Re: No TBAA before ptr_derefs_may_alias_p?

[Richard Biener <rguenther at suse dot de>]

On 1/31/14 4:02 PM, Bingfeng Mei wrote:
> Hi,
> I got this simple example to vectorize. Somehow, GCC (4.8) generates loop version because
> it cannot determine alias between acc[i] write and x[i].real read. It is pretty obvious to me that they are not aliased based on TBAA information.
> 
> typedef struct
> {
>    short real;
>    short imag;
> } complex16_t;
> 
> void
> libvector_AccSquareNorm_ref (unsigned long long  *acc,
>                              const complex16_t *x, unsigned len)
> {
>     for (unsigned i = 0; i < len; i++)
>     {
>         acc[i] +=
>             ((unsigned long long)((int)x[i].real * x[i].real)) +
>             ((unsigned long long)((int)x[i].imag * x[i].imag));
>     }
> }
> 
> Tracing into how the alias information is calculated, I found it hits the following code
> by calling ptr_derefs_may_alias_p and return true. ptr_derefs_may_alias_p doesn't contain
> TBAA disambiguation code. Should we add check before that? 
> 
>   /* If we had an evolution in a MEM_REF BASE_OBJECT we do not know
>      the size of the base-object.  So we cannot do any offset/overlap
>      based analysis but have to rely on points-to information only.  */
>   if (TREE_CODE (addr_a) == MEM_REF
>       && DR_UNCONSTRAINED_BASE (a))
>     {
>       if (TREE_CODE (addr_b) == MEM_REF
> 	  && DR_UNCONSTRAINED_BASE (b))
> 	return ptr_derefs_may_alias_p (TREE_OPERAND (addr_a, 0),
> 				       TREE_OPERAND (addr_b, 0));
>       else
> 	return ptr_derefs_may_alias_p (TREE_OPERAND (addr_a, 0),
> 				       build_fold_addr_expr (addr_b));
>     }
>   else if (TREE_CODE (addr_b) == MEM_REF
> 	   && DR_UNCONSTRAINED_BASE (b))
>     return ptr_derefs_may_alias_p (build_fold_addr_expr (addr_a),
> 				   TREE_OPERAND (addr_b, 0));
> 
>   /* Otherwise DR_BASE_OBJECT is an access that covers the whole object
>      that is being subsetted in the loop nest.  */
>   if (DR_IS_WRITE (a) && DR_IS_WRITE (b))
>     return refs_output_dependent_p (addr_a, addr_b);
>   else if (DR_IS_READ (a) && DR_IS_WRITE (b))
>     return refs_anti_dependent_p (addr_a, addr_b);
>   return refs_may_alias_p (addr_a, addr_b);
> 
> This issue can be reproduced on trunk x86-64 gcc. 

True, you can add a

 if (flag_strict_aliasing
    && DR_IS_WRITE (a) && DR_IS_READ (b)
    && !alias_sets_conflict_p (get_alias_set (DR_REF (a)), get_alias_set
(DR_REF (b)))
   return false;

before the ptr_derefs_may_alias_p calls.  TBAA is only valid for
true dependences.

Richard.

> Cheers,
> Bingfeng Mei
>