Comment by amluto
4 days ago
I have very mixed opinions about the custom syntax. IMO the correct asm syntax, with very few exceptions, is the one in the manual. This is why Intel syntax is right and AT&T syntax is wrong: the ISA comes from Intel, the docs are from Intel and AMD, and those docs use Intel syntax.
So I was kind of hoping that the custom syntax would at least result in a very, very strong checker, at least as good as Fil-C’s. Maybe with an escape hatch to say something like “I know it looks like I clobbered xyz, but I promise I really didn’t.
Sadly, the CPUID example in the article apparently compiles, but IMO it shouldn’t have: CPUID takes two inputs, in EAX and ECX, and the example forgot to bind ECX as an input. One might argue that CPUID takes even more inputs if you’re on a VM and doing something special, but ECX is really quite unambiguous.
Following the vendor syntax may be useful if you only ever program in assembly for only a single target ISA.
Otherwise, all the vendor syntaxes are different and all of them have many ugly quirks, for various historical reasons.
If you ever have to write assembly for at least 2 ISAs, e.g. x86-64 and Aarch64, then it is much more productive to use a unified, and better syntax, like the one described in TFA.
Nothing makes more likely the appearance of bugs than having to alternate frequently between two or more slightly different syntaxes.
Even with only one target ISA, if you frequently intersperse inline assembly within a high-level language source code, it is better to have harmonized syntaxes, as explained in TFA.
And that is how you end up with Plan 9 Assembly, that requires mapping to every single ISA.
It is better to put this burden on a small team of language developers than putting the burden on every future user of the language
The correct assembly syntax is a matter of convention and personal preference – not a category error.
Historically, some ISA's have adopted the «src, dst» convention, whilst others have preferred «dst, src». We should be grateful that no engineer, in a moment of excessive creativity, attempted boustrophedon – a conceptual device solely appropriate to the likes of INTERCAL. Then we have 3 operand RISC instructions.
As with so many technical orthodoxies, the first convention one encounters tends thereafter to acquire the status of natural law.
The consistent application of the same convention is useful nevertheless. It reduces the unnecessary cognitive overload once one starts jumping across multiple hardware architectures.
> the CPUID example in the article [...] forgot to bind ECX as an input.
I'm not really familiar with this stuff, but the example uses what it calls a "pin" (which in their docs is a type of "binding") on ECX before calling CPUID.
You don't really need to be familiar with either "this stuff" or Odin to spot that this clearly takes a single parameter named "leaf" and that's the input, which goes in EAX. However CPUID may care about ECX as input and that's only used as an output in this uh, "template".
Here's Rust implementing this same feature:
https://doc.rust-lang.org/src/core/stdarch/crates/core_arch/...
Rust provides this for both x86-64 and the original 32-bit x86 and this is a function, not an Odin-style "template" but hopefully this helps show what you're supposed to do.
[Edited to add the Rust example]
I'm not sure the example template was supposed to be canonical, vs demonstrating multiple output destructuring. Certainly the actual instruction definition in the checker library seems to understand that there could be two possible inputs:
https://github.com/odin-lang/Odin/blob/4247507dd5e31c9fd8716...
But I'm also not entirely sure why the example should not have compiled. It seems to me that the idea here is to be able to define a typed set of something equivalent to a function that inlines some assembly, but nothing about that inherently requires that the number of input or output parameters to the template match the parameters in the underlying assembly calls. There's no reason (in my mind anyway) why this shouldn't be a perfectly valid template:
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The syntax in the manual is embarrassingly outdated. Like, it's actually a disgrace and shameful for our profession that assembly languages and tooling are stuck in the previous century. There is absolutely zero logical reason we should be constrained to such primitiveness.