Basic kill analysis for modref

Richard Biener rguenther@suse.de
Fri Nov 12 11:00:56 GMT 2021


On Fri, 12 Nov 2021, Jan Hubicka wrote:

> > 
> > I wonder why we bother producing summaries for things that do not
> > bind locally?  The summary->kills.length () has an upper bound?
> 
> Because of local aliases.
> The size of the array is capped by param_max_modref_accesses which is
> 16.
> > 
> > > +	  && summary->kills.length ())
> > > +	{
> > > +	  tree base = ao_ref_base (ref);
> > > +	  for (unsigned int i = 0; i < summary->kills.length (); i++)
> > > +	    {
> > > +	      modref_access_node &a = summary->kills[i];
> > > +	      tree op;
> > > +	      poly_offset_int off1_adj = 0, off2_adj = 0;
> > > +	      poly_int64 off1, off2;
> > > +	      tree base_ptr = NULL;
> > > +	      tree base_decl = NULL;
> > > +
> > > +	      if (a.parm_index >= 0)
> > > +		op = gimple_call_arg (stmt, a.parm_index);
> > > +	      else if (a.parm_index == MODREF_STATIC_CHAIN_PARM)
> > > +		op = gimple_call_chain (stmt);
> > > +	      else
> > > +		gcc_unreachable ();
> > 
> > I wonder if we can abstract this to a modref_access_node method?
> 
> Something like get_param (stmt)? Yes, it looks like a good idea.
> > 
> > > +
> > > +	      off2_adj += a.parm_offset * BITS_PER_UNIT;
> > 
> > wasn't there a parm_offset unknown? ...
> Yes, but we do not insert those accesses to kills since they are
> unknown.
> > 
> > > +	      if (!(off2_adj + a.offset).to_shwi (&off2))
> > > +		continue;
> > > +	      if (TREE_CODE (base) == MEM_REF)
> > > +		{
> > > +		  off1_adj = mem_ref_offset (base) << LOG2_BITS_PER_UNIT;
> > > +		  if (TREE_CODE (TREE_OPERAND (base, 0)) == ADDR_EXPR)
> > > +		    base_decl = TREE_OPERAND (TREE_OPERAND (base, 0), 0);
> > 
> > 'base' will be the decl in this case, apart from when the constant
> > offset doesn't fit ao_ref.offset, so I think you can spare this
> > special-case and give up on non-SSA base_ptr
> 
> I tought we wrap decls to modrefs in lto streaming when type merging
> fails?

ao_ref_base returns the result of get_ref_base_and_extent which
unwraps this

> > 
> > > +		  else
> > > +		    base_ptr = TREE_OPERAND (base, 0);
> > > +		}
> > > +	      /* Give up on TMRs for now.  */
> > > +	      else if (TREE_CODE (base) == TARGET_MEM_REF)
> > > +		break;
> > > +	      else
> > > +		base_decl = base;
> > > +
> > > +	      gcc_checking_assert (!base_decl || DECL_P (base_decl));
> > > +	      gcc_checking_assert (!base_ptr
> > > +				   || TREE_CODE (base_ptr) == SSA_NAME);
> > > +
> > > +	      /* OP is a pointer and we have access range from its
> > > +		 dereference.  */
> > > +	      if (TREE_CODE (op) == ADDR_EXPR)
> > > +		{
> > > +		  poly_int64 size, offset, max_size;
> > > +		  bool reverse;
> > > +		  tree op_base = get_ref_base_and_extent
> > > +			  (TREE_OPERAND (op, 0), &offset, &size,
> > > +			   &max_size, &reverse);
> > 
> > I think you want get_addr_base_and_unit_offset here.  All
> > variable indexed addresses are in separate stmts.  That also means
> > you can eventually work with just byte sizes/offsets?
> 
> Will do.  The access range in modref summary is bit based (since we want
> to disabiguate bitfields like we do in rest of alias oracle) but indeed
> this part cna be in bytes.
> > 
> > > +		  if (!known_size_p (size) || !known_eq (size, max_size))
> > > +		    continue;
> > > +		  off2_adj += offset;
> > > +		  /* &str->foo are not passed as gimple operands directly,
> > > +		     would need to look up the def stmt.  */
> > > +		  gcc_checking_assert (TREE_CODE (op_base) != MEM_REF);
> > > +		  if (!base_decl
> > > +		      || compare_base_decls (op_base, base_decl) != 1)
> > > +		    continue;
> > > +		}
> > > +	      else if (!base_ptr || !operand_equal_p (base_ptr, op))
> > > +		continue;
> > > +
> > > +	      if (!(off1_adj + ref->offset).to_shwi (&off1))
> > > +		continue;
> > > +	      if (!(off2_adj + a.offset).to_shwi (&off2))
> > > +		continue;
> > > +
> > > +	      if (known_subrange_p (off1, ref->max_size, off2, a.size)
> > > +		  && dbg_cnt (ipa_mod_ref))
> > > +		{
> > > +		  /* For store to be killed it needs to not be used earlier.  */
> > > +		  if (ref_maybe_used_by_call_p_1 (as_a <gcall *> (stmt), ref,
> > > +						  true))
> > 
> > Hmm, so moderf says p->x is killed when we have
> > 
> > foo (struct X *p)
> > {
> >   int tem = p->x;
> >   p->x = 0;
> >   return tem;
> > }
> > 
> > ?  Or even
> Yep, this will currently land in kills.  I can add loop pruning kills
> with known load ranges incrementally.
> > 
> > foo (struct X *p)
> > {
> >   bar ();
> >   p->x = 0;
> > }
> > 
> > ?
> Here we will end up with reading global memory and that will turn kills
> empty in modref.
> 
> The check is still needed to verify that ref is not passed as aggregate
> parameter.
> 
> I will update patch.
> Thanks,
> Honza
> > 
> > Otherwise it looks sensible.
> > 
> > Richard.
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)


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