[RFC/RFT] Improving SMS by data dependence export

[Alexander Monakov]

On Fri, 07 Dec 2007 23:49:28 +0300, Daniel Berlin <dberlin@dberlin.org>  
wrote:

> On 12/7/07, Alexander Monakov <amonakov@ispras.ru> wrote:
>> Hi.
>>
>> Attached is the patch that allows to save dependence info obtained on  
>> tree
>> level by data-reference analysis for usage on RTL level (for RTL memory
>> disambiguation and dependence graph construction for modulo scheduling).
>> It helps for RTL disambiguation on platforms without base+offset memory
>> addressing modes, and impact on SMS is described below.  We would like  
>> to
>> see it in 4.4 mainline.
>>
>> We have tested this patch with modulo scheduling on ia64, using SPEC
>> CPU2000 benchmark suite.  It allows to apply software pipelining to more
>> loops, resulting in ~1-2% speedup (compared to SMS without exported
>> info).  The most frequent improvements are removal of cross-iteration
>> memory dependencies, as currently SMS adds such dependencies for all  
>> pair
>> of memory references, even in cases when they cannot alias (for example,
>> for different arrays or different fields of a struct).  As I understand,
>> SMS does not use RTL alias analysis here because pairs that do not alias
>> within one iteration, but may alias when cross-iteration movement is
>> performed (like a[i] and a[i+1]), should be marked as dependent.  So,  
>> SMS
>> data dependence analysis can be greatly improved even without
>> data-dependence export patch by using RTL-like memory disambiguation,  
>> but
>> without pointer arithmetic analysis.
>>
>> There are currently two miscompiled SPEC tests with this patch; in one  
>> of
>> them, the problem is related to generation of register moves in the
>> prologue of software pipelined loop (which was not pipelined without the
>> patch).  The problem is reported and discussed with Revital Eres from  
>> IBM
>> Haifa.
>>
>> We would like to ask people interested in SMS performance on PowerPC and
>> Cell SPU to conduct tests with this patch.  Any feedback is greatly
>> appreciated.
>>
>
> I see a few random unrelated changes, like, for example:
>
>    if (may_eliminate_iv (data, use, cand, &bound))
> -    {
> -      elim_cost = force_var_cost (data, bound, &depends_on_elim);
> -      /* The bound is a loop invariant, so it will be only computed
> -        once.  */
> -      elim_cost /= AVG_LOOP_NITER (data->current_loop);
> -    }
> +    elim_cost = force_var_cost (data, bound, &depends_on_elim);
>    else
>      elim_cost = INFTY;
>
>
> Please pull these out into separate patches or don't do them :)
> also, i see
> +  /* We do not use operand_equal_p for ORIG_EXPRs because we need to
> +     distinguish memory references at different points of the loop  
> (which
> +     would have different indices in SSA form, like a[i_1] and a[i_2],  
> but
> +     were later rewritten to same a[i]).  */
> +         && (p->orig_expr == q->orig_expr));
>
> This doesn't do enough to distinguish memory references at different
> points of the loop, while also eliminating from consideration that
> *are* the same.
>
> What if they are regular old VAR_DECL?
> This will still return true, but they may be different accesses at
> different points in the loop.
>
> In any case, this doesn't belong in mem_attrs_htab_eq, because if they
> are operand_equal_p, for purposes of memory attributes, they *are*
> equal.  They may still be different accesses, which is something you
> have to discover later on.
>
> IE You should be doing this check somewhere else, not in a hashtable
> equality function :)
>
>
> DDR will mark them as data refs
>> Thanks.
>>
>> --
>> Alexander Monakov
>>