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Subject: [Bug middle-end/48124] [4.3/4.4/4.5/4.6/4.7 Regression] likely wrong code bug
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48124

--- Comment #5 from Aldy Hernandez <aldyh at redhat dot com> 2011-03-15 12:42:36 UTC ---
> struct S
> {
>    signed a : 26;
>    signed b : 16;
>    signed c : 10;
>    volatile signed d : 14;
>    int e;
> } s;
> I think you can't just modify s.e when writing s.d (I think it is fine to
> modify
> adjacent bitfields though, Aldy?).

No, you can't modify s.e when writing to s.d.  However, you can modify 
adjacent bitfields.  All contiguous bitfields can be considered a single 
memory location for the purpose of introducing data races.  The only 
exception is when bitfields are separated by a zero-length bitfield, or 
when they happen to be contiguous but occur in different 
structures/unions.  Those conditions force alignments on those fields.