Re: immediate strings #2

[Ken Raeburn <raeburn@raeburn.org>]

On Nov 28, 2011, at 17:25, Andreas Schwab wrote:
> Stefan Monnier <monnier@IRO.UMontreal.CA> writes:
> 
>> Not that it's relevant to Emacs, but I don't know which part of the
>> C standard would force a C compiler to layout all unions at an
>> "addressable" offset.
> 
> 6.7.2.1#13

Well, I think Stefan's technically right... the "as-if" rule lets the compiler get away with a lot, if it can analyze enough of the program to figure out that it wouldn't make a difference to the semantics ("no one will notice", as Stefan put it).  For example, gcc can make some variables that have their addresses taken still live in registers anyways.  But few or none of the compilers we care about right now will do that when dealing with structure layouts and multiple source files as in Emacs; they'll implement something close enough to the abstract machine description in the standard that the union would have to be addressable.

Ken