What is the LLVM
The LLVM Project is a collection of modular and reusable compiler and toolchain technologies that can be used to develop a frontend for any programming language and a backend for any instruction set architecture. LLVM is designed around a language-independent intermediate representation (IR) that serves as a portable, high-level assembly language that can be optimized with a variety of transformations over multiple passes.[5] The name LLVM originally stood for Low Level Virtual Machine, though the project has expanded and the name is no longer officially an initialism.
LLVM is written in C++ and is designed for compile-time, link-time, runtime, and "idle-time" optimization.
Setup
We would install LLVM on Windows 11
First, we need download the install the tools as below:
MinGW I installed both 32bit and 64bit
LLVM-16.0.6-win64 This installer just includes the toolchain. So we need download the development lib in addition.
clang+llvm-16.0.6-x86_64-windows.tar.xz Copy bin, include and lib folder to LLVM folder after unzip.
Demo
After setup, we would print “hello world” by creating 2 strings in different way. The comments described the details. and I think you can catch it completely.
// Include the used lib files
#include <llvm-c/Core.h>
#include <llvm-c/ExecutionEngine.h>
#include <llvm-c/Target.h>
#include <llvm-c/Analysis.h>
#include <llvm-c/BitWriter.h>
#include <llvm-c/Transforms/PassManagerBuilder.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
int main() {
// Create a new LLVM module and a builder
LLVMModuleRef module = LLVMModuleCreateWithName("my_module");
LLVMBuilderRef builder = LLVMCreateBuilder();
// Declare a string type in LLVM IR
LLVMTypeRef stringType = LLVMPointerType(LLVMInt8Type(), 0);
// Declare the printf function in LLVM
LLVMTypeRef printfArgTypes[] = { stringType }; // printf takes char* as argument
LLVMTypeRef printfReturnType = LLVMInt32Type(); // printf returns int
LLVMTypeRef printfFuncType = LLVMFunctionType(printfReturnType, printfArgTypes, 1, 1); // isVarArg = true
LLVMValueRef printfFunc = LLVMAddFunction(module, "printf", printfFuncType);
// Create a local variable (pointer) in the entry block of a function
LLVMTypeRef mainFuncType = LLVMFunctionType(LLVMVoidType(), NULL, 0, 0);
LLVMValueRef mainFunc = LLVMAddFunction(module, "main", mainFuncType);
LLVMBasicBlockRef entry = LLVMAppendBasicBlock(mainFunc, "entry");
LLVMPositionBuilderAtEnd(builder, entry);
// Create a global string constant for "hello"
LLVMValueRef helloStr = LLVMAddGlobal(module, LLVMArrayType(LLVMInt8Type(), 7), "helloStr");
LLVMSetInitializer(helloStr, LLVMConstString("hello", 6, 0));
LLVMSetGlobalConstant(helloStr, 1);
LLVMSetLinkage(helloStr, LLVMPrivateLinkage);
// Allocate space for the pointer and store the address of the global string in it
LLVMValueRef strPtr = LLVMBuildAlloca(builder, stringType, "strPtr");
LLVMBuildStore(builder, LLVMBuildBitCast(builder, helloStr, stringType, ""), strPtr);
// Declare a string variable in LLVM IR
LLVMValueRef strVar = LLVMBuildAlloca(builder, stringType, "strVar");
// Assign the string "world" to the variable in LLVM IR
LLVMValueRef helloString = LLVMBuildGlobalStringPtr(builder, "world", "helloString");
LLVMBuildStore(builder, helloString, strVar);
// Load the string pointer and pass it to printf
LLVMValueRef loadedStrPtr = LLVMBuildLoad2(builder, LLVMTypeOf(strPtr), strPtr, "");
LLVMValueRef loadedvariablePtr = LLVMBuildLoad2(builder, LLVMTypeOf(strVar), strVar, "");
LLVMValueRef format = LLVMBuildGlobalStringPtr(builder,"%s %s.\n","format");
// Call the printf function with arguments
LLVMValueRef printfArgs[] = {format, loadedStrPtr, loadedvariablePtr};
LLVMBuildCall2(builder, LLVMGlobalGetValueType(printfFunc), printfFunc, printfArgs, 3, "");
// Finish the function
LLVMBuildRetVoid(builder);
// Verify the module
char *error = NULL;
LLVMVerifyModule(module, LLVMPrintMessageAction, &error);
LLVMDisposeMessage(error);
// Print the LLVM IR to stdout
LLVMDumpModule(module);
// Print the LLVM IR to a local file
LLVMPrintModuleToFile(module, "out_p1.ll", &error);
// Compile IR file to exe file
system("set PATH=%MINGW32%\bin;%PATH%");
system("clang out_p1.ll -o out_p1.exe -lLLVM-C -target x86_64-pc-windows-gnu -I \"C:\\tools\\llvm\\include\" -L \"C:\\tools\\LLVM\\lib\"");
// Initialize LLVM
LLVMInitializeNativeTarget();
LLVMInitializeNativeAsmPrinter();
LLVMInitializeNativeAsmParser();
// Create an execution engine
error = NULL;
LLVMExecutionEngineRef engine;
LLVMCreateExecutionEngineForModule(&engine, module, &error);
if (error) {
fprintf(stderr, "Failed to create Execution Engine: %s\n", error);
LLVMDisposeMessage(error);
//return 1;
exit(EXIT_FAILURE);
}
// Look up the main function using the execution engine
void (*main)() = (void (*)())LLVMGetFunctionAddress(engine, "main");
main(); // Call the function
// Clean up
LLVMDisposeBuilder(builder);
LLVMDisposeModule(module);
return 0;
}
Compile and Run
- Use 3 steps
Compile the C source code to LLVM IR using Clang
clang -S -emit-llvm ir.c -o ir.ll -target x86_64-pc-windows-gnu -v -I "C:\tools\llvm\include"
Compile LLVM IR to object file using LLVM toolchain
llvm-llc ir.ll -filetype=obj -o ir.o
Link object file with Clang and create the executable
gcc ir.o -o ir.exe -I "C:\tools\llvm\include" -L "C:\tools\LLVM\lib" -lLLVM-C
Or link object file with MinGW and create the executable
clang ir.o -o ir.exe -target x86_64-pc-windows-gnu -I "C:\tools\llvm\include" -L "C:\tools\LLVM\lib" -lLLVM-C
- just use 2 commands with Clang
Compile the C source code to LLVM IR using Clang
clang -S -emit-llvm ir.c -o ir.ll -target x86_64-pc-windows-gnu -v -I "C:\tools\llvm\include"
Compile LLVM IR and link to create the executable
clang ir.ll -o ir.exe -lLLVM-C -target x86_64-pc-windows-gnu -I "C:\tools\llvm\include" -L "C:\tools\LLVM\lib"
- Or just use 1 command with MinGW
Compile and link to create the executable
gcc p1.c -o p1 -I "C:\tools\llvm\include" -L "C:\tools\LLVM\lib" 
