llvm::Error Rich Error Handling in LLVM Error Handling History - - PowerPoint PPT Presentation

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Sep 30, 2022 182 likes •274 views

llvm::Error Rich Error Handling in LLVM Error Handling History LLVMs APIs historically used ad-hoc approaches bools, nullptrs, string errors std::error_code C++ standard library error type Enumerable errors only Lack

SLIDE 1

llvm::Error

Rich Error Handling in LLVM

SLIDE 2

Error Handling History

LLVM’s APIs historically used ad-hoc approaches
bools, nullptrs, string errors…
std::error_code
C++ standard library error type
Enumerable errors only
Lack of enforcement — easy to drop errors

SLIDE 3

Exceptions

User defined error types
Type safe handlers
Once thrown, impossible to forget

However…

Not actually zero-cost
Turned off in LLVM

SLIDE 4

llvm::Error

Error as return value:

Error foo(…);  Expected<T> bar(…); 

User defined error types:

class MyError  : public ErrorInfo<MyError> { … }; 

Errors must be checked before destruction

SLIDE 5

Idiomatic Usage

Error foo(…);  if (auto Err = foo(…))  return Err;

Conversion to bool  “checks” error

SLIDE 6

Idiomatic Usage

Error foo(…); foo(…);

Destruction of unchecked  Error triggers abort

SLIDE 7

Type-safe Handlers

Error foo(…);  handleErrors( foo(…), [](MyError &M) {  }, [](SomeOtherError &S) {  }, …,  );

SLIDE 8

Benefits

Safer: Avoid vulnerabilities due to missed errors
More descriptive:
LLVM ERROR: Malformed MachO file.

becomes

truncated or malformed object

(bad section index: 66 for symbol at index 8)

Supports error hierarchies (e.g. ObjectFileError)

SLIDE 9

Conclusion

Many utilities:
Interoperability with std::error_code and ErrorOr
Standard error types (StringError)
Exit-on-error idiom support for tool code
Check out Programmers Manual for usage