源码解读---mem2reg源码（3）

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这篇文章接着之前写的。。
源码解读—mem2reg源码（1）
源码解读—mem2reg源码（2）

本文主要介绍在插入phi节点后的重命名。

重命名中第一个核心函数是RenamePass这个函数，看注释：

/// Recursively traverse the CFG of the function, renaming loads and
/// stores to the allocas which we are promoting.
///
/// IncomingVals indicates what value each Alloca contains on exit from the
/// predecessor block Pred.
void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
                                RenamePassData::ValVector &IncomingVals,
                                RenamePassData::LocationVector &IncomingLocs,
                                std::vector<RenamePassData> &Worklist) {
NextIteration:
  // If we are inserting any phi nodes into this BB, they will already be in the
  // block.
  if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
    // If we have PHI nodes to update, compute the number of edges from Pred to
    // BB.
    if (PhiToAllocaMap.count(APN)) {
      // We want to be able to distinguish between PHI nodes being inserted by
      // this invocation of mem2reg from those phi nodes that already existed in
      // the IR before mem2reg was run.  We determine that APN is being inserted
      // because it is missing incoming edges.  All other PHI nodes being
      // inserted by this pass of mem2reg will have the same number of incoming
      // operands so far.  Remember this count.
      // 这一堆文字说的是，我们的PHI节点可能源程序就有，所以为了区别源程序中的PHI节点和我们添加的PHI节点，
      //	我们获取它的操作数，我们添加的PHI节点的操作数一定相等，且初始值为0，即没有操作数。
      unsigned NewPHINumOperands = APN->getNumOperands();

      unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
      assert(NumEdges && "Must be at least one edge from Pred to BB!");

      // Add entries for all the phis.
      BasicBlock::iterator PNI = BB->begin();
      do {
        unsigned AllocaNo = PhiToAllocaMap[APN];

        // Update the location of the phi node.
        updateForIncomingValueLocation(APN, IncomingLocs[AllocaNo],
                                       APN->getNumIncomingValues() > 0);

        // Add N incoming values to the PHI node.
        // 为PHI节点添加操作数
        for (unsigned i = 0; i != NumEdges; ++i)
          APN->addIncoming(IncomingVals[AllocaNo], Pred);		<---------- (1)

        // The currently active variable for this block is now the PHI.
        IncomingVals[AllocaNo] = APN;
        for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[AllocaNo])
          ConvertDebugDeclareToDebugValue(DII, APN, DIB);

        // Get the next phi node.
        ++PNI;
        APN = dyn_cast<PHINode>(PNI);
        if (!APN)
          break;

        // Verify that it is missing entries.  If not, it is not being inserted
        // by this mem2reg invocation so we want to ignore it.
      } while (APN->getNumOperands() == NewPHINumOperands);
    }
  }

  // Don't revisit blocks.
  if (!Visited.insert(BB).second)
    return;
	// 下面的for循环是在做替换。
	// 比如 %a = alloca i32
	//		%0 = load i32 %a
	//		store i32 5, i32* %0
	//		store i32* %0, i32* %1
	//		那么就会变成：
	//		store i32 5, i32* %1
	//		else分支中的store同上，昨晚这一切之后，删除相对应的load和store
  for (BasicBlock::iterator II = BB->begin(); !II->isTerminator();) {
    Instruction *I = &*II++; // get the instruction, increment iterator

    if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
      AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
      if (!Src)
        continue;

      DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
      if (AI == AllocaLookup.end())
        continue;

      Value *V = IncomingVals[AI->second];

      // If the load was marked as nonnull we don't want to lose
      // that information when we erase this Load. So we preserve
      // it with an assume.
      if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
          !isKnownNonZero(V, SQ.DL, 0, AC, LI, &DT))
        addAssumeNonNull(AC, LI);

