从llvm删除完整分支ir

Question

在ir中有一个分支我想完全删除（condtion + branch + true_basic_block + false_basic_block）。它看起来像这样：

%4 = icmp sge i32 %2, %3 
br i1 %4, label %5, label %7 

; <label>:5          ; preds = %0 
%6 = load i32* %x, align 4 
store i32 %6, i32* %z, align 4 
br label %9 

; <label>:7          ; preds = %0 
%8 = load i32* %y, align 4 
store i32 %8, i32* %z, align 4 
br label %9 

; <label>:9          ; preds = %7, %5 
%10 = call dereferenceable(140) %"class.std::basic_ostream"*@_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(%"class.std::basic_ostream"* dereferenceable(140) @_ZSt4cout, i8* getelementptr inbounds ([5 x i8]* @.str, i32 0, i32 0)) 
%11 = load i32* %z, align 4 
%12 = call dereferenceable(140) %"class.std::basic_ostream"* @_ZNSolsEi(%"class.std::basic_ostream"* %10, i32 %11) 
%13 = call dereferenceable(140) %"class.std::basic_ostream"* @_ZNSolsEPFRSoS_E(%"class.std::basic_ostream"* %12, %"class.std::basic_ostream"* (%"class.std::basic_ostream"*)* @_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_) 
ret i32 0 

现在把它删除，是否有removeBranch功能，或者我需要删除的指令一个接一个。我一直在尝试后一种方式，但我已经看到从“基本块在主没有终止符”的每个错误“使用仍然当def被破坏”，以及更多..我使用了erasefromparent，replaceinstwithvalue，replaceinstwithinst，removefromparent等

任何人都可以足以指出我在正确的方向吗？

这是我function_pass：

bool runOnFunction(Function &F) override { 
    for (auto& B : F) 
    for (auto& I : B) 
      if(auto* brn = dyn_cast<BranchInst>(&I))  
       if(brn->isConditional()){ 
        Instruction* cond = dyn_cast<Instruction>(brn->getCondition()); 
        if(cond->getOpcode() == Instruction::ICmp){ 
         branch_vector.push_back(brn); 
         //removeConditionalBranch(dyn_cast<BranchInst>(brn)); 
        } 
       } 

/*For now just delete the branches in the vector.*/ 
for(auto b : branch_vector) 
     removeConditionalBranch(dyn_cast<BranchInst>(b)); 

return true; 
} 

这是输出：

Answer 1

我不知道任何RemoveBranch实用功能的，但这样的事情应该工作。这个想法是删除分支指令，然后删除任何因此而死的东西，然后将初始块与连接块合并。

// for DeleteDeadBlock, MergeBlockIntoPredecessor 
#include "llvm/Transforms/Utils/BasicBlockUtils.h" 
// for RecursivelyDeleteTriviallyDeadInstructions 
#include "llvm/Transforms/Utils/Local.h" 

void removeConditionalBranch(BranchInst *Branch) { 
    assert(Branch && 
     Branch->isConditional() && 
     Branch->getNumSuccessors() == 2); 

    BasicBlock *Parent = Branch->getParent(); 
    BasicBlock *ThenBlock = Branch->getSuccessor(0); 
    BasicBlock *ElseBlock = Branch->getSuccessor(1); 

    BasicBlock *ThenSuccessor = ThenBlock->getUniqueSuccessor(); 
    BasicBlock *ElseSuccessor = ElseBlock->getUniqueSuccessor(); 
    assert(ThenSuccessor && ElseSuccessor && ThenSuccessor == ElseSuccessor); 

    Branch->eraseFromParent(); 
    RecursivelyDeleteTriviallyDeadInstructions(Branch->getCondition()); 
    DeleteDeadBlock(ThenBlock); 
    DeleteDeadBlock(ElseBlock); 

    IRBuilder<> Builder(Parent); 
    Builder.CreateBr(ThenSuccessor); 
    bool Merged = MergeBlockIntoPredecessor(ThenSuccessor); 
    assert(Merged); 
} 

此代码只处理您已经证明了简单的情况下，与当时和else块都无条件地转移到共同的跳跃加入块（它将失败，对任何事情更复杂的断言错误）。更复杂的控制流将会有点棘手，但您仍应该能够使用此代码作为起点。