[Bug tree-optimization/42652] vectorizer created unaligned vector insns

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from law at redhat dot com  2010-01-11 17:14 -------
Subject: Re:  vectorizer created unaligned vector
 insns

On 01/10/10 01:22, irar at il dot ibm dot com wrote:
> ------- Comment #5 from irar at il dot ibm dot com  2010-01-10 08:22 -------
> In vector_alignment_reachable_p() we check if an access is packed using
> contains_packed_reference(). For packed accesses we return false, meaning
> alignment is unreachable and peeling cannot be used.
>
> In the attached testcase contains_packed_reference() returns false for
> palette_5.
>    
Right.  If you look more closely at the way the code works, there's no 
way contains_packed_reference is ever going to return true.  It simply 
isn't passed the right information.  It'd need to be looking at the base 
address's type (not the base object).  Furthermore, 
contains_packed_reference simply isn't designed to dive into types.  It 
primarily works on exprs and only references the underlying type when it 
encounters a COMPONENT_REF.

As Richard apparently pointed out in the past, this code is not 
conservatively correct -- if it can't prove the item is properly 
aligned, then a runtime check is required.

So while I think we might be able to fix this specific instance by 
improving the code around the call to contains_packed_reference, I think 
we have a bigger problem as it's fairly trivial to change the test so 
that the addresses of interest are parameters and alignment info is 
effectively unknown.


Jeff


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42652