The 5.7 kernel is out [LWN.net]

The 5.7 kernel is out

Posted Jun 1, 2020 20:37 UTC (Mon) by quotemstr (subscriber, #45331)
In reply to: The 5.7 kernel is out by Wol
Parent article: The 5.7 kernel is out

> Yup. Simply put, C++ is a high-level language. It is not designed for resource-constrained environments.

I have to completely disagree with this perspective. C++ lets you control the machine as precisely as C does. There's nothing in the language, nothing at all, that prevents your writing low-level and efficient code. C++ isn't Python. Is your position just that people writing C++ can't avoid the temptation to use inefficient patterns? That's a social problem, not a technical one.

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The 5.7 kernel is out

Posted Jun 2, 2020 0:24 UTC (Tue) by Wol (subscriber, #4433) [Link] (3 responses)

There's nothing in the language that prevents you - correct.

But how many programmers actually have a clue about which features are efficient, and which are not? The other massive problem with the question "which features are efficient", is probably that the answer is "it depends on the compiler".

If you *need* a program to be efficient, giving programmers a compiler that can be inefficient may be a social problem, but stopping those programmers from using those features is probably an intractable problem ... I don't want a Ryzen 7 to be the minimum recommended processor for running linux ...

Cheers,
Wol

The 5.7 kernel is out

Posted Jun 2, 2020 0:48 UTC (Tue) by quotemstr (subscriber, #45331) [Link] (2 responses)

The criticisms you mention all apply to C too. I don't see C++ being special here --- to write good code, you have to know what you're doing, and if you're a bozo, you're going to wreck things in either C or C++. That's why we do code review.

The 5.7 kernel is out

Posted Jun 2, 2020 8:21 UTC (Tue) by Wol (subscriber, #4433) [Link] (1 responses)

And how much - even kernel code - is code-reviewed? And of those reviewers, how many have the *experience* to know what is good or bad?

You only need some badly written and poorly written driver to start evicting L1 cache and the whole system suffers.

I think it's a case of "C: if it looks efficient it probably is. C++: If it looks efficient, who knows what the hell is going on behind the scenes ..."

Now if you apply C++ discipline to C code that's a whole 'nother ball game!

My expertise is databases. And I like Pick because there is a consistent model right through from hardware to application (at least, there is if the programmer knows what they're doing! :-) Just like C, there's plenty of rope to hang yourself.

Relational - C&D to be precise - is a whole pile of poo. Firstly, there's the logical inconsistency within the rules themselves. Then there's the model second-guessing you to try and protect you from yourself. Then there's the implementation - severely hamstrung by the model itself if it tries to implement it faithfully. Pascal, SQL, C++, ... great for novices, and quite good languages, but expensive in wasted computer power as it tries to protect you from yourself.

Cheers,
Wol

The 5.7 kernel is out

Posted Jun 2, 2020 12:21 UTC (Tue) by adobriyan (guest, #30858) [Link]

Inefficiency of the language can be trivially demonstrated by absence of multiple return values.

Given

int f(const char *str, int *val);

int val;
int rv = f(str, &val);
if (rv < 0)
return rv;
[use "val"]

C ABI and single return value forces to allocate data on a stack and a write.
Now top of stack is optimized so it doesn't matter for performance, but the act of writing to [rsp+] has second order effects.
Stack protector sees that function modifies memory and starts placing canaries, bloating code and issuing reads and adding
mandatory branch for stack smashing.

Could this be solved in C? Yes, by using 2-value structure which fully fits into 2x8-byte registers.
C ABI even has provision for this by referring to RDX as a second return value.
Which of course nobody uses because there is no syntax for destructuring.

The 5.7 kernel is out

Posted Jun 2, 2020 2:04 UTC (Tue) by mathstuf (subscriber, #69389) [Link]

Is `-fno-exceptions` really part of C++ or just a mechanism that Linux would have to disable? I guess Linux would have to avoid `new` anyways as it does `malloc`, so maybe it wouldn't be *that* far off. But without `std::optional` and some nice wrapper around `std::variant` to act as Haskell's `Either` or Rust's `Result`, the algorithms aren't that useful. I don't know. At that point, you may as well just adopt some Golang `defer`-like C extension rather than try and strip C++ down since that's mostly what would be wanted from what's left anyways. Sure templates for data structures would be nice, but if what you get is just a bit more rigor than what the macros are already doing, unit tests solve most of that problem too (which you'd want *anyways*).