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Subject: Re: [boost] Reforming Boost.System and <system_error> round 2
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2018-01-16 09:40:01

> Myself and Peter feel this is worth fixing despite the increased runtime
> overhead. Others, including one of its original inventors, feel it is
> too much overhead.
> I must ask once again how do we know it is "too much" overhead? Where is
> the real-world program which would be impossible or difficult to write
> otherwise?
> For some reason I never get an actual answer, and instead it's always
> something along the lines of "I'm an expert, trust me, I know". Well, I
> don't. Sorry.

A little unfair. I've supplied numbers to you before, but as with all
such numbers, they're totally arbitrary and mostly meaningless. All
anyone can really say is that with some real world application X, that Y
was the case with that application. And that's about it.

This particular debate comes down to how expensive is a virtual function
call, and how frequently is code going to be doing `if(ec) ...`.

As is fairly obvious by now, I consider virtual function call overhead
to be pretty much in the stochastic noise of processor timings except on
in-order CPUs [1]. As in, can be generally ignored as unimportant. I
also think that `if(ec) ...` is not going to be called frequently, no
more than twice per failing operation, and usually once.

So to me, my proposed fix solves a real problem of correctness in lots
of code out there in the wild which is currently subtly broken for
virtually no cost. Therefore, it's a slam dunk. It should be implemented
and the C++ standard upgraded to match. Obviously others disagree with me.

> This applies to C++ exception handling and error handling in general.
> Yes, I know, there is overhead -- but where is the proof that 1) it
> matters, and 2) it's not worth it?

The "it matters" part depends on your attitude to the semantics behind
observation of error codes. What does the code being 0 actually mean?

Also, as Peter has explained several times now, composition of
error_code returning functions in generic code where the category is
unknown is hard without a generic facility to determine if some
error_code means failure or not. For that alone, this change should be
implemented. It breaks no existing code whatsoever, but fixes the
semantics into what the programmer probably intended.


[1]: Of course raw benchmarks show virtual function calls are slower to
no function call due to inlining in some artificial benchmark. But in
real world code, the cost of a virtual function relative *to doing any
work at all* is insignificant. While your doing work code is stalled on
memory or whatever, the CPU is off busy executing virtual function calls
and so on. Once you average it out, virtual function calls approach free
in most real world code use cases on modern out-of-order CPUs.

ned Productions Limited Consulting

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