Messages in this thread |  | | Date | Tue, 25 Feb 2003 11:17:55 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: [PATCH] s390 (7/13): gcc 3.3 adaptions. |
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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.
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