Re: [patch][version 6] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

[Richard Biener <rguenther@suse.de>]

On Mon, 16 Aug 2021, Qing Zhao wrote:

> My current code for expand_DEFERRED_INIT is like the following, could you check and see whether there is any issue for it:
> 
> #define INIT_PATTERN_VALUE  0xFE
> static void
> expand_DEFERRED_INIT (internal_fn, gcall *stmt)
> {
>   tree lhs = gimple_call_lhs (stmt);
>   tree var_size = gimple_call_arg (stmt, 0);
>   enum auto_init_type init_type
>     = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
>   bool is_vla = (bool) TREE_INT_CST_LOW (gimple_call_arg (stmt, 2));
> 
>   tree var_type = TREE_TYPE (lhs);
>   gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
> 
>   if (is_vla || (!use_register_for_decl (lhs)))
>     {
>       if (TREE_CODE (lhs) == SSA_NAME)
>         lhs = SSA_NAME_VAR (lhs);

this should not be necessary (in fact you shouldn't see a SSA_NAME
here, if you do then using SSA_NAME_VAR is wrong)

>     /* If this is a VLA or the variable is not in register,
>        expand to a memset to initialize it.  */
>       tree var_addr = NULL_TREE;
>       if (is_vla)
>         var_addr = TREE_OPERAND (lhs, 0);
>       else
>         {
>           TREE_ADDRESSABLE (lhs) = 1;
>           var_addr = build_fold_addr_expr (lhs);
>         }

use, independent of is_vla

         mark_addressable (lhs);
         var_addr = build_fold_addr_expr (lhs);

> 
>       tree value = (init_type == AUTO_INIT_PATTERN) ?
>                     build_int_cst (unsigned_char_type_node,
>                                    INIT_PATTERN_VALUE) :
>                     build_zero_cst (unsigned_char_type_node);

since memset has an integer argument for the value use
integer_zero_node for the zero case and build_int_cst (integer_type_node, 
...) for the pattern case

>       tree m_call = build_call_expr (builtin_decl_implicit (BUILT_IN_MEMSET),
>                                      3, var_addr, value, var_size);
>       /* Expand this memset call.  */
>       expand_builtin_memset (m_call, NULL_RTX, TYPE_MODE (var_type));
>     }
>   else
>     {
>     /* If this variable is in a register, use expand_assignment might
>        generate better code.  */
>       tree pattern = NULL_TREE;
>       unsigned HOST_WIDE_INT total_bytes
>         = tree_to_uhwi (TYPE_SIZE_UNIT (var_type));
> 
>       if (init_type == AUTO_INIT_PATTERN)
>         {
>           if (can_native_interpret_type_p (var_type))
>             {
>               unsigned char *buf = (unsigned char *) xmalloc (total_bytes);
>               memset (buf, INIT_PATTERN_VALUE, total_bytes);
>               pattern = native_interpret_expr (var_type, buf, total_bytes);
>               gcc_assert (pattern);
>             }
>           else
>             {
>               tree index_type = build_index_type (size_int (total_bytes - 1));
>               tree array_type = build_array_type (unsigned_char_type_node,
>                                                   index_type);
>               tree element = build_int_cst (unsigned_char_type_node,
>                                             INIT_PATTERN_VALUE);
>               vec<constructor_elt, va_gc> *elts = NULL;
>               for (unsigned int i = 0; i < total_bytes; i++)
>                 CONSTRUCTOR_APPEND_ELT (elts, NULL_TREE, element);
>               pattern = build_constructor (array_type, elts);
>               pattern = build1 (VIEW_CONVERT_EXPR, var_type, pattern);
>             }
>         }
> 
>       tree init = (init_type == AUTO_INIT_PATTERN) ?
>                    pattern :
>                    build_zero_cst (var_type);

maybe conditionally initialize init instead of pattern and init?
Thus replace pattern by init and do

        else
          init = build_zero_cst (var_type);


the above should work, as said the RTL expansion part can possibly
be improved but we can do this as followup as well.

>       expand_assignment (lhs, init, false);
>     }
> }
> 
> Thanks.
> 
> Qing
> 
> 
> 
> 

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