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

[Richard Biener <rguenther@suse.de>]

On Mon, 9 Aug 2021, Qing Zhao wrote:

> Hi, Richard,
> 
> Thanks a lot for you review.
> 
> Although these comments are not made on the latest patch (7th version) :-), all the comments are valid since the parts you commented
> are not changed in the 7th version.
> 
> 
> > On Aug 9, 2021, at 9:09 AM, Richard Biener <rguenther@suse.de> wrote:
> > 
> > On Tue, 27 Jul 2021, Qing Zhao wrote:
> > 
> >> Hi,
> >> 
> >> This is the 6th version of the patch for the new security feature for GCC.
> >> 
> >> I have tested it with bootstrap on both x86 and aarch64, regression testing on both x86 and aarch64.
> >> Also compile CPU2017 (running is ongoing), without any issue. (With the fix to bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101586).
> >> 
> >> Please take a look and let me know any issue.
> > 
> > +/* Handle an "uninitialized" attribute; arguments as in
> > +   struct attribute_spec.handler.  */
> > +
> > +static tree
> > +handle_uninitialized_attribute (tree *node, tree name, tree ARG_UNUSED 
> > (args),
> > +                               int ARG_UNUSED (flags), bool 
> > *no_add_attrs)
> > +{
> > +  if (!VAR_P (*node))
> > +    {
> > +      warning (OPT_Wattributes, "%qE attribute ignored", name);
> > +      *no_add_attrs = true;
> > +    }
> > 
> > you are documenting this attribute for automatic variables but
> > here you allow placement on globals as well (not sure if at this
> > point TREE_STATIC / DECL_EXTERNAL are set correctly).
> 
> Right, I should warn when the attribute is placed for globals or static variables. 
> I will try TREE_STATIC/DECL_EXTERNAL to see whether it’s work or not.
> 
> > 
> > +  /* for languages that do not support BUILT_IN_CLEAR_PADDING, create the
> > +     function node for padding initialization.  */
> > +  if (!fn)
> > +    {
> > +      tree ftype = build_function_type_list (void_type_node,
> > +                                            ptr_type_node,
> > 
> > the "appropriate" place to do this would be 
> > tree.c:build_common_builtin_nodes
> 
> Sure, will move the creation of  function node of BUILT_IN_CLEAR_PADDING for Fortran etc. to tree.c:build_common_builtin_nodes.
> 
> > 
> > You seem to marshall the is_vla argument as for_auto_init when
> > expanding/folding the builtin and there it's used to suppress
> > diagnostics (and make covered pieces not initialized?).
> 
> Yes, I added an extra argument “for_auto_init” for “BUILT_IN_CLEAR_PADDING”, this argument is added to suppress errors emitted during folding
> BUILT_IN_CLEAR_PADDING for flexible array member . Such errors should Not be emitted when “BUILT_IN_CLEAR_PADDING” is called with compiler automatic initialization.
> Please see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101586, comment #6 from Jakub Jelinek.
> 
> >  I suggest
> > to re-name is_vla/for_auto_init to something more descriptive.
> 
> Okay, I will. 
> > 
> > +   gimple_fold_builtin_clear_padding. If FOR_AUTO_INIT,
> > +   not emit some of the error messages since doing that
> > +   might confuse the end user.  */
> > 
> > doesn't explain to me whether errors still might be raised or
> > what the actual behavior is.
> 
> Okay, will make this more clear in the comments.
> 
> > 
> > +static gimple *
> > +build_deferred_init (tree decl,
> > +                    enum auto_init_type init_type,
> > +                    bool is_vla)
> > +{
> > +  gcc_assert ((is_vla && TREE_CODE (decl) == WITH_SIZE_EXPR)
> > +             || (!is_vla && TREE_CODE (decl) != WITH_SIZE_EXPR));
> > 
> > so the is_vla parameter looks redundant (and the assert dangerous?).
> > Either the caller knows it deals with a VLA, then that should be
> > passed through - constant sizes can also later appear during
> > optimization after all - or is_vla should be determined here
> > based on whether the size at gimplification time is constant.
> 
> The routine “build_deferred_init” is ONLY called during gimplification phase by the routine “gimple_add_init_for_auto_var", at this place,
> Is_vla should be determined by the caller to check the size of the DECL. If it’s a vla, the “maybe_with_size_expr” will be applied for
> DECL to make it to a WITH_SIZE_EXPR.  So, the assertion is purely to make sure this at gimplification phase.
> 
> Yes, the size of the VLA decl might become a constant later due to constant propagation, etc.  but during the gimplification phase, the assertion should be true.
> > 
> > +         /* If the user requests to initialize automatic variables, we
> > +            should initialize paddings inside the variable. Add a call to
> > +            __BUILTIN_CLEAR_PADDING (&object, 0, for_auto_init = true) to
> > +            initialize paddings of object always to zero regardless of
> > +            INIT_TYPE.  */
> > +         if (opt_for_fn (current_function_decl, flag_auto_var_init)
> > +               > AUTO_INIT_UNINITIALIZED
> > +             && VAR_P (object)
> > +             && !DECL_EXTERNAL (object)
> > +             && !TREE_STATIC (object))
> > +           gimple_add_padding_init_for_auto_var (object, false, pre_p);
> > +         return ret;
> > 
> > I think you want to use either auto_var_p (object) or
> > auto_var_in_fn_p (object, current_function_decl).  Don't you also
> > want to check for the 'uninitialized' attribute here?  I suggest
> > to abstract the check on whether 'object' should be subject
> > to autoinit to a helper function.
> 
> Thanks for the suggestion, I will do this.
> 
> 
> > 
> > There's another path above this calling gimplify_init_constructor
> > for the case of
> > 
> > const struct S x = { ... };
> > struct S y = x;
> > 
> > where it will try to init 'y' from the CTOR directly, it seems you
> > do not cover this case.
> 
> Yes, you are right, this case was not covered right now, and this should be covered.
> 
> Looks like that I need to move the “gimple_add_padding_init_for_auto_var” inside the routine “gimplify_init_constructor” to
> Cover all the cases. 
> 
> >  I also think that the above place applies
> > to all aggregate assignment statements, not only to INIT_EXPRs?
> 
> > So don't you want to restrict clear-padding emit here?
> 
> You are right, I might need to restrict it Only to INIT_EXPR. 
> Will update.
> 
> > 
> > +static void
> > +expand_DEFERRED_INIT (internal_fn, gcall *stmt)
> > +{
> > +  tree var = gimple_call_lhs (stmt);
> > +  tree size_of_var = gimple_call_arg (stmt, 0);
> > +  tree vlaaddr = NULL_TREE;
> > +  tree var_type = TREE_TYPE (var);
> > +  bool is_vla = (bool) TREE_INT_CST_LOW (gimple_call_arg (stmt, 2));
> > +  enum auto_init_type init_type
> > +    = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
> > +
> > +  gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
> > +
> > +  /* if this variable is a VLA, get its SIZE and ADDR first.  */
> > +  if (is_vla)
> > +    {
> > +      /* The temporary address variable for this vla should have been
> > +        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)) == 
> > INDIRECT_REF);
> > +      /* 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?
> 
> The LHS of the call to .DEFERRED_INIT is the DECL itself whatever it’s a VLA or not. 
> 
> In order to create a memset call, we need the Address of this DECL as the first argument. 
> If the DECL is not a VLA, we just simply apply “build_fold_addr_expr” on this DECL to get its address,
> However, for VLA, during gimplification phase “gimplify_vla_decl”, we have already created a temporary
> address variable for this DECL, and recorded this address variable with “DECL_VALUE_EXPR(DECL), 
> We should use this already created address variable  for VLAs. 

