RE: [PATCH] check alias sets in add_inter_loop_mem_dep (ddg.c)

[Revital1 Eres <ERES at il dot ibm dot com>]

Hello,

> I asked on the GCC mailing list about the DDG export patch a few months
ago. I
> tried to merged it recently into the mainline, without much success. A
> immediate issue I have with modulo scheduling is that it doesn't handle
> restrict keyword. I just adapt these the walker functions in the above
patch,
> which I think is generic and simple.

OK, thanks.
I am a little confused regarding how alias_sets_conflict_p will handle
cross-iteration dependencies when constructing DDG for SMS.

>From previous talks with Andrey I understood that data dependency
graph construction for SMS can not use rtl alias analysis because of the
following explanation:
When adding cross-iteration dependencies, because rtl AA includes
base+offset disambiguation, which allows to disambiguate a[i] and a[i+1]
on targets with base+offset memory addressing; while a[i] and a[i+1]
are independent within one iteration, cross-iteration dependency still
must be added; thus, DDG construction adds dependency edges for all
pairs of memory references.

Revital

>
> Cheers,
> Bingfeng
>
> > -----Original Message-----
> > From: Revital1 Eres [mailto:ERES@il.ibm.com]
> > Sent: 19 January 2009 06:27
> > To: Bingfeng Mei
> > Cc: Andrey Belevantsev; Ayal Zaks; gcc-patches@gcc.gnu.org
> > Subject: Re: [PATCH] check alias sets in
> > add_inter_loop_mem_dep (ddg.c)
> >
> > Hello,
> >
> > It seems that a similar function can be found in the export
> > DDG patch: http://gcc.gnu.org/ml/gcc/2007-08/msg00342.html
> > (please see independent_by_exported_info () in
> > http://gcc.gnu.org/ml/gcc/2007-08/txt00017.txt)
> > However, alias_sets_conflict_p is not used there.
> >
> > Andrey, I appreciate it if you could explain the difference between
> > those two functions.
> >
> > Thanks,
> > Revital
> >
> > > I have a following patch to detect if memory references of two
> > instructions
> > > belong to non-conflicting alias sets. It is used in
> > inter-loop dependence
> >
> > > analysis (ddg.c) by modulo scheduler. For the following
> > testcase, GCC
> > will
> > > produce better code due to better memory dependency info. In fact,
> > powperpc-
> > > gcc cannot software pipeline this loop without the patch. OK for
> > mainline?
> > > Should I also include this test in the patch?
> > >
> > > /* {dg-do run} */
> > > /* {dg-options "-O2 -fmodulo-sched -w"} */
> > > void foo( int * __restrict__ a, int * __restrict__ b, int *
> > __restrict__
> > c)
> > > {
> > >    int i;
> > >    for(i = 0; i < 100; i+=4)
> > >      {
> > >        a[i] = b[i] * c[i];
> > >        a[i+1] = b[i+1] * c[i+1];
> > >        a[i+2] = b[i+2] * c[i+2];
> > >        a[i+3] = b[i+3] * c[i+3];
> > >      }
> > > }
> > >
> > >
> > > int a[100], b[100], c[100];
> > > int main()
> > > {
> > >   int i, res;
> > >   for(i = 0; i < 100; i++)
> > >     {
> > >       b[i] = c[i] = i;
> > >     }
> > >   foo(a, b, c);
> > >
> > >   res = 0;
> > >   for(i = 0; i < 100; i++)
> > >     {
> > >       res += a[i];
> > >     }
> > >   if(res != 328350)
> > >     abort();
> > >
> > >   return 0;
> > > }
> > >
> > > This is the first time I try to submit a patch. Hope it
> > conforms to the
> > coding
> > > style, patch format, etc. Additionally, my company
> > (Broadcom) has already
> >
