Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!cmcl2!adm!smoke!gwyn
From: gwyn@smoke.BRL.MIL (Doug Gwyn )
Newsgroups: comp.lang.c
Subject: Re: Register unions
Message-ID: <8741@smoke.BRL.MIL>
Date: 23 Oct 88 21:16:19 GMT
References: <322@hrc.UUCP> <2699@hound.UUCP> <976@l.cc.purdue.edu> <8724@smoke.BRL.MIL> <981@l.cc.purdue.edu>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 15

>> In article <976@l.cc.purdue.edu> cik@l.cc.purdue.edu (Herman Rubin) writes:
>> >One complaint that I have about the C compilers I have used is that they
>> >do not support register unions.

The point is, on many architectures not all basic data types could actually
share a register.  Your original example was of an integer and a floating
type sharing a register, which doesn't work on any machines I'm aware of.
And obviously a union of large types would not fit into a register.

Simply permitting "register" to be crammed on the front of "union" would
do no good; the hardware would have to actually support the usage for
this to be worthwhile.  If the hardware DOES support this, its C
implementation is allowed to use an actual register (so long as the
address is not taken).  Good optimizing C compilers ignore the "register"
specifiers and do their own register allocation anyway.