llvm：将 i8* 类型值转换为 double 类型值

Question

I have a char pointer (Type::getInt8PtrTy(Context) ) and trying to convert a value (string in other words) which holds this pointer into double type (Type::getDoubleTy(Context)). The way I tried is:

Value* stringValue = ... // Value which type Type::getInt8PtrTy(Context)
auto doubleValue = CastInst::CreatePointerCast(stringValue, Type::getDoubleTy(Context), Builder.GetInsertBlock());
AllocaInst *alloca = .. //alloc memory for element which type is Type::getDoubleTy(Context)
Builder.CreateStore(doubleValue, alloca);

The problem is when I'm trying to print doubleValue using external C function (printf), looks like that I got the address but not double value. In other words, I want to write code which do the same like C atoi function. I'm using C++ API.

I would be grateful for the answers.

Answer 1

atoi doesn't simply cast a variable to a different type. It actually converts a string value into an integer by means of some algorithm.

If you want to do the same, you'll have to include a function implementing that algorithm in your bitcode and then use call instruction on your pointer to get the double value.

If you really want to interpret string's data as double, use bitcast to cast the pointer from i8* to double* and then use load instruction.