Re: [PATCH] arm: Don't require a runtime check function for targets with unconditional NEON

Hardware checks like this cannot be done at configuration time; they either
have to be done at run time or compile time.  This is because of the way
cellphone OSes handle the requirement to ship single apps for multiple
architectures.  IRC the Google  implementation uses run time checks and the
Apple implementation uses compile time checks via multilib.

In neither of these cases can the architecture be set in configure because
there are multiple architectures in one build.  In the runtime case
explicit code in the binary swaps to the relevant implementation, in the
multilib case the configuration is compiled with multiple passes of the
compiler one for each distinct architecture then combined at the link step
via, I assume, trampolines to the correct implementation (so that can be
done as a loader operation, just like DLL handling).

So the configuration can be used to switch hardware specific code *off *and
can even, perhaps for testing, be used to hard-switch it on but the normal
case is to let the build system determine the right code via compiler set
#defines.  Doing "configure" tests just doesn't work; it freezes the result
to early.

I thought I had fixed this, but that was years ago.  It is perhaps best
placed in an architecture specific header file rather than cluttering up
pngpriv.h; nothing outside architecture specific code should ever depend on
it.  It strikes me as unfortunate and a mistake that #defiens exist in
png.h for architecture specific stuff; this should not be part of the
public API.

_______________________________________________
png-mng-implement mailing list
png-mng-implement-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org
https://lists.sourceforge.net/lists/listinfo/png-mng-implement