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C++ in the Linux Kernel

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Re: C++ in the Linux Kernel

#21

Post by mhh__ »

jlkjaoifnwlekfj wrote: post #10

> A first-year computer science student can tell you that the arguments get pushed onto the stack. In other words, a call to this 3GL function results in the following assembly pseudo code Are people this ignorant when it comes to C/C++ or any systems language? ABI & calling conventions were introduced early in my C & C++ textbooks (age 13 btw, not even close to college years).

Ab initio first years probably just about know what registers are so I can believe that.

Decoupling the compilers optimizations and the ABI (particularly what constitutes a "move" of a struct) has derailed a few conversations I've been involved with - even from very smart devs (although mainly interpretation rather than basic misunderstandings like thinking what is actually due to the ABI is an optimization)

Re: C++ in the Linux Kernel

#22

Post by Pinus »

monocasa wrote: post #5

Lol, part of me likes the effort taken just because, but the kernel devs _really_ do not want C++. One hint: "struct class" https://elixir.bootlin.com/linux/latest/source/include/linux...

Wasn't there a period recently — around 1995 or so — when the Linux kernel had to be compiled with a C++ compiler?

Re: C++ in the Linux Kernel

#23

Post by monocasa »

Pinus wrote: post #22
monocasa wrote: post #5

Lol, part of me likes the effort taken just because, but the kernel devs _really_ do not want C++. One hint: "struct class" https://elixir.bootlin.com/linux/latest/source/include/linux...

Wasn't there a period recently — around 1995 or so — when the Linux kernel had to be compiled with a C++ compiler?

I don't think that's accurate. Maybe you're thinking of how it needs gcc extensions?

Re: C++ in the Linux Kernel

#25

Post by kevin_thibedeau »

jcelerier wrote: post #6

> You see the problem. My C++ code expected the calling convention that pushed arguments on the stack, that would be very weird on Linux. The x86_64 linux ABI mandates that the first arguments go on registers afaik (I'm assuming x86_64 here since the post mentions linux distros which are overwhelmingly x64). What compiler would default to a pure stack-based calling convention ? Certainly not GCC or clang, no ? > and…

AVRs don't have enough storage for templated code to explode into an unmanageable problem.

Re: C++ in the Linux Kernel

#26

Post by ivegotnoaccount »

10000truths wrote: post #7
jcelerier wrote: post #6

> You see the problem. My C++ code expected the calling convention that pushed arguments on the stack, that would be very weird on Linux. The x86_64 linux ABI mandates that the first arguments go on registers afaik (I'm assuming x86_64 here since the post mentions linux distros which are overwhelmingly x64). What compiler would default to a pure stack-based calling convention ? Certainly not GCC or clang, no ? > and…

> that would be very weird on Linux. The x86_64 linux ABI mandates that the first arguments go on registers afaik (I'm assuming x86_64 here since the post mentions linux distros which are overwhelmingly x64). What compiler would default to a pure stack-based calling convention ? Certainly not GCC or clang, no ? The System V i386 ABI passes parameters through the stack. Perhaps that is what the author is referring to,…

In case it is talking about the x64 calling convention, there is actually some kind of an odd case where you would get something that looks like an argument was pushed on the stack:

When a non-trivially-copyable object is passed by value to a function, you need to ensure that through the lifetime of the copy, it's address will never change because the constructor may have stored address of some of the field (for instance, a pointer to a field). The way this is handled at list in the SystemV x84_64 ABI is that the object is created on the caller's stack, and a pointer to it is stored in a register, so just like if it was passed as a pointer.

I have seen several cases where a header would have an "ifdef c++" clause with copy constructors and destructors in them ("It does not add field so it should be OK, right ?"), which make the object non trivially-copyable, leading to clashing calling convention between C and C++ codes. I am curious about if this may be the issue he encountered.

Re: C++ in the Linux Kernel

#27

Post by pjmlp »

kevin_thibedeau wrote: post #25
jcelerier wrote: post #6

> You see the problem. My C++ code expected the calling convention that pushed arguments on the stack, that would be very weird on Linux. The x86_64 linux ABI mandates that the first arguments go on registers afaik (I'm assuming x86_64 here since the post mentions linux distros which are overwhelmingly x64). What compiler would default to a pure stack-based calling convention ? Certainly not GCC or clang, no ? > and…

AVRs don't have enough storage for templated code to explode into an unmanageable problem.

Somehow C64 can deal with them.

"CppCon 2016: Jason Turner “Rich Code for Tiny Computers: A Simple Commodore 64 Game in C++17”"

https://www.youtube.com/watch?v=zBkNBP00wJE

"C++20 For The Commodore 64"

https://www.youtube.com/watch?v=EIKAqcLxtT0

Re: C++ in the Linux Kernel

#28

Post by uyt »

> Kernel developers obsess about speed and performance. The Linux kernel is built using -mregparm=3, which is sometimes called fastcall.

I've never messed with calling convention for the sake of performance before so I found this bit interesting. I found more info about it at: https://en.wikipedia.org/wiki/X86_calling_conventions#Borlan...

Does anyone have benchmarks? Assuming I don't care about ABI stability, what's the fastest calling convention?

Re: C++ in the Linux Kernel

#29

Post by creamytaco »

There's one problem with rants such as these, it's too easy for someone to be exposed as clueless and broadcast his lack of knowledge and assumption-heavy development process to the world. How is that as an advertisement for one's employer?
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