bug#51484: [PATCH] Move runtime check for recent giflib to compile time

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Fri, 29 Oct 2021 07:42:56 -0700
> 
> I'm looking into some bugs in how we handle gifs, and I see that we
> check for GIFLIB_MAJOR at runtime.

Only once, right?

> Is there any reason not to do it at compile-time as in the attached
> patch?

(What patch?)

The compiler converts that into a run-time constant anyway.  The
reason for using such "run-time" testing is usually to have a more
readable code, since #ifdef's make the code harder to read.  There are
no downsides, since the test is compiled away.

> I expect that GCC is smart enough to see that "5 < 4" is always false
> and optimize this all away, and probably also won't include unused
> static variables, so maybe this doesn't matter.  But I think it's nice
> to be a bit more explicit, and I guess it can't hurt to see warnings if
> anyone tries using interlace_start and interlace_increment outside of
> their intended use.

I don't think I understand what scenario you have in mind where having
an #ifdef would be better.  Please elaborate.