examples/Fibonacci/fibonacci.cpp

   1 //===--- examples/Fibonacci/fibonacci.cpp - An example use of the JIT -----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This small program provides an example of how to build quickly a small module
  11 // with function Fibonacci and execute it with the JIT.
  12 //
  13 // The goal of this snippet is to create in the memory the LLVM module
  14 // consisting of one function as follow:
  15 //
  16 //   int fib(int x) {
  17 //     if(x<=2) return 1;
  18 //     return fib(x-1)+fib(x-2);
  19 //   }
  20 //
  21 // Once we have this, we compile the module via JIT, then execute the `fib'
  22 // function and return result to a driver, i.e. to a "host program".
  23 //
  24 //===----------------------------------------------------------------------===//
  25
  26 #include "llvm/LLVMContext.h"
  27 #include "llvm/Module.h"
  28 #include "llvm/DerivedTypes.h"
  29 #include "llvm/Constants.h"
  30 #include "llvm/Instructions.h"
  31 #include "llvm/ModuleProvider.h"
  32 #include "llvm/Analysis/Verifier.h"
  33 #include "llvm/ExecutionEngine/JIT.h"
  34 #include "llvm/ExecutionEngine/Interpreter.h"
  35 #include "llvm/ExecutionEngine/GenericValue.h"
  36 #include "llvm/Support/raw_ostream.h"
  37 using namespace llvm;
  38
  39 static Function *CreateFibFunction(Module *M, LLVMContext &Context) {
  40   // Create the fib function and insert it into module M.  This function is said
  41   // to return an int and take an int parameter.
  42   Function *FibF =
  43     cast<Function>(M->getOrInsertFunction("fib", Type::getInt32Ty(Context),
  44                                           Type::getInt32Ty(Context),
  45                                           (Type *)0));
  46
  47   // Add a basic block to the function.
  48   BasicBlock *BB = BasicBlock::Create(Context, "EntryBlock", FibF);
  49
  50   // Get pointers to the constants.
  51   Value *One = ConstantInt::get(Type::getInt32Ty(Context), 1);
  52   Value *Two = ConstantInt::get(Type::getInt32Ty(Context), 2);
  53
  54   // Get pointer to the integer argument of the add1 function...
  55   Argument *ArgX = FibF->arg_begin();   // Get the arg.
  56   ArgX->setName("AnArg");            // Give it a nice symbolic name for fun.
  57
  58   // Create the true_block.
  59   BasicBlock *RetBB = BasicBlock::Create(Context, "return", FibF);
  60   // Create an exit block.
  61   BasicBlock* RecurseBB = BasicBlock::Create(Context, "recurse", FibF);
  62
  63   // Create the "if (arg <= 2) goto exitbb"
  64   Value *CondInst = new ICmpInst(*BB, ICmpInst::ICMP_SLE, ArgX, Two, "cond");
  65   BranchInst::Create(RetBB, RecurseBB, CondInst, BB);
  66
  67   // Create: ret int 1
  68   ReturnInst::Create(Context, One, RetBB);
  69
  70   // create fib(x-1)
  71   Value *Sub = BinaryOperator::CreateSub(ArgX, One, "arg", RecurseBB);
  72   CallInst *CallFibX1 = CallInst::Create(FibF, Sub, "fibx1", RecurseBB);
  73   CallFibX1->setTailCall();
  74
  75   // create fib(x-2)
  76   Sub = BinaryOperator::CreateSub(ArgX, Two, "arg", RecurseBB);
  77   CallInst *CallFibX2 = CallInst::Create(FibF, Sub, "fibx2", RecurseBB);
  78   CallFibX2->setTailCall();
  79
  80
  81   // fib(x-1)+fib(x-2)
  82   Value *Sum = BinaryOperator::CreateAdd(CallFibX1, CallFibX2,
  83                                          "addresult", RecurseBB);
  84
  85   // Create the return instruction and add it to the basic block
  86   ReturnInst::Create(Context, Sum, RecurseBB);
  87
  88   return FibF;
  89 }
  90
  91
  92 int main(int argc, char **argv) {
  93   int n = argc > 1 ? atol(argv[1]) : 24;
  94
  95   LLVMContext Context;
  96
  97   // Create some module to put our function into it.
  98   Module *M = new Module("test", Context);
  99
 100   // We are about to create the "fib" function:
 101   Function *FibF = CreateFibFunction(M, Context);
 102
 103   // Now we going to create JIT
 104   ExecutionEngine *EE = EngineBuilder(M).create();
 105
 106   errs() << "verifying... ";
 107   if (verifyModule(*M)) {
 108     errs() << argv[0] << ": Error constructing function!\n";
 109     return 1;
 110   }
 111
 112   errs() << "OK\n";
 113   errs() << "We just constructed this LLVM module:\n\n---------\n" << *M;
 114   errs() << "---------\nstarting fibonacci(" << n << ") with JIT...\n";
 115
 116   // Call the Fibonacci function with argument n:
 117   std::vector<GenericValue> Args(1);
 118   Args[0].IntVal = APInt(32, n);
 119   GenericValue GV = EE->runFunction(FibF, Args);
 120
 121   // import result of execution
 122   outs() << "Result: " << GV.IntVal << "\n";
 123   return 0;
 124 }