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Everyone says assembly is untyped—everyone is wrong

46 points by adamrezich ago | 15 comments

AshamedCaptain |next [-]

> AT&T bakes the width into the mnemonic (movb, movw, movl, movq [...] Intel’s syntax is to prefix the memory operand with byte, word, dword, or qword, but Odin’s just uses the Odin type system directly.

In GAS you can skip the width suffix from the mnemonic, and in most Intel assemblers you can skip the memory type operators like byte. They happily guess it from the operands. The problem is that on x86 (but also other ISAs, even if to a lower extent) the different operand sizes have a lot of side effects, which is why everyone just makes the operand size explicit, up to the point that apparently the author/LLM believes that it is mandatory to specify them.

This kind of defeats the headline of the article...

Tomorrow you need to pass a 128 bit int into two registers and your fancy syntax then also becomes a messy bunch of hacks. This is why everyone's inline assembly syntax looks like that, because they want to cover the weird cases (gcc's one is almost like an history book). You're normally using inline assembly for when you have some ridiculous corner case, if not, then what you ought to use is more akin to intrinsics...

Also it forgets Watcom C, which does have a complete but messy syntax for inline assembly (which combines nicely with its ability to specify really weird calling conventions).

amluto |next |previous [-]

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.

genxy |next |previous [-]

An avenuge of research worth being sniped on is Typed Assembly Language

https://en.wikipedia.org/wiki/Typed_assembly_language

https://www.cs.cornell.edu/talc/overview.html

gingerBill |root |parent [-]

TALs are not what I am referring to here. I am arguing that assembly is already typed and does not need extra annotation to be typed.

TALs are also solving an entirely different problem.

genxy |root |parent |next [-]

The technique is good, and compilers that interact with assembly should do this, but as you outline, they basically just shove blobs of text around and hope for the best.

I didn't say you were referring to TALs. Yours is a syntax level check, not type checking of the program in the normative sense. It might be more accurate refer to your technique as an "instruction signature", rather than a type.

I would argue that that are complementary and not entirely different.

I thought it would be interesting for folks.

questionableans |root |parent |previous [-]

But a language being “typed” doesn’t tell us anything useful. Untyped languages are typed too: they’re uni-typed (every expression is an expression).

I think you do your analysis a disservice by focusing on “is assembly language typed?” as the top line question. The more interesting question you examine is what do the type constraints in inline asm offer, and how do they interact with the host language’s type system?

gingerBill |root |parent [-]

I know that "untyped" means a single-type, but assembly operands have multiple different kinds of types (as I state in the article). What makes it really interesting is what you can know about each instruction and what it does (what operands it excepts, what it clobbers, what side-effects its has, etc).

And from that huge table of type information, this can be used to give good error messages and suggestions to the user because the compiler actually knows all of this. The type constraints here allow for a lot more than information that normal assemblers just don't give.

questionableans |root |parent [-]

Yes, and your second paragraph above is the interesting part that I would want the reader to focus on, starting from the title.

Jblx2 |next |previous [-]

Can you get an assemble-time or run-time type-error with assembly? Might be a fine article otherwise without the click-bait headline.

measurablefunc |root |parent [-]

> However, every instruction has a set of valid forms. Each form dictates the kind of each operand (register, memory, immediate, label), the class of each register (general-purpose, vector, mask), the width of each operand, the range each immediate may take, and what the instruction clobbers (flags, memory, particular registers). In x86, a mulps wants a 128-bit vector register; a crc32 in one of its forms wants a 32-bit destination and an 8-bit memory source; div reads and writes rdx and rax whether ask to it do or not.

The instructions have bit-width, arity/source/target requirements so technically there are types whereas an abstract virtual machine that only operates on some fixed set of integer registers is mostly untyped (modulo number of registers).

Krssst |next |previous [-]

Sorry, somewhat of a tangent but regarding:

> The %0 and %1 are positional references into a list you have to count by hand.

You can name your operands in gcc inline assembly.

https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Output-...

Look for "asmSymbolicName".

On a phone so not checking if it builds, but something like `asm("add %[my_out], %[my_in], #3":[my_out]"=r"(outvar):[my_in]"r"(invar):);`.

gingerBill |root |parent [-]

The equivalent Odin syntax looks like this:

add_three :: asm(my_in: u64) -> (my_out: u64) { add my_out, my_in, 3 }

out_var = add_three(in_var)

Which is already infinitely more readable and requires no parochial sigils nor the arcane clobbering syntax.

magicalhippo |next |previous [-]

> But because of its time period, the built-in assembler only ever understood up to 80286 instructions, so the day you wanted a 386 and its 32-bit registers you were sent off to an external assembler anyway.

Or you just prefixed the instructions with "db $66", et voila your instructions were 32bit. I wrote a lot of inline 32bit assembly that way in TP 6.0 and 7.0.

childintime |next |previous [-]

I don't care much about the typed part, I care much more that this is a good take on what an assembler should be, far ahead of the GCC monstrosity, that serves just one purpose well: it screams "don't use me". This feature could make Odin the language of choice for some types of projects, for it seems to remove so much friction.

fithisux |previous [-]

True. It takes some time to grasp but gingerbill is right.