[FFmpeg-devel] Moving if(constant expression) to preprocessor?

[Måns Rullgård]

"Axel Holzinger" <aholzinger at gmx.de> writes:

> M?ns Rullg?rd and nice are mutually exclusive. Does this help?

Please refrain from ad hominem attacks.

>> > I got inspired by their BOOST_PP_IF macro.
>> >
>> > It's more generic as it would be needed here. So reduced to what's
>> > needed it could look like this (compiler specific #fdefs removed for
>> > clarity):
>> 
>> What compiler-specifics would be involved here?  FFmpeg uses 
>> only standard C99 features in non-optional code.

You did not answer this question.

> Here is the code that works and that's tested with gcc:
>
> #define AV_COND_IIF(bit, t, f) AV_COND_IIF_I(bit, t, f)
> #define AV_COND_IIF_I(bit, t, f) AV_COND_IIF_II(AV_COND_IIF_ ## bit(t, f))
> #define AV_COND_IIF_II(id) id
> #define AV_COND_IIF_0(t, f) f
> #define AV_COND_IIF_1(t, f) t
> #define AV_COND_BOOL(x) AV_COND_BOOL_I(x)
> #define AV_COND_BOOL_I(x) AV_COND_BOOL_ ## x
> #define AV_COND_BOOL_0 0
> #define AV_COND_BOOL_1 1
> #define AV_COND_IF(cond, t, f) AV_COND_IIF(AV_COND_BOOL(cond), t, f)

That is quite a bit more obfuscated than it needs to be.  The
following actually works:

$ cat foo.c
#define AV_IF_0(t, f) f
#define AV_IF_1(t, f) t
#define AV_IF(c, t, f)  AV_IF2(c, t, f)
#define AV_IF2(c, t, f) AV_IF_##c(t, f)

#define REGISTER_THING(X, x) { extern int x##_thing; AV_IF(CONFIG_##X, register_thing(&x##_thing),); }

#define CONFIG_FOO 1
#define CONFIG_BAR 0

void init(void) {
    REGISTER_THING(FOO, foo);
    REGISTER_THING(BAR, bar);
}
$ gcc -E foo.c
void init(void) {
    { extern int foo_thing; register_thing(&foo_thing); };
    { extern int bar_thing; ; };
}
$

Even so, there is no good reason to change the code.  We have already
told you why.

> Usage:
> Before:
> #define REGISTER_HWACCEL(X,x) { \
>           extern AVHWAccel x##_hwaccel; \
>           if(CONFIG_##X##_HWACCEL) av_register_hwaccel(&x##_hwaccel);
> }
>
> With the macro:
> #define REGISTER_HWACCEL(X,x) { \
>           extern AVHWAccel x##_hwaccel; \
>           AV_COND_IF(CONFIG_##X##_HWACCEL,
> (av_register_hwaccel(&x##_hwaccel)),) ; }
>
>> However, it is far more convoluted than the current code
>
> What?

10 obfuscated macros are more convoluted than none.

>> for no gain,
>
> It makes it possible to completely disable optimisations.

I do not consider that a gain.

> To date the configure script offers to disable optimisations, but
> doesn't do so.  One could call this a bug, at least it relies on
> compiler specifics that are not mandated by C99.

We rely on a lot of things outside the scope of C99, for example linkers.

>> and Reimar's comments are valid too.
>
> I think Reimar's comments are beside the point. The only thing the
> compiler does, if a runtime if is used instead of a macro, is that it
> checks the syntax of the call, nothing else.

I consider it a good thing having the syntax checked.

-- 
M?ns Rullg?rd
mans at mansr.com