Re: [RFC/RFA] Const variable initializer folding

[Richard Guenther <rguenther at suse dot de>]

On Thu, 2 Sep 2010, Jan Hubicka wrote:

> Hi,
> i re-tested the patch with the rest_of_decl_compilation fix and it does pass now.
> Note that i am not terribly sure in what conditions we want to make the transform.
> This patch chnages previous behaviour in two ways:
> 
> 1) scalars and arrays/aggregates are now handled equivalently (same was as
> scalars previously, so in partiuclar one can not overwrite initializer at
> dynamic linking time)
> 2) COMDATs are handled as initializer being known since I use DECL_REPLACEABLE_P
> instead of targetm.binds_local_p
> 
> OK?

Ok.

Thanks,
Richard.

> Honza
> > 
> > There is one failure with the patch, gfortran.dg/cray_pointers_5.f90.  It is
> > because fortran adds into varpool variable that is !TREE_STATIC &&
> > !DECL_EXTERNAL that seems like a bug to me, but I am not familiar enough with
> > this area of FE to know what it is supposed to do.
> > 
> > Does this seem to make sense?  Patch was bootstrapped/regtested x86_64-linux and
> > darwin.
> > 
> > Honza
> > 
> > 	* cgraph.h (struct varpool_node): Add const_value_known.
> > 	(varpool_decide_const_value_known): Declare.
> > 	* tree-ssa-ccp.c (fold_const_aggregate_ref): Update initializer folding.
> > 	* lto-cgraph.c (lto_output_varpool_node): Store const_value_known.
> > 	(input_varpool_node): Restore const_value_known.
> > 	* tree-ssa-loop-ivcanon (constant_after_peeling): Check varpool for
> > 	initializer folding.
> > 	* ipa.c (ipa_discover_readonly_nonaddressable_var,
> > 	function_and_variable_visibility): Compute const_value_known.
> > 	* gimple-fold.c (get_symbol_constant_value): Use varpool for initializer
> > 	folding.
> > 	* varpool.c (varpool_decide_const_value_known): New function.
> > Index: gcc/cgraph.h
> > ===================================================================
> > --- gcc/cgraph.h	(revision 163636)
> > +++ gcc/cgraph.h	(working copy)
> > @@ -503,6 +503,8 @@ struct GTY((chain_next ("%h.next"), chai
> >       During WPA output it is used to mark nodes that are present in
> >       multiple partitions.  */
> >    unsigned in_other_partition : 1;
> > +  /* True when variable is constant and its value is known.  */
> > +  unsigned int const_value_known : 1;
> >  };
> >  
> >  /* Every top level asm statement is put into a cgraph_asm_node.  */
> > @@ -726,6 +728,7 @@ void varpool_empty_needed_queue (void);
> >  bool varpool_extra_name_alias (tree, tree);
> >  const char * varpool_node_name (struct varpool_node *node);
> >  void varpool_reset_queue (void);
> > +bool varpool_decide_const_value_known (struct varpool_node *node);
> >  
> >  /* Walk all reachable static variables.  */
> >  #define FOR_EACH_STATIC_VARIABLE(node) \
> > Index: gcc/tree-ssa-ccp.c
> > ===================================================================
> > --- gcc/tree-ssa-ccp.c	(revision 163636)
> > +++ gcc/tree-ssa-ccp.c	(working copy)
> > @@ -1348,7 +1348,8 @@ fold_const_aggregate_ref (tree t)
> >  	case VAR_DECL:
> >  	  if (!TREE_READONLY (base)
> >  	      || TREE_CODE (TREE_TYPE (base)) != ARRAY_TYPE
> > -	      || !targetm.binds_local_p (base))
> > +	      || ((TREE_STATIC (base) || DECL_EXTERNAL (base))
> > +		  && !varpool_get_node (base)->const_value_known))
> >  	    return NULL_TREE;
> >  
> >  	  ctor = DECL_INITIAL (base);
> > @@ -1435,7 +1436,8 @@ fold_const_aggregate_ref (tree t)