      // Anything using the load now uses the current value.
      LI->replaceAllUsesWith(V);
      BB->getInstList().erase(LI);
    } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
      // Delete this instruction and mark the name as the current holder of the
      // value
      AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
      if (!Dest)
        continue;

      DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
      if (ai == AllocaLookup.end())
        continue;

      // what value were we writing?
      unsigned AllocaNo = ai->second;
      IncomingVals[AllocaNo] = SI->getOperand(0);

      // Record debuginfo for the store before removing it.
      IncomingLocs[AllocaNo] = SI->getDebugLoc();
      for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[ai->second])
        ConvertDebugDeclareToDebugValue(DII, SI, DIB);
      BB->getInstList().erase(SI);
    }
  }

  // 'Recurse' to our successors.
  succ_iterator I = succ_begin(BB), E = succ_end(BB);
  if (I == E)
    return;

  // Keep track of the successors so we don't visit the same successor twice
  SmallPtrSet<BasicBlock *, 8> VisitedSuccs;

  // Handle the first successor without using the worklist.
  VisitedSuccs.insert(*I);
  Pred = BB;
  BB = *I;
  ++I;

  for (; I != E; ++I)
    if (VisitedSuccs.insert(*I).second)
      Worklist.emplace_back(*I, Pred, IncomingVals, IncomingLocs);

  goto NextIteration;
}

第一个箭头处代码的意思是，我们为PHI 节点加上入口边。什么意思？还记得我们在上篇文章说了，上篇文章放置PHI节点的时候，只是放了一个PHI节点的空壳，还没有数据，这里就是给PHI节点放数据的。比如上篇文章的PHI节点是：

%1 = PHI i32

箭头处加完数据后：

%1 = PHI i32 10, i32* %0

后面一段代码就是做一些代码收尾工作，清除无用的alloca代码。

// Remove the allocas themselves from the function.
  for (Instruction *A : Allocas) {
    // If there are any uses of the alloca instructions left, they must be in
    // unreachable basic blocks that were not processed by walking the dominator
    // tree. Just delete the users now.
    if (!A->use_empty())
      A->replaceAllUsesWith(UndefValue::get(A->getType()));
    A->eraseFromParent();
  }

  // Remove alloca's dbg.declare instrinsics from the function.
  for (auto &Declares : AllocaDbgDeclares)
    for (auto *DII : Declares)
      DII->eraseFromParent();

  // Loop over all of the PHI nodes and see if there are any that we can get
  // rid of because they merge all of the same incoming values.  This can
  // happen due to undef values coming into the PHI nodes.  This process is
  // iterative, because eliminating one PHI node can cause others to be removed.
  bool EliminatedAPHI = true;
  while (EliminatedAPHI) {
    EliminatedAPHI = false;

    // Iterating over NewPhiNodes is deterministic, so it is safe to try to
    // simplify and RAUW them as we go.  If it was not, we could add uses to
    // the values we replace with in a non-deterministic order, thus creating
    // non-deterministic def->use chains.
    for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
             I = NewPhiNodes.begin(),
             E = NewPhiNodes.end();
         I != E;) {
      PHINode *PN = I->second;

      // If this PHI node merges one value and/or undefs, get the value.
      if (Value *V = SimplifyInstruction(PN, SQ)) {		<-----------
        PN->replaceAllUsesWith(V);
        PN->eraseFromParent();
        NewPhiNodes.erase(I++);
        EliminatedAPHI = true;
        continue;
      }
      ++I;
    }
  }

箭头处的代码比较有意思，是看我们插入的PHI节点能不能被简化，这部分代码比较长，建议感兴趣自己去看看。其实影响不会很大。

最后一段代码就不放了，主要就是处理那些不能被遍历到的基本块。基本就是做些收尾工作。

到这里，整个mem2reg的代码分析完成了，其实并不难，难得是要想清楚整个过程，难的是写出健壮、鲁棒性高的代码。。

好了，就这样。。

后面有空会分析其他pass。

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