Re: [PATCH] Improve TBAA with unions

[Richard Biener <rguenther at suse dot de>]

On Wed, 18 May 2016, Richard Biener wrote:

> 
> The following adjusts get_alias_set beahvior when applied to
> union accesses to use the union alias-set rather than alias-set
> zero.  This is in line with behavior from the alias oracle
> which (bogously) circumvents alias-set zero with looking at
> the alias-sets of the base object.  Thus for
> 
> union U { int i; float f; };
> 
> float
> foo (union U *u, double *p)
> {
>   u->f = 1.;
>   *p = 0;
>   return u->f;
> }
> 
> the langhooks ensured u->f has alias-set zero and thus disambiguation
> against *p was not allowed.  Still the alias-oracle did the disambiguation
> by using the alias set of the union here (I think optimizing the
> return to return 1. is valid).
> 
> We have a good place in the middle-end to apply such rules which
> is component_uses_parent_alias_set_from - this is where I move
> the logic that is duplicated in various frontends.
> 
> The Java and Ada frontends do not allow union type punning (LTO does),
> so this patch may eventually pessimize them.  I don't care anything
> about Java but Ada folks might want to chime in.
> 
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
> 
> Ok for trunk?

Ping.

Thanks,
Richard.

> Thanks,
> Richard.
> 
> 2016-05-18  Richard Biener  <rguenther@suse.de>
> 
> 	* alias.c (component_uses_parent_alias_set_from): Handle
> 	type punning through union accesses by using the union alias set.
> 	* gimple.c (gimple_get_alias_set): Remove union type punning case.
> 
> 	c-family/
> 	* c-common.c (c_common_get_alias_set): Remove union type punning case.
> 	
> 	fortran/
> 	* f95-lang.c (LANG_HOOKS_GET_ALIAS_SET): Remove (un-)define.
> 	(gfc_get_alias_set): Remove.
> 
> 	
> Index: trunk/gcc/alias.c
> ===================================================================
> *** trunk.orig/gcc/alias.c	2016-05-18 11:15:41.744792403 +0200
> --- trunk/gcc/alias.c	2016-05-18 11:31:40.139709782 +0200
> *************** component_uses_parent_alias_set_from (co
> *** 619,624 ****
> --- 619,632 ----
>   	case COMPONENT_REF:
>   	  if (DECL_NONADDRESSABLE_P (TREE_OPERAND (t, 1)))
>   	    found = t;
> + 	  /* Permit type-punning when accessing a union, provided the access
> + 	     is directly through the union.  For example, this code does not
> + 	     permit taking the address of a union member and then storing
> + 	     through it.  Even the type-punning allowed here is a GCC
> + 	     extension, albeit a common and useful one; the C standard says
> + 	     that such accesses have implementation-defined behavior.  */
> + 	  else if (TREE_CODE (TREE_TYPE (TREE_OPERAND (t, 0))) == UNION_TYPE)
> + 	    found = t;
>   	  break;
>   
>   	case ARRAY_REF:
> Index: trunk/gcc/c-family/c-common.c
> ===================================================================
> *** trunk.orig/gcc/c-family/c-common.c	2016-05-18 11:15:41.744792403 +0200
> --- trunk/gcc/c-family/c-common.c	2016-05-18 11:31:40.143709828 +0200
> *************** static GTY(()) hash_table<c_type_hasher>
> *** 4734,4741 ****
>   alias_set_type
>   c_common_get_alias_set (tree t)
>   {
> -   tree u;
> - 
>     /* For VLAs, use the alias set of the element type rather than the
>        default of alias set 0 for types compared structurally.  */
>     if (TYPE_P (t) && TYPE_STRUCTURAL_EQUALITY_P (t))
> --- 4734,4739 ----
> *************** c_common_get_alias_set (tree t)
> *** 4745,4763 ****
>         return -1;
>       }
>   
> -   /* Permit type-punning when accessing a union, provided the access
> -      is directly through the union.  For example, this code does not
> -      permit taking the address of a union member and then storing
> -      through it.  Even the type-punning allowed here is a GCC
> -      extension, albeit a common and useful one; the C standard says
> -      that such accesses have implementation-defined behavior.  */
