Re: [RFC/RFT] Improving SMS by data dependence export

["Daniel Berlin" <dberlin@dberlin.org>]

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
>