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

Qing Zhao qing.zhao@oracle.com
Thu Aug 12 19:24:18 GMT 2021

Hi, Richard,

For RTL expansion of call to .DEFERRED_INIT, I changed my code per your suggestions like following:

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 || (!can_native_interpret_type_p (var_type)))
    /* If this is a VLA or the type of the variable cannot be natively
       interpreted, expand to a memset to initialize it.  */
      if (TREE_CODE (lhs) == SSA_NAME)
        lhs = SSA_NAME_VAR (lhs);
      tree var_addr = NULL_TREE;
      if (is_vla)
        var_addr = TREE_OPERAND (lhs, 0);
         TREE_ADDRESSABLE (lhs) = 1;
         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);
      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));
    /* If this is not a VLA and the type of the variable can be natively 
       interpreted, expand to assignment to 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)
          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);

      tree init = (init_type == AUTO_INIT_PATTERN) ?
                   pattern :
                   build_zero_cst (var_type);
      expand_assignment (lhs, init, false);

Now, I used “can_native_interpret_type_p (var_type)” instead of “use_register_for_decl (lhs)” to decide 
whether to use “memset” or use “assign” to expand this function.

However, this exposed an bug that is very hard to be addressed:

*******For the testing case: test suite/gcc.dg/uninit-I.c:

/* { dg-do compile } */
/* { dg-options "-O2 -Wuninitialized" } */

int sys_msgctl (void)
  struct { int mode; } setbuf;
  return setbuf.mode;  /* { dg-warning "'setbuf\.mode' is used" } */

******the above auto var “setbuf” has “struct” type, which “can_native_interpret_type_p(var_type)” is false, therefore, 
Expanding this .DEFERRED_INIT call went down the “memset” expansion route. 

However, this structure type can be fitted into a register, therefore cannot be taken address anymore at this stage, even though I tried:

         TREE_ADDRESSABLE (lhs) = 1;
         var_addr = build_fold_addr_expr (lhs);

To create an address variable for it, the expansion still failed at expr.c: line 8412:
during RTL pass: expand
/home/opc/Work/GCC/latest-gcc/gcc/testsuite/gcc.dg/auto-init-uninit-I.c:6:24: internal compiler error: in expand_expr_addr_expr_1, at expr.c:8412
0xd04104 expand_expr_addr_expr_1
0xd04a95 expand_expr_addr_expr
0xd13592 expand_expr_real_1(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
0xd05142 expand_expr_real(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
0xaed1d3 expand_expr
0xaf0d89 get_memory_rtx
0xafb4fb expand_builtin_memset_args
0xafacde expand_builtin_memset(tree_node*, rtx_def*, machine_mode)
0xe97fb3 expand_DEFERRED_INIT

******That’s the major reason why I chose “use_register_for_decl(lhs)” to decide “memset” expansion or “assign” expansion, “memset” expansion
needs to take address of the variable, if the variable has been decided to fit into a register, then its address cannot taken anymore at this stage.

******using “can_native_interpret_type_p” did make the “pattern” generation part much  cleaner and simpler, however, looks like it didn’t work correctly.

Based on this, I’d like to keep my previous implementation by using “use_register_for_decl” to decide whether to take “memset” expansion or “assign” expansion.
Therefore, I might still need to keep the “UGLY”  implementation of generatting “pattern” constant for different types?

Let me know your opinion on this.

Thanks a lot for the help.


> On Aug 9, 2021, at 9:09 AM, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 27 Jul 2021, Qing Zhao wrote:
> +        created during gimplification phase.  Refer to gimplify_vla_decl
> +        for details.  */
> +      tree var_decl = (TREE_CODE (var) == SSA_NAME) ?
> +                      SSA_NAME_VAR (var) : var;
> +      gcc_assert (DECL_HAS_VALUE_EXPR_P (var_decl));
> +      gcc_assert (TREE_CODE (DECL_VALUE_EXPR (var_decl)) == 
> +      /* Get the address of this vla variable.  */
> +      vlaaddr = TREE_OPERAND (DECL_VALUE_EXPR (var_decl), 0);
> err - isn't the address of the decl represented by the LHS 
> regardless whether this is a VLA or not?  Looking at DECL_VALUE_EXPR
> looks quite fragile since that's not sth data dependence honors.
> It looks you only partly gimplify the build init here?  All
> DECL_VALUE_EXPRs should have been resolved.
> +  if (is_vla || (!use_register_for_decl (var)))
> ...
> +  else
> +    {
> +    /* If this variable is in a register, use expand_assignment might
> +       generate better code.  */
> you compute the patter initializer even when not needing it,
> that's wasteful.  It's also quite ugly, IMHO you should
> use can_native_interpret_type_p (var_type) and native_interpret
> a char [] array initialized to the pattern and if
> !can_native_interpret_type_p () go the memset route.

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