> >  	case VAR_DECL:
> >  	  if (!TREE_READONLY (base)
> >  	      || TREE_CODE (TREE_TYPE (base)) != RECORD_TYPE
> > -	      || !targetm.binds_local_p (base))
> > +	      || ((TREE_STATIC (base) || DECL_EXTERNAL (base))
> > +		  && !varpool_get_node (base)->const_value_known))
> >  	    return NULL_TREE;
> >  
> >  	  ctor = DECL_INITIAL (base);
> > @@ -1509,7 +1511,8 @@ fold_const_aggregate_ref (tree t)
> >  
> >  	  if (!TREE_READONLY (base)
> >  	      || TREE_CODE (TREE_TYPE (base)) != ARRAY_TYPE
> > -	      || !targetm.binds_local_p (base))
> > +	      || ((TREE_STATIC (base) || DECL_EXTERNAL (base))
> > +		  && !varpool_get_node (base)->const_value_known))
> >  	    return NULL_TREE;
> >  
> >  	  ctor = DECL_INITIAL (base);
> > Index: gcc/lto-cgraph.c
> > ===================================================================
> > --- gcc/lto-cgraph.c	(revision 163636)
> > +++ gcc/lto-cgraph.c	(working copy)
> > @@ -575,6 +575,7 @@ lto_output_varpool_node (struct lto_simp
> >    bp_pack_value (&bp, node->force_output, 1);
> >    bp_pack_value (&bp, node->finalized, 1);
> >    bp_pack_value (&bp, node->alias, 1);
> > +  bp_pack_value (&bp, node->const_value_known, 1);
> >    gcc_assert (!node->alias || !node->extra_name);
> >    gcc_assert (node->finalized || !node->analyzed);
> >    gcc_assert (node->needed);
> > @@ -1106,9 +1112,10 @@ input_varpool_node (struct lto_file_decl
> >    node->force_output = bp_unpack_value (&bp, 1);
> >    node->finalized = bp_unpack_value (&bp, 1);
> >    node->alias = bp_unpack_value (&bp, 1);
> > +  node->const_value_known = bp_unpack_value (&bp, 1);
> >    node->analyzed = node->finalized; 
> >    node->used_from_other_partition = bp_unpack_value (&bp, 1);
> >    node->in_other_partition = bp_unpack_value (&bp, 1);
> >    aliases_p = bp_unpack_value (&bp, 1);
> >    if (node->finalized)
> >      varpool_mark_needed_node (node);
> > Index: gcc/tree-ssa-loop-ivcanon.c
> > ===================================================================
> > --- gcc/tree-ssa-loop-ivcanon.c	(revision 163636)
> > +++ gcc/tree-ssa-loop-ivcanon.c	(working copy)
> > @@ -165,9 +165,7 @@ constant_after_peeling (tree op, gimple 
> >        if ((DECL_P (base)
> >        	   && TREE_STATIC (base)
> >  	   && TREE_READONLY (base)
> > -           && (DECL_INITIAL (base)
> > -	       || (!DECL_EXTERNAL (base)
> > -		   && targetm.binds_local_p (base))))
> > +	   && varpool_get_node (base)->const_value_known)
> >  	  || CONSTANT_CLASS_P (base))
> >  	{
> >  	  /* If so, see if we understand all the indices.  */
> > Index: gcc/ipa.c
> > ===================================================================
> > --- gcc/ipa.c	(revision 163636)
> > +++ gcc/ipa.c	(working copy)
> > @@ -561,6 +561,7 @@ ipa_discover_readonly_nonaddressable_var
> >  	    if (dump_file)
> >  	      fprintf (dump_file, " %s (read-only)", varpool_node_name (vnode));
> >  	    TREE_READONLY (vnode->decl) = 1;
> > +	    vnode->const_value_known |= varpool_decide_const_value_known (vnode);
> >  	  }
> >        }
> >    if (dump_file)
> > @@ -767,6 +768,9 @@ function_and_variable_visibility (bool w
> >  	      || ! (ADDR_SPACE_GENERIC_P
> >  		    (TYPE_ADDR_SPACE (TREE_TYPE (vnode->decl))))))
> >  	DECL_COMMON (vnode->decl) = 0;
> > +     /* Even extern variables might have initializers known.
> > +	See, for example testsuite/g++.dg/opt/static3.C  */
