c++: Reject __builtin_clear_padding on non-trivially-copyable types with one exception [PR102586]

  Hi!

As mentioned by Jason in the PR, non-trivially-copyable types (or non-POD
for purposes of layout?) types can be base classes of derived classes in
which the padding in those non-trivially-copyable types can be redused for
some real data members or even the layout can change and data members can
be moved to other positions.
__builtin_clear_padding is right now used for multiple purposes,
in <atomic> where it isn't used yet but was planned as the main spot
it can be used for trivially copyable types only, ditto for std::bit_cast
where we also use it.  It is used for OpenMP long double atomics too but
long double is trivially copyable, and lastly for -ftrivial-auto-var-init=.

The following patch restricts the builtin to pointers to trivially-copyable
types, with the exception when it is called directly on an address of a
variable, in that case already the FE can verify it is the complete object
type and so it is safe to clear all the paddings in it.

Bootstrapped/regtested on powerpc64le-linux, ok for trunk?

Something like the https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c16
will still be needed with adjusted testcase from
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15 such that
__builtin_clear_padding is called directly on var addresses rather than
in separate functions.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102586
gcc/
	* doc/extend.texi (__builtin_clear_padding): Clearify that for C++
	argument type should be pointer to trivially-copyable type unless it
	is address of a variable or parameter.
gcc/cp/
	* call.cc (build_cxx_call): Diagnose __builtin_clear_padding where
	first argument's type is pointer to non-trivially-copyable type unless
	it is address of a variable or parameter.
gcc/testsuite/
	* g++.dg/cpp2a/builtin-clear-padding1.C: New test.


	Jakub
  

On 2/11/22 14:55, Jakub Jelinek wrote:
> Hi!
> 
> As mentioned by Jason in the PR, non-trivially-copyable types (or non-POD
> for purposes of layout?) types can be base classes of derived classes in
> which the padding in those non-trivially-copyable types can be redused for
> some real data members or even the layout can change and data members can
> be moved to other positions.
> __builtin_clear_padding is right now used for multiple purposes,
> in <atomic> where it isn't used yet but was planned as the main spot
> it can be used for trivially copyable types only, ditto for std::bit_cast
> where we also use it.  It is used for OpenMP long double atomics too but
> long double is trivially copyable, and lastly for -ftrivial-auto-var-init=.
> 
> The following patch restricts the builtin to pointers to trivially-copyable
> types, with the exception when it is called directly on an address of a
> variable, in that case already the FE can verify it is the complete object
> type and so it is safe to clear all the paddings in it.
> 
> Bootstrapped/regtested on powerpc64le-linux, ok for trunk?

OK.

