bug#17072: bug#17381: _GL_ATTRIBUTE_PURE in dfa.h

[Paul Eggert]

On 05/01/2014 11:29 AM, Aharon Robbins wrote:
> custom.h is for system customization to override things that Autoconf 
> can't figure out or gets wrong

OK, it's easy to have something else include mbsupport.h instead. 
config.h, say.  The attached patch does that.  It doesn't really matter 
what includes it, so long as it's done before dfa.c and dfa.h start 
using the multibyte functions.

> Requiring gnulib in that header makes it less attractive to other 
> projects that might want to use dfa as a black box. Are there such? I 
> don't know. (I thought I'd heard something about gettext using dfa but 
> I am unsure if that is true.)

gettext uses gnulib, so that's not an issue.

> Does the GL_PURE stuff have to be on every declaration? Or can it just 
> be on the body?

It should be on the declaration for external functions, so that the 
function's caller knows to optimize it.

> What does it even mean

It means the function has no effects except the return value and that 
the return value depends only on the parameters and/or global variables.

> Does whatever optimization it enables *really* make a big difference, 
> or is it just a micro-optimization?

We put it in because GCC nowadays complains if we leave it out, if we 
configure with --enable-gcc-warnings.  The optimization seems to be a 
win in general and (more important) an aid for humans reading the code, 
so we typically just add the pure attribute and move on.

> Yes, I know. I am unsure if your patch, which totally eliminates the 
> ability to compile gawk on systems without multibyte support

It's not supposed to do that.  It's supposed to work on those hosts, by 
supplying substitutes for wchar_t, wctype_t, etc.  Hmm, are you worried 
about hosts that don't even have wchar.h and wctype.h?  If so, that can 
be worked around reasonably easily; please see attached patch.

> I just looked at the patch again. It really doesn't do the trick; 
> there are lots of places where MBS_SUPPORT is checked in the gawk code 
> and pulling mbsupport.h out of awk.h is likely to break things

No, it should still work.  With the revised patch, config.h includes 
mbsupport.h, so MBS_SUPPORT will be defined appropriately for gawk code 
and gawk's other MBS_SUPPORT usages will continue to work as before.

I'll CC: this to Bug#17157 and Bug#17072 as it's following up to the 
last messages in both those threads, too.

0001-awk-simplify-dfa.c-by-having-it-not-include-mbsuppor.patch
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