[llvm-commits] [llvm] r84175 - /llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp

[Chris Lattner]

On Oct 17, 2009, at 5:44 PM, Nick Lewycky wrote:

> Chris Lattner wrote:
>> On Oct 15, 2009, at 12:11 AM, Nick Lewycky wrote:
>>> Author: nicholas
>>> Date: Thu Oct 15 02:11:24 2009
>>> New Revision: 84175
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=84175&view=rev
>>> Log:
>>> Teach basicaa about memcpy/memmove/memset. The length argument can  
>>> be used to
>>> improve alias results if constant, and the source pointer can't be  
>>> modified.
>> Hi Nick,
>> Please write some testcases that fail without this and pass with it.
>
> Added in r84385.
>
>  I'm pretty sure that this is already
>> handled by logic in AliasAnalysis that knows that memset only looks  
>> at its arguments:
>
> No, that isn't enough. memmove/memcpy have an argument that they  
> never write to. The existing logic returns ModRef against that  
> pointer, while we know we can return Ref.
>
> Similarly, when alias() isn't given the size, it has to assume that  
> the possible writer will write over the whole thing, and any two  
> pointers which are computed as offset from the same base are  
> potentially aliasing. By pulling the size out of memmove/memcpy/ 
> memset, we can return NoModRef in cases where the fallback code  
> could not.

Ok, thanks!