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
>
