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Date: Sat, 28 Sep 2002 16:55:00 -0000
From: Per Bothner <per@bothner.com>
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To: Richard Henderson <rth@redhat.com>
CC: "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: C ABI: struct/union inconsistency
References: <20020928100655.A10847@hollebeek.com> <48190000.1033234561@warlock.codesourcery.com> <20020928192325.GA30150@redhat.com>
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Richard Henderson wrote:
> No, that's not what we want.
> 
> 
> 	struct foo {
> 	#ifdef SMP
> 		long long count;
> 		long lock;
> 	#endif
> 	};
> 
> Here, attribute((nopadding)) would leave foo with size 12, 
> instead of the required size 16 when SMP is defined.
> 
> We _only_ want to change the zero-size-ness of this struct.

Well, this can handled:

#ifndef SMP
        __attribute__(nopadding)__
#endif

That is not to say whether re-using nopadding for two (related)
purposes would be a good idea or not, but we *could* do so.

Another (wild) idea:  Allow "negative" dizes - or fields that
don't count to the final size:

struct foo {
   int a;
   __size_cutoff_;
   int b;
};

In this case, sizeof(struct foo)==sizeof(int).  If you access a 'b',
it will be actually be access from the *next* field.  This would
be like:

struct foo {
   int a;
   int __b[0];
};

except b is not an array.

Note:  I don't say this is a terribly useful feature - but then some
would say that empty structs aren't terribly useful either.

Then an empty struct is:
struct foo {
   __size_cutoff;
:;
-- 
	--Per Bothner
per@bothner.com   http://www.bothner.com/per/