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Re: [RFC/RFA] Const variable initializer folding
- From: Richard Guenther <rguenther at suse dot de>
- To: Jan Hubicka <hubicka at ucw dot cz>
- Cc: gcc-patches at gcc dot gnu dot org, rth at redhat dot com
- Date: Fri, 3 Sep 2010 11:00:19 +0200 (CEST)
- Subject: Re: [RFC/RFA] Const variable initializer folding
- References: <20100831230839.GI1664@kam.mff.cuni.cz> <20100902195028.GB30437@atrey.karlin.mff.cuni.cz>
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
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746 - GF: Markus Rex