LLVM Programmer’s Manual
llvm.org
llvm.org
Support for Motorola 68000 has never been finished but this guy's abandoned fork offers an interesting glimpse into how one might start going about it though:
https://github.com/kwaters/llvm-m68k/commits/m68k
A bit over my head for now but I'm using this as a fun occasion to get back into "proper" computer science. Dabbling is my favourite learning mode so consulting the LLVM Programmer's Manual while hacking on a more isolated and really simple task is something I'm very much looking forward to at this point.
Still, if there is anybody with more knowledge who could chip in a bit on supporting m68k or LLVM in general I'd be super curious to read about it :)
On the list of "stuff llvm got wrong", i feel like this wouldn't even make the top hundred.
I think the only situation where exceptions help is in constructors, but there are better solutions to that problem, like explicit object construction methods that can return an error.
I think Go's approach to error handling is the best - explicit and in context. Rust's `Result` looks quite nice though I haven' tried it yet.
I've said it here before, but this is my classification of errors that exceptions are commonly used for:
1) Programming error: null pointer exception, library invariants broken, etc. These generally shouldn't be caught, and could arguably be replaced by fatal assertions, but that's typically not a polite thing to do in a library.
2) Non-deterministic failure condition: something outside the control of the calling code failed in a way that it cannot predict ahead of time. A network connection broke, a file was deleted unexpectedly, a device was detached, database server died, etc. There's something of an argument for annotating functions that can fail this way in the manner of Java's checked exceptions and it has a strong relationship with the IO monad in Haskell (and similar problems with encapsulation and hiding implementation details). These kinds of errors can sometimes be handled, and handling them with exceptions isn't much better or worse than error codes. An advantage of exceptions is that they makes it easy to abort the whole task and go back to the request handler (server code) or event dispatch loop (UI code). Exceptions can carry more state than error codes, too.
3) Business logic problem: the user, for want of a better term, of the program tried to do something that is not allowed / possible, and the program needs to abort the current task and inform the user of the problem. Exceptions are as good a means of aborting as most others. These should only be caught at the top level, whereupon they can be converted into something for human / 422 response / whatever consumption.
In almost all cases, you cannot proceed and there is nothing you can do in reaction to the error. Aborting and unwinding the call stack is usually the correct thing to do.
I haven't used Go (I'm about to for a small project, though) so I can't comment on the "if err != nil" idiom, but it seems like a step back by encouraging code duplication where it's not needed. I'm a fan of the pattern matching in rust, but it's still rather orthogonal to exceptions as it allows you to deal with an error when you try to access state whereas exceptions are most useful when they appear mid operation.
Go's implementation on JSON parsing is also.....
Platform::COMException
https://msdn.microsoft.com/en-us/library/windows/apps/hh7104...
Either your function can complete its task successfully, in which case it satisfies whatever invariants are required of it and returns accordingly, or it can’t, in which case it can throw an exception to indicate that inability to return successfully. In the success case, the calling code can then continue based on whatever returned value and/or side effects the function was supposed to provide. In the failure case, the calling code may well fail itself if it can also no longer complete its task successfully, and so on up the stack until you reach a level in your design that knows how to recover from the failure or stop the entire program gracefully, which is where you catch the exception.
I’ve never quite understood the idea that exceptions are similar to goto. Other than transferring control over potentially long “distances” within a program, they don’t seem to have much in common at all. To me, a much closer analogy would be returning early from a function on success, if you reach a point in your algorithm where you already know the required answer to whatever question your calling code was asking. Another similar situation would be using a break or continue statement to finish an iteration early within a loop, or adding extra guard cases to stop a recursion early, again if you already know the outcome and continuing with the full algorithm has no advantage.
That is the point, but my specific objection is to the dynamic binding and dynamic typing, not the returning. With “return”, even early return, you don’t really need to reason about where you’ll end up at all—it’s always the end of the current function. And the same goes for functions that throw exceptions directly.
The problem is that when you have unchecked exceptions, any function can throw, so every function call in your code is now also a potential early return, and for correct code you need to account for that.
This means that writing state-modifying code becomes unreasonably difficult, as you can no longer just write:
// A and B must be kept in sync and updated together
A = computeA()
B = computeB()
Yes, there are ways round this involving temporary variables. So you write:
tmpA = computeA()
tmpB = computeB()
A = tmpA, B = tmpB;
Personally I don't think that exceptions work very well in the presence of mutable state. Without mutable state, they're exactly equivalent to
Exceptions with mutable state needs to be coded transactionally, yes, and this is an excellent reason to try and avoid mutable state and use persistent data structures where possible.
