Question

我正在玩LLVM。我想过在中间代码中改变常量的值。但是，对于类llvm::ConstantInt，我没有看到setvalue函数。任何想法如何修改IR代码中常量的值？

Answer 1

ConstantInt是一家工厂，不是吗？类具有get method来构造新常量：

     /* ... return a ConstantInt for the given value. */
00069   static Constant *get(Type *Ty, uint64_t V, bool isSigned = false);

因此，我认为，您无法修改现有的ConstantInt。如果要修改IR，则应尝试将指针更改为参数（更改IR本身，而不是常量对象）。

可能你想要这样的东西（请记住，我对LLVM没有经验;我几乎可以肯定的例子是不正确的。）

Instruction *I = /* your argument */;
/* check that instruction is of needed format, e.g: */
if (I->getOpcode() == Instruction::Add) {
   /* read the first operand of instruction */
   Value *oldvalue = I->getOperand(0);

   /* construct new constant; here 0x1234 is used as value */
   Value *newvalue = ConstantInt::get(oldValue->getType(), 0x1234); 

   /* replace operand with new value */
   I->setOperand(0, newvalue);
}

单独“修改”常量有一个解决方案（递增和递减are illustrated）：

 /// AddOne - Add one to a ConstantInt.
 static Constant *AddOne(Constant *C) {
   return ConstantExpr::getAdd(C, ConstantInt::get(C->getType(), 1));
 }

 /// SubOne - Subtract one from a ConstantInt.
 static Constant *SubOne(ConstantInt *C) {
   return ConstantInt::get(C->getContext(), C->getValue()-1);
 }

PS，Constant.h在关于创建和不删除常量http://llvm.org/docs/doxygen/html/Constant_8h_source.html

的乞求中有重要的评论。

00035 /// Note that Constants are immutable (once created they never change) 
00036 /// and are fully shared by structural equivalence.  This means that two 
00037 /// structurally equivalent constants will always have the same address.  
00038 /// Constants are created on demand as needed and never deleted: thus clients 
00039 /// don't have to worry about the lifetime of the objects.
00040 /// @brief LLVM Constant Representation

设置llvm :: ConstantInt的值

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