[userbinator]

Speaking as an Asm programmer for several decades: Calling conventions are stupid. They are the results of mindless stupid-compiler-oriented thinking from a time when compilers produced horrible copy-paste-replace code. The CPU itself couldn't care less which registers you use for what. So many wasted bytes on moving values between registers, just because the calling convention wanted it there, and no other reason. The only need to pay attention to calling conventions is when you're interfacing with compiler-generated code. Modern CPUs are fast, but there's still tons of inefficiency in compiler output.

[timewizard]

> The CPU itself couldn't care less which registers you use for what.

Not all registers encode as operands equivalently (implicit rdx:rax, implicit [rbx+al], limited [rbp/r13+imm8]). Some have other encoding restrictions or special purposes (rdi, rsi, rcx). When segmentation was a thing there were different default segments for each. Some are destroyed when certain opcodes used (syscall: rcx, r11).

> So many wasted bytes on moving values between registers [...] Modern CPUs are fast

Well, they've special cased this anyways, as these will often be caught in the rename stage and not even occupy an execution slot. Since we've long recognized that passing these values in registers instead of the stack is far more efficient, which is why the `fastcall` convention came about and got it's name way back in the x86 days.

> but there's still tons of inefficiency in compiler output.

Which is also why the 'inline' heuristic exists. In which case all of the calling conventions are fully abandoned. I mean, things like ELF dynamic symbol tables, and linux thread local storage annoy me far more than calling conventions ever have.

[userbinator]

Well, they've special cased this anyways, as these will often be caught in the rename stage and not even occupy an execution slot

They still need to be fetched and decoded, and take up space in caches and RAM that could be used for more purposeful instructions.

Which is also why the 'inline' heuristic exists.

Inlining has its own problems too.

I mean, things like ELF dynamic symbol tables, and linux thread local storage annoy me far more than calling conventions ever have.

Don't get me started on the whole ELF and dynamic linking situation...

[lmz]

> The only need to pay attention to calling conventions is when you're interfacing with compiler-generated code.

So, the vast majority of code out there in the wild?

[Spivak]

If you're not interfacing with it, say linking it as a library, then it doesn't matter what you do.

[lmz]

Sure, but this sort of limits the kinds of thing you can realistically build unless you want to build everything from the ground up. Even in the case of code reuse with statically linked assembly files there would be some sort of "convention" about how to call and be called.

[almostgotcaught]

vast majority doesn't even begin to describe it - i would wager 10 years of my salary that the fraction of all currently running CPU instructions that were handwritten is so small that it's within the margin of error (i.e., random bit flips) for whatever computer you use to perform the count.

[RandomBK]

Depending on how you count, the ratio might not be that small. A lot of hot code are written in hand-coded inline assembly, so in terms of CPU cycles run it's probably non-negligible.

i.e. take a look at the glibc implementation of 'strcmp` [0]

[0] https://github.com/bminor/glibc/blob/master/sysdeps/x86_64/m...

[lionkor]

Now how much of that doesn't interface with compiler generated code?

[almostgotcaught]

> A lot of hot code are written in hand-coded inline assembly

I know... I write GPU assembly for a living... And still I make that wager. It's not a lot. It's not even a little. It's an epsilon (overall). And it gets smaller over time.

[Kamq]

What do you think the percentage of computation is with respect to hand-written vs compiler generated?

If small hot loops tend to be disproportionately hand written, and certain programs spend the majority of time in their hot loops, that could still be a decent percentage of time/instructions executed.

[userbinator]

I mean it only matters at the interface.

[almostgotcaught]

do people think this is insightful? do you?

> conventions are stupid

all conventions are stupid when examined through the lens of an isolated island dweller. you might as well be saying something like "you only need to drive on the left-hand side of the road when you're driving on public roads".

[userbinator]

Compilers were stupid, and that's how we ended up with this constant overhead of inefficiency long after they could've done better; it's only within the last decade or so that "custom calling conventions" started being even considered.

[almostgotcaught]

Calling conventions have no more to do with dumb or smart compilers than driving on the left-hand side of the road has to do with dumb or smart urban planners.

[userbinator]

Of course they do. An Asm programmer will naturally use the appropriate registers to minimise data movement (see also: PC BIOS interface - no stupid stack shit) depending on the circumstances, a stupid compiler will just push everything on the stack. A more intelligent compiler will behave more like the human programmer and decide how to pass parameters and save or restore registers on a case-by-case basis.

[antics]

Since no one seems to be pushing back I'll add my 2¢ here as a former compilers engineer. Calling conventions are just like any other style guide. Yes, any particular coding style is stupid, but it's still useful to have one that you are more or less committed to, especially if everyone else in the ecosystem is committed to it too.

Frame pointers are a great example. Having a well-known and generic representation of %rbp is helpful when you go to use or integrate with existing tools like debuggers, link editors, or (say) most of the existing LLVM/GCC/whatever toolchain. Or when you want to expose a stable ABI to consumers for whatever reason (as, e.g., the Linux kernel famously does). Or, or, or.

I think it's reasonable to say this has been mostly uncontroversial since at least the 90s. The discussion has changed a bit since (apparently) Go needed none of these things to succeed—not the LLVM compiler toolchain infrastructure, and also not the user-facing things like the debuggers. To hear Russ Cox tell the tale, this is mostly because they required flexibility, and I suppose they were right, since they did rewrite their linker 3 times, and sure enough, 15 years later, in the year of our lord 2024, most debugging in Go seems to happen by writing ASCII-shaped bytes to some kind of a file, somewhere, and then using the world's most expensive full text search engine to get those bytes so you can physically read them on a screen. A debugger does seem limited in use for that specific workflow, so maybe that was the right call, who knows.

Anyway, I don't think there's ever been any real doubt that something like LLVM imposes a serious integration cost, but now that we have Go, the discussion has mostly shifted to "is it worth it", and seemingly the answer is "mostly yes" since nearly everyone building a new and hip native language uses LLVM or something like it. Every language is different, YMMV, etc., but I personally don't hear a lot of complaining about what a bummer it is that all these tools work pleasantly together instead of secretly sabotaging each other by loading up FP with whatever cursed data Go wanted to use it for. And why would they?

What is more mysterious to me is how an actual assembly programmer came to defend Go's stance on ... assembly. Perhaps I'm the only one who reads these things, but at various points in the ASM docs[1] (which I will heretofore call "Mr Pike's wild ride") the author expresses a view that I think is reasonably well-described as "a tempered but pretty much open contempt for the practice as a whole". cf.,

> Instructions, registers, and assembler directives are always in UPPER CASE to remind you that assembly programming is a fraught endeavor. (Exception: the g register renaming on ARM.)

Even if your feelings are hard to hurt though, if you ever crack open the toolchain and attempt to read the golang kind-of-IL-kind-of-x86, it is hard to walk away thinking "these people really get me and my profession". DI and FP are both normal registers! It uses the unicode interpunct instead of the plain old dot operator! It uses NIL instead of NULL! It is one thing to say calling conventions are stupid, but it's another thing entirely to give a great big hug to an almost-but-not-quite-assembly-code that is convenient neither for humans to type nor for tools to consume.

[1]: https://go.dev/doc/asm

[x-shadowban]

for functions that don't escape the current compilation unit (`static` functions, anonymous namespace functions), can/do compilers ignore calling conventions and do the faster thing? Of course, they can just inline, and that makes this moot.