.. which makes using the STL surprisingly hard, because you can get an out of memory exception all over the place.
In practice, I think this issue gets far more attention than it deserves, particularly within the C++ community. If you’re running in a particularly resource-constrained environment where running out is a relevant problem, you probably already take precautions about how you allocate those resources and adjust how you structure your program accordingly. You’re trying to avoid any possibility of those errors occurring, which means you don’t then have to worry about dealing with them unexpectedly all over your code.
For me, the more interesting discussing is around how we deal with errors that can be properly recognised and handled within normal execution. That means the interesting exceptions are those we throw ourselves and those deliberately thrown by other modules that we depend on, which we hope will be documented with each module’s interface. These things don’t just go off arbitrarily in just about any line of code we write, and they can be systematically structured, and we can write code that controls where mutations and other effects happen and where exceptions happen and how they interact.
It's even harder to get right concurrency with exceptions :D
No it isn't.
The bad idea was making language features optional.
Library writers are forced to either support all possible combinations, don't use any of them or use them and see their library being rejected by those that won't turn on the features even at point gun.
One of the things that made me initially enjoy Java was that there weren't features to turn on or off.
They don't play well with manual memory management or the C semantics C++ was trying to be as compatible as possible.
They don't play well with:
- constructors/destructors
- manual resource management
- low level code like UNIX signals
- allowing every possible type to be used as exception
So all of that made C++ exceptions poor cousins of how exceptions are used in other languages.
I have used exceptions in quite a few languages and rather use them (the alternative being sum types), but do understand that how C++ evolved they are an unwelcome feature.
A ban on C++ exceptions is a huge red flag for me. A project has to be really special if I'm going to contribute to it after it's crippled the language.
void read_file_by_lines(const char* path, void (*callback)(const char* line)) {
FILE* file = fopen(path, "r");
size_t length = 0;
char* line = NULL;
while (-1 != getline(&line, &length, file)) {
callback(line);
}
free(line);
fclose(file);
}
C++ exceptions are predicated on the assumption that all resources are managed using RAII. In the real world, this isn't true, especially considering that C doesn't even have RAII.
Python programmers (including me) love their exceptions, because Python is a dynamic language, and exposes system resources as reference counted objects.
The most general definition of the restriction is that C++ code cannot throw exceptions through exception unsafe code. How do you know if your C++ function was called from exception unsafe code? You can't!
So it's only ever safe to throw an exception when you are expecting it to be caught immediately i.e. you can use exceptions as glorified error codes, but not much else.
In Django I can throw a Http404 exception from deep inside a view and the server will come up with a 404 response. I can assert and the client will get a 500 error but the server won't go down. This is the real power of exceptions, and God help anybody who tries to do it in C++.
In the enterprise context you can hardly count on proper documentation, specially in projects with high attrition of consultants where each one comes, implements something and departures to the next gig.
Not to count the commercial libraries distributed in binary only form.
So one never knows if the code is exception safe, don't work with exceptions or is exception happy.
I think there are examples related to inter-library exceptions that might be much more subtle than this one, but I've never understood the subtlety.
C++ users in particular have strong concerns about things like RTTI and the overhead of exceptions not thrown. The cost of making unthrown exceptions low (near zero) is that throwing exceptions is much more expensive. This turns into a feedback loop, where C++ programs end up performing better if they never throw any exceptions because of the tradeoff.
This feeds into self-hosted compilers. Thing is, certain compiler design techniques can work well with cheap unwinding: constraint solving (more common in languages with better type systems than C++), forward-looking grammar assertions with speculative parsing, IDE-integrated code completion (implement the lexer to add the cursor position as a special token, and jump out of the parser when it's found, where all the context is available to pass along). But C++'s peculiar tradeoffs may make using things like exceptions less than ideal to use for this purpose, not because exceptions are the wrong tool, but because most C++ programs are not compilers and aren't tuned for it.
And OTOH, compilers written in low-level languages like C++ often twist themselves into knots to simulate things like tree pattern matching, multiple dispatch, destructuring binds, etc. Sometimes they even give up and write code generating tools.
So by having exceptions, with code that only knows about C semantics but was compiled with C++ mode, you open the door to all sorts of nasty surprises.
So exceptions can cause resource leaks and other issues like being thrown in the middle of a UNIX signal.
Which leads to the situation that sadly many library writers avoid them.
I still enjoy using C++, but the best place for it is as infrastructure language for the bottom layer when performance requires it, then using something else on the other layers.
That could be a fun thing to have a language competition that way, to see which language "feels" good: easy to read, expressive, etc.
If I wanted to implement a language with Lua-style coroutines, could I target LLVM?