Re: [C++ PATCH]: Shrink struct lang_decl

[Mark Mitchell <mark at codesourcery dot com>]

Nathan Sidwell wrote:

> This patch shrinks lang_decl from 40 to 32 bytes (on an ILP32 machine).
>
> You'll notice that this limits the size of objects that can be thunked
> to 262144 bytes, that is the largest object the standard says a system
> need support.  Mark, Jason, if you think this should be made larger
> let me know.  There are still some spare bits, but I do have plans
> for them :)

I don't really have an opinion about shrinking the maximum object size 
in this way.  I guess it's OK; I certainly can't imagine very many 
people with virtual bases 262K away from non-virtual parts of the same 
object.  But, I'm not sure that we should in general try to improve the 
compiler's performance by limiting ourselves in these sorts of ways; 
surely we don't want identifier length limits! :-)

I agree with Joseph that if we do have such limits, we should have a 
check for overflow.  When using full 32-bit fields, I've not worried 
about that before, on the grounds that virtual memory has probably been 
exhausted by the time we get there, or that some other part of the 
compiler will check for overflow of an INTEGER_CST, but here we really 
could silently generate wrong code for super-big objects.

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com