设置 llvm::ConstantInt 的值

Question

我在玩 LLVM。我考虑过在中间代码中更改常量的值。但是，对于 llvm::ConstantInt 类，我没有看到 setvalue 函数。知道如何修改 IR 代码中的常量值吗？

Answer 1

ConstantInt 是一个工厂，不是吗？类有get method 来构造新的常量：

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

所以，我认为，您不能修改现有的 ConstantInt。如果要修改 IR，则应尝试更改指向参数的指针（更改 IR 本身，而不是常量对象）。

也许你想要这样的东西（请记住，我对 LLVM 的经验为零；而且我几乎可以肯定这个例子是不正确的）。

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);
}

要单独“修改”一个常量，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 有关于创建和不删除Constantshttp://llvm.org/docs/doxygen/html/Constant_8h_source.html的乞求重要评论

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