> -   for (u = t;
> -        TREE_CODE (u) == COMPONENT_REF || TREE_CODE (u) == ARRAY_REF;
> -        u = TREE_OPERAND (u, 0))
> -     if (TREE_CODE (u) == COMPONENT_REF
> - 	&& TREE_CODE (TREE_TYPE (TREE_OPERAND (u, 0))) == UNION_TYPE)
> -       return 0;
> - 
>     /* That's all the expressions we handle specially.  */
>     if (!TYPE_P (t))
>       return -1;
> --- 4743,4748 ----
> Index: trunk/gcc/fortran/f95-lang.c
> ===================================================================
> *** trunk.orig/gcc/fortran/f95-lang.c	2016-05-18 11:15:41.744792403 +0200
> --- trunk/gcc/fortran/f95-lang.c	2016-05-18 11:31:48.623806334 +0200
> *************** static bool global_bindings_p (void);
> *** 74,80 ****
>   static bool gfc_init (void);
>   static void gfc_finish (void);
>   static void gfc_be_parse_file (void);
> - static alias_set_type gfc_get_alias_set (tree);
>   static void gfc_init_ts (void);
>   static tree gfc_builtin_function (tree);
>   
> --- 74,79 ----
> *************** static const struct attribute_spec gfc_a
> *** 110,116 ****
>   #undef LANG_HOOKS_MARK_ADDRESSABLE
>   #undef LANG_HOOKS_TYPE_FOR_MODE
>   #undef LANG_HOOKS_TYPE_FOR_SIZE
> - #undef LANG_HOOKS_GET_ALIAS_SET
>   #undef LANG_HOOKS_INIT_TS
>   #undef LANG_HOOKS_OMP_PRIVATIZE_BY_REFERENCE
>   #undef LANG_HOOKS_OMP_PREDETERMINED_SHARING
> --- 109,114 ----
> *************** static const struct attribute_spec gfc_a
> *** 142,148 ****
>   #define LANG_HOOKS_PARSE_FILE           gfc_be_parse_file
>   #define LANG_HOOKS_TYPE_FOR_MODE	gfc_type_for_mode
>   #define LANG_HOOKS_TYPE_FOR_SIZE	gfc_type_for_size
> - #define LANG_HOOKS_GET_ALIAS_SET	gfc_get_alias_set
>   #define LANG_HOOKS_INIT_TS		gfc_init_ts
>   #define LANG_HOOKS_OMP_PRIVATIZE_BY_REFERENCE	gfc_omp_privatize_by_reference
>   #define LANG_HOOKS_OMP_PREDETERMINED_SHARING	gfc_omp_predetermined_sharing
> --- 140,145 ----
> *************** gfc_init_decl_processing (void)
> *** 503,526 ****
>   }
>   
>   
> - /* Return the typed-based alias set for T, which may be an expression
> -    or a type.  Return -1 if we don't do anything special.  */
> - 
> - static alias_set_type
> - gfc_get_alias_set (tree t)
> - {
> -   tree u;
> - 
> -   /* Permit type-punning when accessing an EQUIVALENCEd variable or
> -      mixed type entry master's return value.  */
> -   for (u = t; handled_component_p (u); u = TREE_OPERAND (u, 0))
> -     if (TREE_CODE (u) == COMPONENT_REF
> - 	&& TREE_CODE (TREE_TYPE (TREE_OPERAND (u, 0))) == UNION_TYPE)
> -       return 0;
> - 
> -   return -1;
> - }
> - 
>   /* Builtin function initialization.  */
>   
>   static tree
> --- 500,505 ----
> Index: trunk/gcc/gimple.c
> ===================================================================
> *** trunk.orig/gcc/gimple.c	2016-05-18 11:15:41.744792403 +0200
> --- trunk/gcc/gimple.c	2016-05-18 11:31:40.143709828 +0200
> *************** gimple_signed_type (tree type)
> *** 2396,2416 ****
>   alias_set_type
>   gimple_get_alias_set (tree t)
>   {
> -   tree u;
> - 
> -   /* Permit type-punning when accessing a union, provided the access
> -      is directly through the union.  For example, this code does not
> -      permit taking the address of a union member and then storing
> -      through it.  Even the type-punning allowed here is a GCC
> -      extension, albeit a common and useful one; the C standard says
> -      that such accesses have implementation-defined behavior.  */
> -   for (u = t;
> -        TREE_CODE (u) == COMPONENT_REF || TREE_CODE (u) == ARRAY_REF;
> -        u = TREE_OPERAND (u, 0))
> -     if (TREE_CODE (u) == COMPONENT_REF
> - 	&& TREE_CODE (TREE_TYPE (TREE_OPERAND (u, 0))) == UNION_TYPE)
> -       return 0;
> - 
>     /* That's all the expressions we handle specially.  */
>     if (!TYPE_P (t))
>       return -1;
> --- 2396,2401 ----
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)