> > > signed coporate copyright form with FSF.
> > >
> > > Bingfeng Mei
> > > Broadcom UK
> > >
> > >
> > > 2009-01-15 Bingfeng Mei <bmei@broadcom.com>
> > >
> > >    *alias.c (walk_mems_1, walk_mems_2,
> > insn_alias_sets_conflict_p): Check
> > >    whether two instructions have memory references that belong to
> > conflicting
> > > alias sets.
> > >    walk_mems_1 and walk_mems_2 are helper functions for traversing
> > >    *alias.h (insn_alias_sets_confilict_p): new prototypes
> > >    *ddg.c (add_inter_loop_mem_dep): call
> > insn_alias_sets_conflict_p not
> > to
> > > draw dependency edge
> > >    for instructions with non-conflicting alias sets.
> > >
> > > Index: gcc/ddg.c
> > > ===================================================================
> > > --- gcc/ddg.c   (revision 143391)
> > > +++ gcc/ddg.c   (working copy)
> > > @@ -345,6 +345,10 @@
> > >  static void
> > >  add_inter_loop_mem_dep (ddg_ptr g, ddg_node_ptr from,
> > ddg_node_ptr to)
> > >  {
> > > +  if (!insn_alias_sets_conflict_p (from->insn, to->insn))
> > > +    /* Do not create edge if memory references have
> > disjoint alias sets.
> > */
> > > +    return;
> > > +
> > >    if (mem_write_insn_p (from->insn))
> > >      {
> > >        if (mem_read_insn_p (to->insn))
> > > Index: gcc/alias.c
> > > ===================================================================
> > > --- gcc/alias.c   (revision 143391)
> > > +++ gcc/alias.c   (working copy)
> > > @@ -344,6 +344,43 @@
> > >    return 0;
> > >  }
> > >
> > > +static int
> > > +walk_mems_2 (rtx *x, rtx mem)
> > > +{
> > > +  if (MEM_P (*x))
> > > +    {
> > > +      if (alias_sets_conflict_p (MEM_ALIAS_SET(*x),
> > MEM_ALIAS_SET(mem)))
> > > +        return 1;
> > > +
> > > +      return -1;
> > > +    }
> > > +  return 0;
> > > +}
> > > +
> > > +static int
> > > +walk_mems_1 (rtx *x, rtx *pat)
> > > +{
> > > +  if (MEM_P (*x))
> > > +    {
> > > +      /* Visit all MEMs in *PAT and check indepedence.  */
> > > +      if (for_each_rtx (pat, (rtx_function) walk_mems_2, *x))
> > > +        /* Indicate that dependence was determined and
> > stop traversal.
> > */
> > > +        return 1;
> > > +
> > > +      return -1;
> > > +    }
> > > +  return 0;
> > > +}
> > > +
> > > +/* Return 1 if two specified instructions have mem expr
> > with conflict
> > alias sets*/
> > > +bool
> > > +insn_alias_sets_conflict_p (rtx insn1, rtx insn2)
> > > +{
> > > +  /* For each pair of MEMs in INSN1 and INSN2 check their
> > independence.
> > */
> > > +  return  for_each_rtx (&PATTERN (insn1), (rtx_function)
> > walk_mems_1,
> > > +          &PATTERN (insn2));
> > > +}
> > > +
> > >  /* Return 1 if the two specified alias sets will always
> > conflict.  */
> > >
> > >  int
> > > Index: gcc/alias.h
> > > ===================================================================
> > > --- gcc/alias.h   (revision 143391)
> > > +++ gcc/alias.h   (working copy)
> > > @@ -41,6 +41,7 @@
> > >  extern int alias_sets_must_conflict_p (alias_set_type,
> > alias_set_type);
> > >  extern int objects_must_conflict_p (tree, tree);
> > >  extern int nonoverlapping_memrefs_p (const_rtx, const_rtx);
> > > +extern bool insn_alias_sets_conflict_p (rtx, rtx);
> > >
> > >  /* This alias set can be used to force a memory to
> > conflict with all
> > >     other memories, creating a barrier across which no
> > memory reference
> >
> >
> >