Date: Tue, 16 Mar 1999 16:43:25 +1100 (EST) From: "Andrew Reilly" <andrew@lake.com.au> To: dillon@apollo.backplane.com Cc: andrew@lake.com.au, sjr@home.net, freebsd-hackers@FreeBSD.ORG Subject: Re: Use of "register" in code Message-ID: <19990316054325.40786.qmail@areilly.bpc-users.org> In-Reply-To: <199903160527.VAA06458@apollo.backplane.com>
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Matthew Dillon said: > No, only the auto variable that you took the address of. If an array or > structure, the entire array or structure is put on a real stack. Other > auto variables are left uneffected. > > While it is true that in the olden days people would take the address of > one auto variable in order to reference another by indexing it, in modern > days the alignment and reordering optimizations made by compilers pretty > much nixes such uses on auto variables. Such uses are considered > 'bad code' these days, so compilers have no problem assuming that other > auto variables can remain optimized. Fair enough. I was under the impression that the standard or the compilers had been written to avoid breakage of that "bad code", and so therefore had to be pessimistic. If that's not the case, and they can get away with making those optimisations anyway, then that's good. I've had to use some pretty ordinary compilers. They still exist, and are still shipped with new systems. > specific processor or compiler - usually the VAX. As kernel code started > to migrate to other processors, such optimizations have historically > produced *WORSE* code on the other processors. So people stopped using > 'register' and started expecting compilers to optimize it themselves. But how is using 'register' with gcc in FreeBSD going to make performance _worse_, given that gcc is quite happy to move register variables to and from the stack? I'll shut up now. -- Andrew To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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