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From: "ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/51628] __attribute__((packed)) is unsafe in some cases
Date: Wed, 03 Apr 2013 08:51:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51628

--- Comment #21 from Eric Botcazou <ebotcazou at gcc dot gnu.org> 2013-04-03 08:51:44 UTC ---
> One of the C frontend issues is that the type of the address of
> the field of the packed struct is int *, not int attribute((aligned(1))) 
> *.  And this is so because nothing adjusts the type of the FIELD_DECL
> to be a less aligned type.  That is, we have
> 
>  <field_decl 0x7ffff6d245f0 i
>     type <integer_type 0x7ffff6d175e8 int public SI
>         size <integer_cst 0x7ffff6d1a0c0 constant 32>
>         unit size <integer_cst 0x7ffff6d1a0e0 constant 4>
>         align 32 symtab 0 alias set -1 canonical type 0x7ffff6d175e8 
> precision 32 min <integer_cst 0x7ffff6d1a060 -2147483648> max <integer_cst 
> 0x7ffff6d1a080 2147483647>
>         pointer_to_this <pointer_type 0x7ffff6d1f2a0>>
>     packed SI file t.c line 2 col 9 size <integer_cst 0x7ffff6d1a0c0 32> 
> unit size <integer_cst 0x7ffff6d1a0e0 4>
>     align 8 offset_align 128
>     offset <integer_cst 0x7ffff6d02d80 type <integer_type 0x7ffff6d17000 
> sizetype> constant 0>
>     bit offset <integer_cst 0x7ffff6d02e00 type <integer_type 
> 0x7ffff6d170a8 bitsizetype> constant 0> context <record_type 
> 0x7ffff6e1c3f0 Foo>>

This is on x86, right?  If the alignment of the field cannot be guaranteed to
be that of its type, then it should be made a bit-field.  Maybe it's already
made a bit-field on strict-alignment targets.