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      1 //===-- examples/HowToUseJIT/HowToUseJIT.cpp - An example use of the JIT --===//
      2 //
      3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
      4 // See https://llvm.org/LICENSE.txt for license information.
      5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
      6 //
      7 //===----------------------------------------------------------------------===//
      8 //
      9 //  This small program provides an example of how to quickly build a small
     10 //  module with two functions and execute it with the JIT.
     11 //
     12 // Goal:
     13 //  The goal of this snippet is to create in the memory
     14 //  the LLVM module consisting of two functions as follow:
     15 //
     16 // int add1(int x) {
     17 //   return x+1;
     18 // }
     19 //
     20 // int foo() {
     21 //   return add1(10);
     22 // }
     23 //
     24 // then compile the module via JIT, then execute the `foo'
     25 // function and return result to a driver, i.e. to a "host program".
     26 //
     27 // Some remarks and questions:
     28 //
     29 // - could we invoke some code using noname functions too?
     30 //   e.g. evaluate "foo()+foo()" without fears to introduce
     31 //   conflict of temporary function name with some real
     32 //   existing function name?
     33 //
     34 //===----------------------------------------------------------------------===//
     35 
     36 #include "llvm/ADT/STLExtras.h"
     37 #include "llvm/ExecutionEngine/ExecutionEngine.h"
     38 #include "llvm/ExecutionEngine/GenericValue.h"
     39 #include "llvm/ExecutionEngine/MCJIT.h"
     40 #include "llvm/IR/Argument.h"
     41 #include "llvm/IR/BasicBlock.h"
     42 #include "llvm/IR/Constants.h"
     43 #include "llvm/IR/DerivedTypes.h"
     44 #include "llvm/IR/Function.h"
     45 #include "llvm/IR/IRBuilder.h"
     46 #include "llvm/IR/Instructions.h"
     47 #include "llvm/IR/LLVMContext.h"
     48 #include "llvm/IR/Module.h"
     49 #include "llvm/IR/Type.h"
     50 #include "llvm/Support/Casting.h"
     51 #include "llvm/Support/ManagedStatic.h"
     52 #include "llvm/Support/TargetSelect.h"
     53 #include "llvm/Support/raw_ostream.h"
     54 #include <algorithm>
     55 #include <cassert>
     56 #include <memory>
     57 #include <vector>
     58 
     59 using namespace llvm;
     60 
     61 int main() {
     62   InitializeNativeTarget();
     63   LLVMInitializeNativeAsmPrinter();
     64 
     65   LLVMContext Context;
     66 
     67   // Create some module to put our function into it.
     68   std::unique_ptr<Module> Owner = std::make_unique<Module>("test", Context);
     69   Module *M = Owner.get();
     70 
     71   // Create the add1 function entry and insert this entry into module M.  The
     72   // function will have a return type of "int" and take an argument of "int".
     73   Function *Add1F =
     74       Function::Create(FunctionType::get(Type::getInt32Ty(Context),
     75                                          {Type::getInt32Ty(Context)}, false),
     76                        Function::ExternalLinkage, "add1", M);
     77 
     78   // Add a basic block to the function. As before, it automatically inserts
     79   // because of the last argument.
     80   BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", Add1F);
     81 
     82   // Create a basic block builder with default parameters.  The builder will
     83   // automatically append instructions to the basic block `BB'.
     84   IRBuilder<> builder(BB);
     85 
     86   // Get pointers to the constant `1'.
     87   Value *One = builder.getInt32(1);
     88 
     89   // Get pointers to the integer argument of the add1 function...
     90   assert(Add1F->arg_begin() != Add1F->arg_end()); // Make sure there's an arg
     91   Argument *ArgX = &*Add1F->arg_begin();          // Get the arg
     92   ArgX->setName("AnArg");            // Give it a nice symbolic name for fun.
     93 
     94   // Create the add instruction, inserting it into the end of BB.
     95   Value *Add = builder.CreateAdd(One, ArgX);
     96 
     97   // Create the return instruction and add it to the basic block
     98   builder.CreateRet(Add);
     99 
    100   // Now, function add1 is ready.
    101 
    102   // Now we're going to create function `foo', which returns an int and takes no
    103   // arguments.
    104   Function *FooF =
    105       Function::Create(FunctionType::get(Type::getInt32Ty(Context), {}, false),
    106                        Function::ExternalLinkage, "foo", M);
    107 
    108   // Add a basic block to the FooF function.
    109   BB = BasicBlock::Create(Context, "EntryBlock", FooF);
    110 
    111   // Tell the basic block builder to attach itself to the new basic block
    112   builder.SetInsertPoint(BB);
    113 
    114   // Get pointer to the constant `10'.
    115   Value *Ten = builder.getInt32(10);
    116 
    117   // Pass Ten to the call to Add1F
    118   CallInst *Add1CallRes = builder.CreateCall(Add1F, Ten);
    119   Add1CallRes->setTailCall(true);
    120 
    121   // Create the return instruction and add it to the basic block.
    122   builder.CreateRet(Add1CallRes);
    123 
    124   // Now we create the JIT.
    125   ExecutionEngine* EE = EngineBuilder(std::move(Owner)).create();
    126 
    127   outs() << "We just constructed this LLVM module:\n\n" << *M;
    128   outs() << "\n\nRunning foo: ";
    129   outs().flush();
    130 
    131   // Call the `foo' function with no arguments:
    132   std::vector<GenericValue> noargs;
    133   GenericValue gv = EE->runFunction(FooF, noargs);
    134 
    135   // Import result of execution:
    136   outs() << "Result: " << gv.IntVal << "\n";
    137   delete EE;
    138   llvm_shutdown();
    139   return 0;
    140 }
    141