Hi All,
I’ve been thinking lately about how to improve LLVM’s error model and error reporting. A lack of good error reporting in Orc and MCJIT has forced me to spend a lot of time investigating hard-to-debug errors that could easily have been identified if we provided richer error information to the client, rather than just aborting. Kevin Enderby has made similar observations about the state of libObject and the difficulty of producing good error messages for damaged object files. I expect to encounter more issues like this as I continue work on the MachO side of LLD. I see tackling the error modeling problem as a first step towards improving error handling in general: if we make it easy to model errors, it may pave the way for better error handling in many parts of our libraries.
At present in LLVM we model errors with std::error_code (and its helper, ErrorOr) and use diagnostic streams for error reporting. Neither of these seem entirely up to the job of providing a solid error-handling mechanism for library code. Diagnostic streams are great if all you want to do is report failure to the user and then terminate, but they can’t be used to distinguish between different kinds of errors, and so are unsuited to many use-cases (especially error recovery). On the other hand, std::error_code allows error kinds to be distinguished, but suffers a number of drawbacks:
- It carries no context: It tells you what went wrong, but not where or why, making it difficult to produce good diagnostics.
- It’s extremely easy to ignore or forget: instances can be silently dropped.
- It’s not especially debugger friendly: Most people call the error_code constructors directly for both success and failure values. Breakpoints have to be set carefully to avoid stopping when success values are constructed.
In fairness to std::error_code, it has some nice properties too:
- It’s extremely lightweight.
- It’s explicit in the API (unlike exceptions).
- It doesn’t require C++ RTTI (a requirement for use in LLVM).
To address these shortcomings I have prototyped a new error-handling scheme partially inspired by C++ exceptions. The aim was to preserve the performance and API visibility of std::error_code, while allowing users to define custom error classes and inheritance relationships between them. My hope is that library code could use this scheme to model errors in a meaningful way, allowing clients to inspect the error information and recover where possible, or provide a rich diagnostic when aborting.
The scheme has three major “moving parts”:
- A new ‘TypedError’ class that can be used as a replacement for std::error_code. E.g.
std::error_code foo();
becomes
TypedError foo();
The TypedError class serves as a lightweight wrapper for the real error information (see (2)). It also contains a ‘Checked’ flag, initially set to false, that tracks whether the error has been handled or not. If a TypedError is ever destructed without being checked (or passed on to someone else) it will call std::terminate(). TypedError cannot be silently dropped.
- A utility class, TypedErrorInfo, for building error class hierarchies rooted at ‘TypedErrorInfoBase’ with custom RTTI. E.g.
// Define a new error type implicitly inheriting from TypedErrorInfoBase.
class MyCustomError : public TypedErrorInfo {
public:
// Custom error info.
};
// Define a subclass of MyCustomError.
class MyCustomSubError : public TypedErrorInfo<MyCustomSubError, MyCustomError> {
public:
// Extends MyCustomError, adds new members.
};
- A set of utility functions that use the custom RTTI system to inspect and handle typed errors. For example ‘catchAllTypedErrors’ and ‘handleTypedError’ cooperate to handle error instances in a type-safe way:
TypedError foo() {
if (SomeFailureCondition)
return make_typed_error();
}
TypedError Err = foo();
catchAllTypedErrors(std::move(Err),
handleTypedError(
(std::unique_ptr E) {
// Handle the error.
return TypedError(); // ← Indicate success from handler.
}
)
);
If your initial reaction is “Too much boilerplate!” I understand, but take comfort: (1) In the overwhelmingly common case of simply returning errors, the usage is identical to std::error_code:
if (TypedError Err = foo())
return Err;
and (2) the boilerplate for catching errors is usually easily contained in a handful of utility functions, and tends not to crowd the rest of your source code. My initial experiments with this scheme involved updating many source lines, but did not add much code at all beyond the new error classes that were introduced.
I believe that this scheme addresses many of the shortcomings of std::error_code while maintaining the strengths:
-
Context - Custom error classes enable the user to attach as much contextual information as desired.
-
Difficult to drop - The ‘checked’ flag in TypedError ensures that it can’t be dropped, it must be explicitly “handled”, even if that only involves catching the error and doing nothing.
-
Debugger friendly - You can set a breakpoint on any custom error class’s constructor to catch that error being created. Since the error class hierarchy is rooted you can break on TypedErrorInfoBase::TypedErrorInfoBase to catch any error being raised.
-
Lightweight - Because TypedError instances are just a pointer and a checked-bit, move-constructing it is very cheap. We may also want to consider ignoring the ‘checked’ bit in release mode, at which point TypedError should be as cheap as std::error_code.
-
Explicit - TypedError is represented explicitly in the APIs, the same as std::error_code.
-
Does not require C++ RTTI - The custom RTTI system does not rely on any standard C++ RTTI features.
This scheme also has one attribute that I haven’t seen in previous error handling systems (though my experience in this area is limited): Errors are not copyable, due to ownership semantics of TypedError. I think this actually neatly captures the idea that there is a chain of responsibility for dealing with any given error. Responsibility may be transferred (e.g. by returning it to a caller), but it cannot be duplicated as it doesn’t generally make sense for multiple people to report or attempt to recover from the same error.
I’ve tested this prototype out by threading it through the object-creation APIs of libObject and using custom error classes to report errors in MachO headers. My initial experience is that this has enabled much richer error messages than are possible with std::error_code.
To enable interaction with APIs that still use std::error_code I have added a custom ECError class that wraps a std::error_code, and can be converted back to a std::error_code using the typedErrorToErrorCode function. For now, all custom error code classes should (and do, in the prototype) derive from this utility class. In my experiments, this has made it easy to thread TypedError selectively through parts of the API. Eventually my hope is that TypedError could replace std::error_code for user-facing APIs, at which point custom errors would no longer need to derive from ECError, and ECError could be relegated to a utility for interacting with other codebases that still use std::error_code.
So - I look forward to hearing your thoughts. 
Cheers,
Lang.
Attached files:
typed_error.patch - Adds include/llvm/Support/TypedError.h (also adds anchor() method to lib/Support/ErrorHandling.cpp).
error_demo.tgz - Stand-alone program demo’ing basic use of the TypedError API.
libobject_typed_error_demo.patch - Threads TypedError through the binary-file creation methods (createBinary, createObjectFile, etc). Proof-of-concept for how TypedError can be integrated into an existing system.
typed_error.patch (22.2 KB)
error_demo.tgz (1.25 KB)
) there has been probably many other attempts to address this.