在llvm / clang中没有按设计使用C ++异常

Question

llvm/clang are considered good C++ code bases. I wonder why C++ exceptions arenot used in them at all?

Memory is managed using something like pools, and erros are reported with returnd values and codes like in C. They even wrapping operator new to be placement new that returns error and not exception when no memory.

Do you have idea why llvm philosophy is not to use C++ exceptions when most books recommend using them?

Answer 1

Chris Lattner recently clarified this issue in the LLVM project coding standards .

Not using exceptions and RTTI reduces executable size and reduces overhead. (You might argue that zero-cost exceptions have no overhead unless thrown. At a minimum, they actually break up the code into smaller basic blocks and inhibit some types of code motion.)

Answer 2

Writing exception safe c++ code is a difficult task.

Turning off exceptions can speed up code execution and reduce code size .

Maybe this is related.

Answer 3

Depending on your compiler and your code, a program that uses exceptions can be faster or slower than an equivalent program that disables and doesn't use exceptions. Also the one that uses exceptions may be larger or smaller.

Every error handling strategy incurs some cost, and I expect that the LLVM developers considered their situation and found that disabling exceptions was the better decision for LLVM.

My recommendation, and the recommendation I have most seen from experts, is to use exceptions to report failures unless you have some specific, solid reason not to. If your reason is performance, it would be wise to base your choice on profiling. Remember that it is vital to compare code that uses exceptions to code that doesn't, but still handles errors correctly .

Answer 4

I think this stems from another guideline: Use assert liberally

• Normal error conditions are treated using error codes.
• Exceptional error conditions are treated via assert .

I would say that an assert is an even harder exception: you definitely cannot ignore it ;)

Answer 5

Do most books recommend using them? I know most books on C++ programming cover them because they are part of the language, but I don't think I have seen a book that said to prefer them to error codes or other methods of error handling. In fact I would say most books implicitly do not recommend using exceptions because they don't cover how to write good exception safe code.

As far as LLVM being good code and it being not exception based code, the two concepts are largely orthogonal. Code can be cleanly written with or without exceptions.

Answer 6

Seems it is not a llvm philosophy to avoid exceptions. At least I found nothing about exceptions in the coding standard (http://llvm.org/docs/CodingStandards.html), so it is up to developer.

Why it is not widely used? AFAIK exception support was implemented in llvm not so far, so it was not possible to compile llvm for llvm itself :). So it can be just a historical reason to avoid exceptions.