建議可搭配 LLVM 寫一個 pass - 教學入門篇 一起玩
另外這是 GCC 的對照組GCC : Control Flow Graph
0. 使用前注意
通常在 LLVM 裡面你會是在某個 Pass 裡面去走訪 CFG,而這個時候你必須明確告訴 LLVM 你有使用 CFG, 然後他在呼叫你的 pass 之前就一定會幫你準備好
要作到這件事則必須把以下函數加入到你的 Pass 的那個 struct/class 中
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
1. 走訪該函數所有的 Basic Block
Function F;
for (Function::iterator itr = F.begin();
itr != F.end();
++itr) {
BasicBlock &bb = *itr;
}
2. 走訪 basic block 的所有 successor
BasicBlock bb;
for (succ_iterator itr = succ_begin(&bb);
itr != succ_end(&bb);
++itr) {
BasicBlock *succ = *itr;
}
3. 走訪 basic block 的所有 Predecessor
BasicBlock bb;
for (pred_iterator itr = pred_begin(&bb);
itr != pred_end(&bb);
++itr) {
BasicBlock *pred = *itr;
}
4. 取得 Function 的 Entry Basic Block
要注意的是收的變數最好是 Reference !Function F; BasicBlock &bb = F.getEntryBlock();
5. Basic Block中所有指令
BasicBlock bb;
for (BasicBlock::iterator itr = bb.begin();
itr != bb.end();
++itr) {
Instruction &inst = *itr;
}
跳過所有 PHI Node
BasicBlock bb;
for (BasicBlock::iterator itr = bb.getFirstNonPHI();
itr != bb.end();
++itr) {
Instruction &inst = *itr;
}
另外還有getFirstNonPHIOrDbg () 及 getFirstNonPHIOrDbgOrLifetime () 可以跳過除錯資訊及Lifetime 的資訊
6. 走訪一個函數裡面所有 BB 中的指令之FunctionPass 範例程式碼
#define DEBUG_TYPE "hello"
#include "llvm/Pass.h"
#include "llvm/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/CFG.h"
using namespace llvm;
STATISTIC(HelloCounter, "Counts number of functions greeted");
namespace {
// Hello - The first implementation, without getAnalysisUsage.
struct Hello : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
Hello() : FunctionPass(ID) {}
virtual bool runOnFunction(Function &F) {
++HelloCounter;
errs() << "Hello: ";
errs() << F.getName() << '\n';
for (Function::iterator itr = F.begin();
itr != F.end();
++itr) {
BasicBlock &bb = *itr;
errs() << "Basic block :" << bb.getName () << "\n";
errs() << "successor : {";
for (succ_iterator ssitr = succ_begin(&bb);
ssitr != succ_end(&bb);
++ssitr) {
BasicBlock *ss = *ssitr;
errs() << ss->getName() <<", ";
}
errs() << "}\n";
errs() << "predecessors : {";
for (pred_iterator ssitr = pred_begin(&bb);
ssitr != pred_end(&bb);
++ssitr) {
BasicBlock *ss = *ssitr;
errs() << ss->getName() <<", ";
}
errs() << "}\n";
for (BasicBlock::iterator iitr = bb.begin();
iitr != bb.end();
++iitr) {
Instruction &inst = *iitr;
errs() << inst << '\n';
}
}
return false;
}
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
}
};
}
char Hello::ID = 0;
static RegisterPass<Hello> X("hello", "Hello World Pass");
