[LLVMdev] Reassociating expressions involving GEPs

[Chris Lattner]

looks good to me, applied, thanks!
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20090309/074908.html

-Chris

On Mar 9, 2009, at 8:34 AM, Stefanus Du Toit wrote:

> On 9-Mar-09, at 12:14 AM, Chris Lattner wrote:
>> GEP overflow is undefined, but this sentence means that *accesses* to
>> an array must be within its bounds.  It is fine to GEP outside the
>> array as long as you readjust the pointer back before access.
>
> Thanks, that was my hope. Maybe this should be spelled out in the
> langref, especially since it's inconsistent with C, where merely doing
> pointer arithmetic that yields an out-of-bounds pointer (except for
> "one element past the end") yields undefined behaviour.
>
> Perhaps something like: (apologies for the inline patch):
>
> Index: LangRef.html
> ===================================================================
> --- LangRef.html	(revision 66420)
> +++ LangRef.html	(working copy)
> @@ -3637,11 +3637,13 @@
>    }
>  </pre>
>
> -<p>Note that it is undefined to access an array out of bounds: array
> and
> -pointer indexes must always be within the defined bounds of the array
> type.
> -The one exception for this rule is zero length arrays.  These arrays
> are
> -defined to be accessible as variable length arrays, which requires
> access
> -beyond the zero'th element.</p>
> +<p>Note that it is undefined to access an array out of bounds: array
> +and pointer indexes must always be within the defined bounds of the
> +array type when accessed with an instruction that dereferences the
> +pointer (e.g. a load or store instruction).  The one exception for
> +this rule is zero length arrays.  These arrays are defined to be
> +accessible as variable length arrays, which requires access beyond  
> the
> +zero'th element.</p>
>
>  <p>The getelementptr instruction is often confusing.  For some more
> insight
>  into how it works, see <a href="GetElementPtr.html">the getelementptr
>
> --
> Stefanus Du Toit <stefanus.dutoit at rapidmind.com>
>   RapidMind Inc.
>   phone: +1 519 885 5455 x116 -- fax: +1 519 885 1463
>
>
>
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