2014-05-10

Particles of coroutines

Following the very same idea of the previous post, I've implemented the same stuff for x86. No worry about details — I am not a x86 fan and lover — except that there are few tests I've not done in the m68k version (namely the latter assumed the compressed stream is not corrupted). But it's just noise, not worth considering.

Intel x86 assembly instructions suck, but I admit I don't know it very well and likely I haven't used some cool feature and I don't know any cool feature which, once I'll know it, will make my mind change. Rants end.

Since x86 has not too many registers, and since I've used C library (compiled with nasm, linked with gcc/ld) and x86 calling conventions apply, and since I wanted to avoid special purpose registers (ECX, ESI, EDI…) as “global” variable storage, there are extra push and pop to keep values between calls to library functions, while each coroutine assumes also that the register it's interested in, are not trashed.

The register EBP can be used for “shared” (or global) storage; in fact, I've used it to store the pointer to the token buffer, and the “continuation” address.

The yield/resume feature is done with this code (kept into a macro):

    mov ebx,[ebp]
    mov dword [ebp],$+9
    jmp ebx

First the next address is put into EBX, then it's replaced by the address of the instruction following the jump, then the jump to the address in EBX is performed.

That's all. Readers interested in the whole code can find it at this gist, but I doubt it's worth it. It'd be far more interesting to study an implementation that could be used for really, as the result of compiling high level language code.

Different calling conventions can make it easier, but then you need extra code to call external functions — sticking to common C calling convention on a system is the key to access a lot of code without the need for any kind of glue — almost. Rants end, again; guess when they began.

This may work fine for two coroutines. Let's reason about a third coroutine. Does it work? No. If you need to create another cooperation, e.g. between the parser which extracts tokens (a “lexical scanner” indeed) and a grammar parser (i.e. a parser), you'll be fucked up.

E.g. our parser at some point, instead of got_token, need to give control to another routine, namely the one which understands the grammar. Thus, for each coroutine pair we need a “slot” similar to the one in [ebp]. A theoretical JCONT macro would be more complex, and take into account at least the coroutine we want to give control to. E.g.

 parse:
     JCONT   parse,getc
     test    eax,eax
     ...
 .wend:
     mov     eax,TWORD
     ; the grammar_parser'd like to have ptr to buffer too
     ; ... but this could be a global, as it is
     JCONT   parse,grammar_parser
     ...

If there's a hashmap for each coroutine, then we need to initialize it first, and the somewhere likely we could need a reset too. The macro could look, in pseudocode, something like

    get_slot_of    %2
    mov            ebx,[ebp]
    mov            dword [ebp],$+9
    store_slot_for %1
    jmp            ebx

Just an idea, at a very late hour.

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