addressability checks in the gimplifier

[Olivier Hainque <hainque at adacore dot com>]

Hello,

First a short description of a problem we are seeing, then a couple
of related questions on addressability checks in the gimplifier.

>From a simple Ada testcase which I can provide if need be, the front-end
is producing a MODIFY_EXPR with a lhs of the following shape when we get
to gimplify_modify_expr: 

 <array_range_ref 
    type <array_type p5__process__T6b
        size <var_decl D.632

    arg 0 <component_ref
        type <array_type p5__short_message__T4b
       
        arg 0 <var_decl sm type <record_type 
            BLK 
            size <integer_cst constant invariant 40>

        arg 1 <field_decl data type <array_type 
            external packed bit-field nonaddressable SI 
            align 1 offset_align 128
            offset <integer_cst constant invariant 0>
            bit offset <integer_cst constant invariant 1>

    arg 1 <integer_cst 0x401d9048 constant invariant 1>

in short, a variable size array_range_ref within a bitfield record component.

The lhs remains of similar shape after gimplification, the rhs is of
variable size as well, and we end up at this point in gimplify_modify_expr:

  /* If we've got a variable sized assignment between two lvalues (i.e. does
     not involve a call), then we can make things a bit more straightforward
     by converting the assignment to memcpy or memset.  */
  if (TREE_CODE (*from_p) == WITH_SIZE_EXPR)
    {
      tree from = TREE_OPERAND (*from_p, 0);
      tree size = TREE_OPERAND (*from_p, 1);

      if (TREE_CODE (from) == CONSTRUCTOR)
	return gimplify_modify_expr_to_memset (expr_p, size, want_value);
      if (is_gimple_addressable (from))
	{
	  *from_p = from;
	  return gimplify_modify_expr_to_memcpy (expr_p, size, want_value);
	}
    }

We get down into gimplify_modify_expr_to_memcpy, which builds ADDR_EXPRs
for both operands, which ICEs later on from expand_expr_addr_expr_1 because
the operand sketched above is not byte-aligned.

The first puzzle to me is that there is no check made that the target
is a valid argument for an ADDR_EXPR.  AFAICS, it has been gimplified
with is_gimple_lvalue/fb_lvalue as the predicate/fallback pair, but
this currently doesn't imply the required properties.

I first thought that a is_gimple_addressable (*to_p) addition to the
outer condition would help, but it actually does not because the
predicate is shallow and only checks a very restricted set of
conditions (e.g.  any ARRAY_RANGE_REF or COMPONENT_REF is considered
"addressable"). This is actually the reason why the gimplified lhs
tree is considered is_gimple_lvalue, from:

   bool
   is_gimple_lvalue (tree t)
   {
     return (is_gimple_addressable (t)
	     || TREE_CODE (t) == WITH_SIZE_EXPR
	     /* These are complex lvalues, but don't have addresses, so they
		go here.  */
	     || TREE_CODE (t) == BIT_FIELD_REF);

Assuming that the initial tree is valid GENERIC, it would seem that a more
sophisticated addressability checker (recursing down some inner refs and
checking DECL_BIT_FIELD on field decls in COMPONENT_REFs) might be required.

I'm unclear whether this could/should be is_gimple_addressable, as
comments from http://gcc.gnu.org/ml/gcc/2004-07/msg01255.html indicate
that it not designed for this sort of operation.

I'm pretty sure I'm missing implicit assumptions and/or bits of design
intents in various places, so would appreciate input on the case and
puzzles described above.

Thanks very much in advance for your help,

With Kind Regards,

Olivier