Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 25 Feb 2003 11:05:38 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 25 Feb 2003 11:05:38 -0500 Received: from chaos.analogic.com ([204.178.40.224]:31108 "EHLO chaos.analogic.com") by vger.kernel.org with ESMTP id ; Tue, 25 Feb 2003 11:05:34 -0500 Date: Tue, 25 Feb 2003 11:17:55 -0500 (EST) From: "Richard B. Johnson" Reply-To: root@chaos.analogic.com To: Linus Torvalds cc: Andreas Schwab , Jeff Garzik , Martin Schwidefsky , linux-kernel@vger.kernel.org Subject: Re: [PATCH] s390 (7/13): gcc 3.3 adaptions. In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2894 Lines: 82 On Tue, 25 Feb 2003, Linus Torvalds wrote: > > On Tue, 25 Feb 2003, Andreas Schwab wrote: > > |> > > |> The point is that the compiler should see that the run-time value of i is > > |> _obviously_never_negative_ and as such the warning is total and utter > > |> crap. > > > > This requires a complete analysis of the loop body, which means that the > > warning must be moved down from the front end (the common type of the > > operands only depends on the type of the operands, not of any current > > value of the expressions). > > So? Gcc does that anyway. _Any_ good compiler has to. > > And if the compiler isn't good enough to do it, then the compiler > shouldn't be warning about something that it hasn't got a clue about. > > > |> and anybody who writes 'array[5UL]' is considered a stupid git and a > > |> geek. Face it. > > > > But array[-1] is wrong. An array can never have a negative index (I'm > > *not* talking about pointers). > > You're wrong. > > Yes, when declaring an array, you cannot use "array[-1]". But that's not > because the thing is unsigned: the standard says that the array > declaration has to be a "integer value larger than zero". It is not > unsigned: it's _positive_. > > However, in _indexing_ an array (as opposed to declaring it), "array[-1]" > is indeed perfectly fine, and is defined by the C language to be exactly > the same as "*(array-1)". And negative values are perfectly fine, even for > arrays. Trivial example: > > int x[2][2]; > > int main(int argc, char **argv) > { > return x[1][-1]; > } > > > the above is actually a well-defined C program, and 100% > standards-conforming ("strictly conforming"). > > Linus > I'm glad you tackled that. I was going to write a response, but backed off because I'm getting meek in my old age. I do wish to add that, even in ix86 assembly language, indexing by using a register and a memory oprand is a signed displacement, i.e, here: .byte 0 foo: .byte 0 movl $0xffffffff, %ebx movl foo,(%ebx), %eax ... accesses a memory location one byte before label foo, i.e., foo[-1] or here[0]. This means that with a N-bit addressing, indexed addressing cannot access more than 2^(N-1) memory locations. Something to consider when trying to access large arrays. For instance, if an operating system were to allow flat-mode access from 0x00000000 to 0xffffffff memory addresses in 32-bit user-mode, could you actually use all that address space in 'C'? Cheers, Dick Johnson Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips). Why is the government concerned about the lunatic fringe? Think about it. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/