ConstantInt 是一个工厂，不是吗？类有获取方法 http://llvm.org/docs/doxygen/html/classllvm_1_1ConstantInt.html#a1c51933b9fb89c364a098fad544cba96构造新常量：
/* ... return a ConstantInt for the given value. */
00069 static Constant *get(Type *Ty, uint64_t V, bool isSigned = false);
所以，我认为，你不能修改现有的 ConstantInt。如果你想修改 IR，你应该尝试改变指向参数的指针（改变 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示例是不正确的）。
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”常量，有一个解决方案（递增和递减有插图 http://llvm.org/doxygen/InstCombineAddSub_8cpp_source.html#l00023):
/// 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 在请求创建和不删除常量时有重要注释http://llvm.org/docs/doxygen/html/Constant_8h_source.html http://llvm.org/docs/doxygen/html/Constant_8h_source.html
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
