RE: No TBAA before ptr_derefs_may_alias_p?

[Richard Biener <rguenther at suse dot de>]

On January 31, 2014 6:01:36 PM GMT+01:00, Bingfeng Mei <bmei@broadcom.com> wrote:
>Unfortunately this patch doesn't work because the memory dependency is
>Anti in this
>case. 
>
>Why TBAA cannot handle anti- & output- dependencies? I check GCC bug
>database, and 
>found pr38503 & pr38964.  I don't fully understand it, but seems to me
>is related
>in handling C++ new operator. But this example is pretty clear and has
>nothing to
>do with C++ and new statement. Isn't it too conservative to disable
>TBAA for anti-
>& output- dependency here? 

Because the gcc memory model allows the dynamic type of a memory location to change by a store.

That in turn is the only sensible way of supporting c++ placement new.

Richard.

>
>Bingfeng
>
>-----Original Message-----
>From: Richard Biener [mailto:rguenther@suse.de] 
>Sent: 31 January 2014 15:24
>To: Bingfeng Mei; gcc@gcc.gnu.org
>Subject: Re: No TBAA before ptr_derefs_may_alias_p?
>
>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
>>