[C++0x] contiguous bitfields race implementation

[Aldy Hernandez]

Seeing that the current C++ draft has been approved, I'd like to submit 
this for mainline, and get the proper review everyone's being quietly 
avoiding :).

To refresh everyone's memory, here is the problem:

struct
{
     unsigned int a : 4;
     unsigned char b;
     unsigned int c: 6;
} var;


void seta(){
       var.a = 12;
}


In the new C++ standard, stores into <a> cannot touch <b>, so we can't 
store with anything wider (e.g. a 32 bit store) that will touch <b>. 
This problem can be seen on strictly aligned targets such as ARM, where 
we store the above sequence with a 32-bit store. Or on x86-64 with <a> 
being volatile (PR48124).

This patch fixes both problems, but only for the C++ memory model. This 
is NOT a generic fix PR48124, only a fix when using "--param 
allow-store-data-races=0".  I will gladly change the parameter name, if 
another is preferred.

The gist of this patch is in max_field_size(), where we calculate the 
maximum number of bits we can store into. In doing this calculation I 
assume we can store into the padding without causing any races. So, 
padding between fields and at the end of the structure are included.

Tested on x86-64 both with and without "--param 
allow-store-data-races=0", and visually inspecting the assembly on 
arm-linux and ia64-linux.

OK for trunk?
Aldy
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