Patchwork [PR43513,1/3] Replace vla with array - Implementation.

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Submitter Tom de Vries
Date Sept. 24, 2011, 11:30 p.m.
Message ID <4E7E6830.9020507@mentor.com>
Download mbox | patch
Permalink /patch/116262/
State New
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Comments

Tom de Vries - Sept. 24, 2011, 11:30 p.m.
On 09/24/2011 05:29 PM, Eric Botcazou wrote:
>> This is an updated version of the patch. I have 2 new patches and an
>> updated testcase which I will sent out individually.
>>
>> Patch set was bootstrapped and reg-tested on x86_64.
>>
>> Ok for trunk?
>>
>> Thanks,
>> - Tom
>>
>> 2011-07-30  Tom de Vries  <tom@codesourcery.com>
>>
>> 	PR middle-end/43513
>> 	* Makefile.in (tree-ssa-ccp.o): Add $(PARAMS_H) to rule.
>> 	* tree-ssa-ccp.c (params.h): Include.
>> 	(fold_builtin_alloca_for_var): New function.
>> 	(ccp_fold_stmt): Use fold_builtin_alloca_for_var.
> 
> We have detected another fallout on some Ada code: the transformation replaces 
> a call to __builtin_alloca with &var, i.e. it introduces an aliased variable, 
> which invalidates the points-to information of some subsequent call, fooling 
> DSE into thinking that it can eliminate a live store.
> 
> The brute force approach
> 
> Index: tree-ssa-ccp.c
> ===================================================================
> --- tree-ssa-ccp.c      (revision 179038)
> +++ tree-ssa-ccp.c      (working copy)
> @@ -2014,7 +2014,10 @@ do_ssa_ccp (void)
>    ccp_initialize ();
>    ssa_propagate (ccp_visit_stmt, ccp_visit_phi_node);
>    if (ccp_finalize ())
> -    return (TODO_cleanup_cfg | TODO_update_ssa | TODO_remove_unused_locals);
> +    return (TODO_cleanup_cfg
> +           | TODO_update_ssa
> +           | TODO_rebuild_alias
> +           | TODO_remove_unused_locals);
>    else
>      return 0;
>  }
> 
> works, but we might want to be move clever.  Thoughts?
> 

How about attached (untested) patch implementing a conservative, but
runtime-efficient approach?

Thanks,
- Tom
Eric Botcazou - Sept. 25, 2011, 7:30 a.m.
> How about attached (untested) patch implementing a conservative, but
> runtime-efficient approach?

This doesn't work.  My understanding is that you need to recompute far more 
than that, in particular the points-to information for _all_ the calls in the 
function.  I don't know enough of the machinery to tell whether this can be 
done incrementally, but I wouldn't think so, so passing TODO_rebuild_alias 
might be the only solution (in a less brutal way of course).

I'll post a testcase, but it depends on some other, not yet installed changes.

Patch

Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c (revision 179043)
+++ gcc/tree-ssa-ccp.c (working copy)
@@ -1729,6 +1729,7 @@  fold_builtin_alloca_for_var (gimple stmt
   array_type = build_array_type_nelts (elem_type, n_elem);
   var = create_tmp_var (array_type, NULL);
   DECL_ALIGN (var) = align;
+  pt_solution_add_var (&get_ptr_info (lhs)->pt, var);
 
   /* Fold alloca to the address of the array.  */
   return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));
Index: gcc/tree-ssa-alias.h
===================================================================
--- gcc/tree-ssa-alias.h (revision 179043)
+++ gcc/tree-ssa-alias.h (working copy)
@@ -125,6 +125,7 @@  extern void dump_alias_stats (FILE *);
 
 /* In tree-ssa-structalias.c  */
 extern unsigned int compute_may_aliases (void);
+extern void pt_solution_add_var (struct pt_solution *, tree);
 extern bool pt_solution_empty_p (struct pt_solution *);
 extern bool pt_solution_includes_global (struct pt_solution *);
 extern bool pt_solution_includes (struct pt_solution *, const_tree);
Index: gcc/tree-ssa-structalias.c
===================================================================
--- gcc/tree-ssa-structalias.c (revision 179043)
+++ gcc/tree-ssa-structalias.c (working copy)
@@ -5952,6 +5952,14 @@  pt_solution_ior_into (struct pt_solution
   bitmap_ior_into (dest->vars, src->vars);
 }
 
+void
+pt_solution_add_var (struct pt_solution *dest, tree var)
+{
+  struct pt_solution var_pt;
+  pt_solution_set_var (&var_pt, var);
+  pt_solution_ior_into (dest, &var_pt);
+}
+
 /* Return true if the points-to solution *PT is empty.  */
 
 bool