> > +     vnode->const_value_known |= varpool_decide_const_value_known (vnode);
> >      }
> >    for (vnode = varpool_nodes_queue; vnode; vnode = vnode->next_needed)
> >      {
> > @@ -801,6 +805,7 @@ function_and_variable_visibility (bool w
> >  	  gcc_assert (in_lto_p || whole_program || !TREE_PUBLIC (vnode->decl));
> >  	  cgraph_make_decl_local (vnode->decl);
> >  	}
> > +     vnode->const_value_known |= varpool_decide_const_value_known (vnode);
> >       gcc_assert (TREE_STATIC (vnode->decl));
> >      }
> >    pointer_set_destroy (aliased_nodes);
> > Index: gcc/gimple-fold.c
> > ===================================================================
> > --- gcc/gimple-fold.c	(revision 163636)
> > +++ gcc/gimple-fold.c	(working copy)
> > @@ -38,9 +38,9 @@ along with GCC; see the file COPYING3.  
> >  tree
> >  get_symbol_constant_value (tree sym)
> >  {
> > -  if (TREE_STATIC (sym)
> > -      && (TREE_READONLY (sym)
> > -	  || TREE_CODE (sym) == CONST_DECL))
> > +  if ((TREE_STATIC (sym) || DECL_EXTERNAL (sym))
> > +      && (TREE_CODE (sym) == CONST_DECL
> > +	  || varpool_get_node (sym)->const_value_known))
> >      {
> >        tree val = DECL_INITIAL (sym);
> >        if (val)
> > @@ -65,8 +65,6 @@ get_symbol_constant_value (tree sym)
> >  	 have zero as the initializer if they may not be
> >  	 overridden at link or run time.  */
> >        if (!val
> > -	  && !DECL_EXTERNAL (sym)
> > -	  && targetm.binds_local_p (sym)
> >            && (INTEGRAL_TYPE_P (TREE_TYPE (sym))
> >  	       || SCALAR_FLOAT_TYPE_P (TREE_TYPE (sym))))
> >  	return fold_convert (TREE_TYPE (sym), integer_zero_node);
> > Index: gcc/varpool.c
> > ===================================================================
> > --- gcc/varpool.c	(revision 163636)
> > +++ gcc/varpool.c	(working copy)
> > @@ -357,6 +357,37 @@ decide_is_variable_needed (struct varpoo
> >    return true;
> >  }
> >  
> > +/* Return if NODE is constant and its initial value is known (so we can do
> > +   constant folding).  The decision depends on whole program decisions
> > +   and can not be recomputed at ltrans stage for variables from other
> > +   partitions.  For this reason the new value should be always combined
> > +   with the previous knowledge.  */
> > +
> > +bool
> > +varpool_decide_const_value_known (struct varpool_node *node)
> > +{
> > +  tree decl = node->decl;
> > +
> > +  gcc_assert (TREE_STATIC (decl) || DECL_EXTERNAL (decl));
> > +  gcc_assert (TREE_CODE (decl) == VAR_DECL);
> > +  if (!TREE_READONLY (decl))
> > +    return false;
> > +  /* Variables declared 'const' without an initializer
> > +     have zero as the initializer if they may not be
> > +     overridden at link or run time.  */
> > +  if (!DECL_INITIAL (decl)
> > +      && (DECL_EXTERNAL (decl)
> > +	  || DECL_REPLACEABLE_P (decl)))
> > +    return false;
> > +
> > +  /* Variables declared `const' with an initializer are considered
> > +     to not be overwritable with different initializer by default. 
> > +
> > +     ??? Previously we behaved so for scalar variables but not for array
> > +     accesses.  */
> > +  return true;
> > +}
> > +
> >  /* Mark DECL as finalized.  By finalizing the declaration, frontend instruct the
> >     middle end to output the variable to asm file, if needed or externally
> >     visible.  */
> 
> 

-- 
Richard Guenther <rguenther@suse.de>
Novell / SUSE Labs
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