在LLVM自定义优化过程中删除Segfault指令

Question

这是我写的优化过程。它将任何以零作为其操作数之一的乘法替换为简单的零。

struct FoldConstant : public llvm::FunctionPass {
    static char ID;
    FoldConstant() : llvm::FunctionPass(ID) {}
    bool runOnFunction(llvm::Function &func) override {
        for (auto &block : func) {
            for (auto &inst : block) {
                if (inst.isBinaryOp()) {
                    llvm::Value *left = inst.getOperand(0);
                    llvm::Value *right = inst.getOperand(1);
                    if (inst.getOpcode() == 17) {
                        if (llvm::ConstantInt *lc =
                                llvm::dyn_cast<llvm::ConstantInt>(left)) {
                            if (lc->getSExtValue() == 0) {
                                inst.replaceAllUsesWith(left);
                                inst.eraseFromParent();
                            }
                        }
                        if (llvm::ConstantInt *rc =
                                llvm::dyn_cast<llvm::ConstantInt>(right)) {
                            if (rc->getSExtValue() == 0) {
                                inst.replaceAllUsesWith(right);
                                inst.eraseFromParent();
                            }
                        }
                    }
                }
            }
        }
        return true;
    }
};

现在，当我在下面的IR源上运行它时

; ModuleID = 'bootleg c compiler'
source_filename = "bootleg c compiler"

define i32 @constant_folding(i32 %b) {
entry:
  %b1 = alloca i32
  store i32 %b, i32* %b1
  %b2 = load i32, i32* %b1
  %mul = mul i32 %b2, 0
  ret i32 %mul
}

它给出了一个段错误，下面是堆栈跟踪。

sumit@HAL9001:~/Coding/cc$ opt -S -load ./opt.so -foldconst < output.ll
Stack dump:
0.  Program arguments: opt -S -load ./opt.so -foldconst 
1.  Running pass 'Function Pass Manager' on module '<stdin>'.
2.  Running pass 'constant folding' on function '@constant_folding'
 #0 0x00007f5e0522e4ff llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x9814ff)
 #1 0x00007f5e0522c7b0 llvm::sys::RunSignalHandlers() (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x97f7b0)
 #2 0x00007f5e0522eac5 (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0x981ac5)
 #3 0x00007f5e08f563c0 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x153c0)
 #4 0x00007f5e08f73986 llvm::Value::getValueID() const /usr/lib/llvm-10/include/llvm/IR/Value.h:485:12
 #5 0x00007f5e08f73ae2 llvm::Instruction::getOpcode() const /usr/lib/llvm-10/include/llvm/IR/Instruction.h:125:54
 #6 0x00007f5e08f73b04 llvm::Instruction::isBinaryOp() const /usr/lib/llvm-10/include/llvm/IR/Instruction.h:130:46
 #7 0x00007f5e08f74857 FoldConstant::runOnFunction(llvm::Function&) /home/sumit/Coding/cc/c.opt.cpp:75:17
 #8 0x00007f5e05333d76 llvm::FPPassManager::runOnFunction(llvm::Function&) (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xa86d76)
 #9 0x00007f5e05333ff3 llvm::FPPassManager::runOnModule(llvm::Module&) (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xa86ff3)
#10 0x00007f5e053344a0 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/lib/x86_64-linux-gnu/libLLVM-10.so.1+0xa874a0)
#11 0x00000000004bf3f8 main (/usr/lib/llvm-10/bin/opt+0x4bf3f8)
#12 0x00007f5e043970b3 __libc_start_main /build/glibc-ZN95T4/glibc-2.31/csu/../csu/libc-start.c:342:3
#13 0x00000000004ad0de _start (/usr/lib/llvm-10/bin/opt+0x4ad0de)
Segmentation fault (core dumped)

到目前为止，我已经尝试将代码更改为使用

llvm::BasicBlock::iterator iter(inst);
llvm::ReplaceInstWithValue(block.getInstList(), iter, right);

但这并没有什么不同。任何帮助都将不胜感激。

Answer 1

就像@500-internal-server-error在我的问题的注释中指出的那样，由于我在遍历容器(基本块)时从容器(基本块)中移除了元素(指令)，所以出现了server。我现在正在使用下面的代码，它可以正常工作。

struct FoldConstant : public llvm::FunctionPass {
    static char ID;
    FoldConstant() : llvm::FunctionPass(ID) {}
    bool runOnFunction(llvm::Function &func) override {
        bool changed = false;
        vector<llvm::Instruction *> instsToDelete;

        for (auto &block : func) {
            for (auto &inst : block) {
                if (inst.isBinaryOp()) {
                    llvm::Value *left = inst.getOperand(0);
                    llvm::Value *right = inst.getOperand(1);
                    if (inst.getOpcode() == 17) {
                        if (llvm::ConstantInt *lc =
                                llvm::dyn_cast<llvm::ConstantInt>(left)) {
                            if (lc->getSExtValue() == 0) {
                                instsToDelete.push_back(&inst);
                                inst.replaceAllUsesWith(left);
                                changed = true;
                            }
                        }
                        if (llvm::ConstantInt *rc =
                                llvm::dyn_cast<llvm::ConstantInt>(right)) {
                            if (rc->getSExtValue() == 0) {
                                instsToDelete.push_back(&inst);
                                inst.replaceAllUsesWith(right);
                                changed = true;
                            }
                        }
                    }
                    
                }
            }
        }
        for (auto inst : instsToDelete) {
            inst->eraseFromParent();
        }
        return changed;
    }
};

我将指向需要删除的指令的指针存储在一个向量中，并在以后删除它们。