Comment by WalterBright | Hacker News Reader

Comment by WalterBright | Hacker News Reader

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> forward declaration requirement enables a single-pass compiler to emit code on-the-fly.

True, I know all about that. My Zortech C and C++ compiler was one pass (after the multiple preprocessing passes). The ground up ImportC C compiler completed a couple years ago has a separate parse pass.

So I well know the tradeoffs. The parser being stand-alone means it is much simpler to understand and unittest. I found no advantage to a single pass compiler. It isn't any faster.

> simply adding things onto C gets you C++

C++ doesn't allow forward declarations either.

Successfully doing a parse-only on C code doesn't quite work. It turns out the grammar relies on a symbol table. Fortunately, only a symbol table of the typedefs. Once adding that in, ImportC worked. (I really tried to make it work without the typedef symbol table!)

C++ added a bunch more syntax that relies on the symbol table. I would not even try fixing it to work as parse-only.

> in C you don't solve problems by adding features, but by writing more code in C

The trouble with such sayings is like following a google map that says cross this bridge, but wasn't updated with news that the bridge is out.

> Those things aren't that obvious,

They are once you use another language that doesn't have those restrictions.

> and I'd rather not have them added to C.

C adds new things all the time to the Standard, like normalized Unicode identifiers, which are a complete waste of time. Every C compiler also adds a boatload of extensions, some good, some wacky, many ineptly documented, all incompatible with every other C compiler extensions.

> The parser being stand-alone means it is much simpler to understand and unittest.

Stand-aloneness and single-passness are orthogonal.

> I found no advantage to a single pass compiler. It isn't any faster.

A gigantic advantage: a single-pass-compilable language is simpler. By definition.

Implementations may or may not be simpler or faster.

> C++ doesn't allow forward declarations either.

Well, that's not what I meant.

C++ is "C with just this thing" done way too many times.

> The trouble with such sayings is like following a google map that says cross this bridge, but wasn't updated with news that the bridge is out.

TBH, I didn't really get this. Is this about sticking to C as is, but it is outdated as is?

C would be outdated if it didn't have, say, long long for 64-bit numbers. Having "true" be a keyword instead of a macro doesn't change how outdated it is or isn't, just like compile-time evaluation also doesn't.

> They are once you use another language that doesn't have those restrictions.

I have used many, and I still don't find them obvious.

> C adds new things all the time to the Standard, like normalized Unicode identifiers, which are a complete waste of time.

I agree that many/most are a waste of time, and shouldn't be added to C. The fact of C adding things to the standard all the time shouldn't justify adding even more things, but make one question if those are needed at all, and how to accomplish the goal without it.

> Every C compiler also adds a boatload of extensions, some good, some wacky, many ineptly documented, all incompatible with every other C compiler extensions.

I know about that, and my position is the same: just don't.

I don't use them also.

> A gigantic advantage: a single-pass-compilable language is simpler. By definition.

That's only "by definition" if you take a language that needs multiple passes, then remove the features that need multiple passes, and don't replace them with anything else to compensate.

The "by definition simpler" version of C would not only disallow forward references, it would have no forward declarations either. As-is, forward declarations add some complexity of their own.

(Also, if you can figure out a way to emit jump instructions in a single pass, you can probably figure out a way to call unknown functions in a single pass.)

I have the impression you're mixing single-pass compilation and O(1) memory use of the compiler.

As is, C already is single-pass compilable, modulo some unnecessary syntax ambiguities.

As the compiler reads the text, it marks some character strings as tokens, these tokens are grouped as a fragment of code, and some fragments of code are turned into machine code. A simple function of a 100 lines doesn't need to be parsed until the end for the compiler to start emitting machine code.

Like the parser, this requires memory to keep tabs of information and doesn't work for all types of constructs, like a jump instruction to a label defined later in a function. The code emitter soaks input untill it is possible, and does so, like when the label is already known and can be jumped to.

You cannot do any optimization when generating machine code that way. That's fine for a primitive compiler built for a school project, but not much else. (Even "no optimization" switch settings on a compile do a lot of optimizations, because otherwise the code quality is execrable.)

> That's fine for a primitive compiler built for a school project, but not much else.

Not true.

On the one hand, just see how many non-compiled languages are used outside of primitive school projects.

On the other hand, this simpler approach is actually faster for writing actually fqst compilers. Many modern compiled languages have compilers that work on the order of ~100ms on a simple file with 1k LoC, when it could (and arguably should) work on the order of ~1ms, IOW, imperceptible given the syscalls overhead.

A 100x faster compiler that generates meh code is more useful 99% of the time: when one is recompiling all the time during development.

Doing jump instructions in a single pass is done by creating a patch list, and when the compilation is done walking the patch list and "fixing them up".

Doing this with functions is a lot more difficult, because one cannot anticipate the argument types and return types, which downstream influence the code generation. Of course, early C would just assume such forward references had integer arguments and integer types, but that has long since fallen by the wayside.

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