFPM->add(createPromoteMemoryToRegisterPass());
TheFPM->run(*TheFunction);

(TheFunction should be declared before.)

or maybe addPass function (I am not sure.)
https://llvm.org/doxygen/classllvm_1_1PassManager.html#a314ff184ce4ace8801e1158ef909e22e

or like using an analysis pass inside another different pass:

void LegacyAnotherPass::getAnalysisUsage(AnalysisUsage &Info) const {
  Info.addRequired<LegacyMyAnalysisPass>();
}


I thought we could other command line operations inside our code as this. I
am trying to find a function for my desire. For example as in the pass
example above:

clang -S -emit-llvm example.c -o example.ll
clang -c example.ll -o example.o


And code equivalent (extremely making up :) ):
for llvm ir:

llvmir = file.getLLVMIRCode();


for object code:

objcode = llvmir.getObjectCode();


Is there a way like this in simply and how I can?

Maybe you can say "write a bash script and do operations step by step" but
I want to write this complete tool using llvm and its functions inside. If
you can answer I will be satisfied and please don't forget I am newbie if I
am mistaken on some points.

Thanks,
Seyyid
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