Re: SEGV in do_simple_structure_copy

[Olivier Hainque <hainque at adacore dot com>]

Daniel Berlin wrote:
> IOW, you are lying to the middle-end about the size of the fields.
> Why is the type not a 6 bit integer?

 Because we avoid creating a different type for every possible
 bitsize, which seems to be the purpose of DECL_SIZE in the
 first place and is explicitly expected by the low-level story-layout
 circuitry:

  layout_decl (tree decl, unsigned int known_align)
  ...
  /* Usually the size and mode come from the data type without change,
     however, the front-end may set the explicit width of the field, so its
     size may not be the same as the size of its type.  This happens with
     bitfields, of course (an `int' bitfield may be only 2 bits, say), but it
     also happens with other fields.

 The C front-end behaves similarily, creating

   <field_decl 0xb7429398 x type <integer_type 0xb7429450>
      external packed bit-field nonaddressable decl_4 QI file tt.c line 3
      size <integer_cst 0xb742fd98 constant invariant 3> 
      unit size <integer_cst 0xb73aa228 1>

   <integer_type 0xb7429450 sizes-gimplified public QI
      size <integer_cst constant invariant 8>
      unit size <integer_cst constant invariant 1>
      align 8 symtab 0 alias set -1
      precision 3 min <integer_cst -4> max <integer_cst 3>>

 for 

    typedef struct  __attribute__ ((packed)) {
    int x:3;
 
> >Another way would be to compute the incoming 'size' argument from decl
> >information when appropriate. This seems more involved at first sight.
> 
> This is the correct fix, however, if you are going to lie to the  
> middle end about TYPE_SIZE so that the TYPE_SIZE and DECL_SIZE do not  
> match.

 Well, I'm actually not sure we have a choice. And I'm not sure it is
 really a lie. A restricted set of values happen to fit in a lower number
 of bits than what some type allows, it is not obvious that it makes those
 values of a different type.

 Thanks for your feedback. 

 Olivier