> Something like the https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c16
> will still be needed with adjusted testcase from
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102586#c15 such that
> __builtin_clear_padding is called directly on var addresses rather than
> in separate functions.
> 
> 2022-02-11  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/102586
> gcc/
> 	* doc/extend.texi (__builtin_clear_padding): Clearify that for C++
> 	argument type should be pointer to trivially-copyable type unless it
> 	is address of a variable or parameter.
> gcc/cp/
> 	* call.cc (build_cxx_call): Diagnose __builtin_clear_padding where
> 	first argument's type is pointer to non-trivially-copyable type unless
> 	it is address of a variable or parameter.
> gcc/testsuite/
> 	* g++.dg/cpp2a/builtin-clear-padding1.C: New test.
> 
> --- gcc/doc/extend.texi.jj	2022-02-09 15:16:03.336783697 +0100
> +++ gcc/doc/extend.texi	2022-02-11 13:22:39.846157538 +0100
> @@ -13993,6 +13993,11 @@ bits that are padding bits for all the u
>   This built-in-function is useful if the padding bits of an object might
>   have intederminate values and the object representation needs to be
>   bitwise compared to some other object, for example for atomic operations.
> +
> +For C++, @var{ptr} argument type should be pointer to trivially-copyable
> +type, unless the argument is address of a variable or parameter, because
> +otherwise it isn't known if the type isn't just a base class whose padding
> +bits are reused or laid out differently in a derived class.
>   @end deftypefn
>   
>   @deftypefn {Built-in Function} @var{type} __builtin_bit_cast (@var{type}, @var{arg})
> --- gcc/cp/call.cc.jj	2022-02-09 20:13:51.523305107 +0100
> +++ gcc/cp/call.cc	2022-02-11 12:58:19.168301395 +0100
> @@ -10398,6 +10398,27 @@ build_cxx_call (tree fn, int nargs, tree
>         if (!check_builtin_function_arguments (EXPR_LOCATION (fn), vNULL, fndecl,
>   					     orig_fndecl, nargs, argarray))
>   	return error_mark_node;
> +      else if (fndecl_built_in_p (fndecl, BUILT_IN_CLEAR_PADDING))
> +	{
> +	  tree arg0 = argarray[0];
> +	  STRIP_NOPS (arg0);
> +	  if (TREE_CODE (arg0) == ADDR_EXPR
> +	      && DECL_P (TREE_OPERAND (arg0, 0))
> +	      && same_type_ignoring_top_level_qualifiers_p
> +			(TREE_TYPE (TREE_TYPE (argarray[0])),
> +			 TREE_TYPE (TREE_TYPE (arg0))))
> +	    /* For __builtin_clear_padding (&var) we know the type
> +	       is for a complete object, so there is no risk in clearing
> +	       padding that is reused in some derived class member.  */;
> +	  else if (!trivially_copyable_p (TREE_TYPE (TREE_TYPE (argarray[0]))))
> +	    {
> +	      error_at (EXPR_LOC_OR_LOC (argarray[0], input_location),
> +			"argument %u in call to function %qE "
> +			"has pointer to a non-trivially-copyable type (%qT)",
> +			1, fndecl, TREE_TYPE (argarray[0]));
> +	      return error_mark_node;
> +	    }
> +	}
>       }
>   
>     if (VOID_TYPE_P (TREE_TYPE (fn)))
> --- gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C.jj	2022-02-11 13:13:49.125471991 +0100
> +++ gcc/testsuite/g++.dg/cpp2a/builtin-clear-padding1.C	2022-02-11 13:13:43.403550851 +0100
> @@ -0,0 +1,50 @@
> +// PR tree-optimization/102586
> +// { dg-do compile }
> +// { dg-options "-Wno-inaccessible-base" }
> +
> +struct C0 {};
> +struct C1 {};
> +struct C2 : C1, virtual C0 {};
> +struct C3 : virtual C2, C1 {};
> +struct C4 : virtual C3, C1 {};
> +struct C5 : C4 {};
> +struct C6 { char c; };
> +struct C7 : virtual C6, virtual C3, C1 {};
> +struct C8 : C7 {};
> +
> +void
> +foo (C0 *c0, C1 *c1, C2 *c2, C3 *c3, C4 *c4, C5 *c5, C6 *c6, C7 *c7, C8 *c8)
> +{
> +  __builtin_clear_padding (c0);
> +  __builtin_clear_padding (c1);
> +  __builtin_clear_padding (c2);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C2\\\*'\\\)" }
> +  __builtin_clear_padding (c3);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C3\\\*'\\\)" }
> +  __builtin_clear_padding (c4);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C4\\\*'\\\)" }
> +  __builtin_clear_padding (c5);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C5\\\*'\\\)" }
> +  __builtin_clear_padding (c6);
> +  __builtin_clear_padding (c7);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C7\\\*'\\\)" }
> +  __builtin_clear_padding (c8);	// { dg-error "argument 1 in call to function '__builtin_clear_padding' has pointer to a non-trivially-copyable type \\\('C8\\\*'\\\)" }
> +}
> +
> +void
> +bar ()
> +{
> +  C0 c0;
> +  C1 c1;
> +  C2 c2;
> +  C3 c3;
> +  C4 c4;
> +  C5 c5;
> +  C6 c6;
> +  C7 c7;
> +  C8 c8;
> +  __builtin_clear_padding (&c0);
> +  __builtin_clear_padding (&c1);
> +  __builtin_clear_padding (&c2);
> +  __builtin_clear_padding (&c3);
> +  __builtin_clear_padding (&c4);
> +//  __builtin_clear_padding (&c5);
> +  __builtin_clear_padding (&c6);
> +  __builtin_clear_padding (&c7);
> +  __builtin_clear_padding (&c8);
> +}
> 
> 	Jakub
>