Re: Assembly code parametriaztion

["Piotr Wyderski" <piotr dot wyderski at hoga dot pl>]

Andi Kleen wrote:

> You can do it basically two ways (both are used in the linux kernel):
>
> - Make your include files assembler clean, usually with moderate
> use of #ifdef __ASSEMBLY__ and include them in the assembly
> files. This works well for #define, but not for things like structure
> offsets.

It also doesn't work for enumerated constants, if C++ is used.
So, in fact, this solution is virtually the same as two independent
include files. The same problem with code synchronization remains
 -- you can modify a constant for C, but forget about its associated
assembly definition -- a possible source of very dangerous bugs.
My solution is able to make use of C/C++ constants directly.

> - For structure offsets or complicated expression have a separate file
>   that includes the include files does something like:
>
> asm volatile("-- #define struct_field %0" :: "i" (offsetof(struct
structure, field)));

But such a construction must be defined inside a C function...

>   Compile this program with -S, then run a small postprocessor over
>   the assembly file that just filters out everything behind --
>   Put that into an include file and use it from your assembly.

A truly comfortable solution... ;-))) All due respect (looking at
the e-mail address :-)), but Linux sources are everything but
a standard of clean code -- my built-in aesthetic monitor
throws "E_TOO_MANY_HACKS" exceptions constantly. :-)

Seriously: my set of macros allows the programmer to use it without
such combinations, you just need to type "EMIT(name,value)" and
that's all. But it's not a good solution, why can't we simply extend
GCC to accept parameters for free-standing assembly or to define
assembly constants easily, without helper functions, which are later
removed by the /**/ comments?

>   Nice thing is that it also works fine for cross compiling.

Mine too, but I don't need postprocessors.

>   Only tricky thing is that you have to get the dependencies right
>   in the Makefile, otherwise there can be subtle failures with -j
>   compilation.

A built-in mechanism would prevent such bugs...

    Best regards
    Piotr Wyderski



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