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

[Axel Holzinger]

M?ns Rullg?rd wrote:
> "Axel Holzinger" <aholzinger at gmx.de> writes:
> 
> > M?ns Rullg?rd wrote:
> >> Sent: Sunday, September 19, 2010 7:00 PM
> >> To: FFmpeg development discussions and patches
> >> Subject: Re: [FFmpeg-devel] Moving if(constant expression) to 
> >> preprocessor?
> >> 
> >> "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.
> >
> > That was just a replique on your attack on boost, so forget it.
> 
> I never forget.

Your backpack must already be quite immense :-)

> >> > 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)
> >
> > Smaller and simpler, perfect. I'm fine with that and works for me.
> 
> Whether or not you are "fine" with anything is irrelevant as 
> far as we are concerned.
> 
> >> > 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.
> >
> > There are a lot more multiline obfuscated macros already in
FFmpeg.
> 
> They all provide some advantage over not being there.
> 
> > The singleline ones aren't that complicated.
> 
> A stack of one-line macros is often much harder to understand 
> than a single, large one.
> 
> > And you even got it more simple. So go ahead and use them.
> 
> You will not tell me what to do.

You make it too easy to be predicted :-)

> >> >> for no gain,
> >> >
> >> > It makes it possible to completely disable optimisations.
> >> 
> >> I do not consider that a gain.
> >
> > Isn't having the oportunity to have the choice a gain?
> 
> A choice is only a gain if the alternatives provide mutually 
> exclusive advantages.

Not at all, it's not only black and white. I think the issue is more
that you personally don't have an interest in having an optimisation
free debug build and although the cost is cheap, I tend to say very
cheap, you don't want to move.

Okay, that's your choice. But it would be much easier, if you say so,
instead of finding fruitless arguments.

> >> > 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.
> >
> > Come on, that's silly. Relying on dead code elimination is really 
> > obfuscating, because you can't understand from reading the 
> code that 
> > this only works with optimisation. Having a conditional 
> preprocessor 
> > if makes it much more clear, that the code will not be compiled,
if 
> > the condition is not met.
> 
> Dead code elimination is an implementation detail.  The 
> high-level logic is quite clear.

With depending on it you just loose this. Now it's a prerequisite and
not only an implementation detail.

> >> >> 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.
> >
> > It's far more probable that a potential issue is inside a function

> > call than in the function call itself, whereas most of the
function 
> > calls we're talking about (the ones in the register macros) 
> don't have 
> > parameters at all. So I still tend to think that this not really
an 
> > argument and is beside the point.
> 
> Having some checks is still better than having none.
> 
> --
> M?ns Rullg?rd
> mans at mansr.com
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