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

所以,我认为,你无法修改现有的ConstantInt.如果要修改IR,则应尝试将指针更改为参数(更改IR本身,但不更改常量对象).

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);
}

要“修改”一个常量,就有一个解决方案(增量和减量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的请求中有重要的评论

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