So the issue is that the LHS of the .DEFERRED_INIT call is not properly
gimplified.  We should not have such decl there but I see we do not
have IL verification that covers this.

The gimplifier usually does this in gimplify_var_or_parm_decl,
but you can of course substitute DECL_VALUE_EXPR yourself if the
decl was already gimplified (was it?)

> 
> >  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.
> 
> Don’t quite understand here. you mean that all the “DECL_VALUE_EXPRs” have been resolved at the phase RTL expansion,
> So I cannot use this to get the address variable of the VLA?
> 
> (However, my unit testing cases for VLAs are all looks fine).
> 
> > 
> > +  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.
> 
> Okay, I will restrict the pattern initializer computation when really needed. 
> 
> >  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.
> 
> Thanks for the suggestion. 
> 
> Will try this. 
> 
> > 
> > +  /* We will not verify the arguments for the calls to .DEFERRED_INIT.
> > +     Such call is not a real call, just a placeholder for a later
> > +     initialization during expand phase.
> > +     This is mainly to avoid assertion failure for the following
> > +     case:
> > +
> > +     uni_var = .DEFERRED_INIT (var_size, INIT_TYPE, is_vla);
> > +     foo (&uni_var);
> > +
> > +     in the above, the uninitialized auto variable "uni_var" is
> > +     addressable, therefore should not be in registers, resulting
> > +     the assertion failure in the following argument verification.  */
> > +  if (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
> > +    return false;
> > +
> >   /* ???  The C frontend passes unpromoted arguments in case it
> >      didn't see a function declaration before the call.  So for now
> >      leave the call arguments mostly unverified.  Once we gimplify
> >      unit-at-a-time we have a chance to fix this.  */
> > 
> > -  for (i = 0; i < gimple_call_num_args (stmt); ++i)
> > 
> > isn't that from the time there was a decl argument to .DEFERRED_INIT?
> 
> You mean this issue is only there when the decl is the first argument (the old design for .DEFERRED_INIT).
> With the new design, this issue is not there anymore?

I think so, yes - the change should no longer be needed.

Ricahrd.

> > 
> > +  if (gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
> > +    {
> > +      tree size_of_arg0 = gimple_call_arg (stmt, 0);
> > +      tree size_of_lhs = TYPE_SIZE_UNIT (TREE_TYPE (lhs));
> > +      tree is_vla_node = gimple_call_arg (stmt, 2);
> > +      bool is_vla = (bool) TREE_INT_CST_LOW (is_vla_node);
> > +
> > +      if (TREE_CODE (lhs) == SSA_NAME)
> > +       lhs = SSA_NAME_VAR (lhs);
> > +
> > 
> > 'lhs' is not looked at after this, no need to look at SSA_NAME_VAR.
> 
> Okay, will update this.
> 
> > 
> > 
> > Thanks and sorry for the delay in reviewing this (again).
> 
> Thanks again for your detailed review and suggestions.
> 
> I will update the patch accordingly and send the updated patch soon.
> 
> Qing
> > 
> > Richard.
> > 
> > 
> >> Thanks
> >> 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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