[RFC][patch] If-conversion of COMPONENT_REFs

[Richard Guenther]

On Wed, Mar 30, 2011 at 2:22 PM, Ira Rosen <ira.rosen@linaro.org> wrote:
> On 30 March 2011 12:59, Richard Guenther <richard.guenther@gmail.com> wrote:
>> On Wed, Mar 30, 2011 at 11:13 AM, Ira Rosen <ira.rosen@linaro.org> wrote:
>>> Hi,
>>>
>>> With this patch a data-ref is marked as unconditionally read or
>>> written also if its adjacent field is read or written unconditionally
>>> in the loop.
>>> My concern is that this is not safe enough, even though the fields
>>> have to be non-pointers and non-aggregates, and this optimization is
>>> applied only with -ftree-loop-if-convert-stores flag.
>>>
>>> Bootstrapped on powerpc64-suse-linux and tested on x86_64-suse-linux.
>>>
>>> OK for trunk?
>>
>> The restrictions do not make too much sense to me.  For the C++
>> memory model we can't do speculative stores at all, but for the
>> rest I'd say just checking if the data-refs access the same object
>> is enough, thus, instead of same_data_refs (a, b) simply check
>> operand_equal_p (DR_BASE_ADDRESS (a), DR_BASE_ADDRESS (b), 0)
>> or operand_equal_p (get_base_address (DR_REF (a)), get_base_address
>> (DR_REF (b)), 0), whatever makes more sense (I always confuse what
>> the contents of the various DR fields are).
>
> But what about
>
> int a[10], b[100], c[100];
> for (i = 0; i < 100; i++)
>  {
>     if (i < 10)
>       t = a[i];
>    else
>       t = b[i];
>
>    c[i] = t + a[1];
>   }
>
> We can't transform this to
>
> int a[10], b[100], c[100];
> for (i = 0; i < 100; i++)
>  {
>     t1 = a[i];
>     t2 = b[i];
>     t = (i < 10) ? t1 : t2;
>     c[i] = t + a[1];
>   }
>
> since we create accesses to a[10:100], right?

That's correct - we may only ever create "valid" accesses, but any
valid access to a is ok after we see an unconditional access to a.

So we probably have to restrict ourselves to DECL_P (get_base_address ())
objects and make sure we don't access past it.

Thus, I see what you were trying to do - may I suggest sth like

  ref_base_a = DR_REF (a);
  while (TREE_CODE (ref_base_a) == COMPONENT_REF
           || TREE_CODE (ref_base_a) == IMAGPART_EXPR
           || TREE_CODE (ref_base_a) == REALPART_EXPR)
    ref_base_a = TREE_OPERAND (ref_base_a, 0);

... same for DR_REF (b) ...

  if (operand_equal_p (ref_base_a, ref_base_b, 0))
    ok

that is, strip all non-variable offset handled_component_refs and compare
the remaining base of the two accesses.  If they are equal we are ok.

Any hole in that?

Thanks,
Richard.

> Thanks,
> Ira
>
>>
>> Thanks,
>> Richard.
>>
>>> Thanks,
>>> Ira
>>>
>>>
>>> ChangeLog:
>>>
>>>         * tree-if-conv.c (memrefs_read_or_written_unconditionally): Return true
>>>         if an adjacent field of the data-ref is accessed unconditionally.
>>>
>>> testsuite/ChangeLog:
>>>
>>>         * gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c: New test.
>>>         * gcc.dg/vect/vect.exp: Run if-cvt-stores-vect* tests with
>>>         -ftree-loop-if-convert-stores.
>>>
>>
>