Re: [C++0x] contiguous bitfields race implementation

[Aldy Hernandez <aldyh at redhat dot com>]

On 05/18/11 16:58, Jason Merrill wrote:
> It seems like you're calculating maxbits correctly now, but an access
> doesn't necessarily start from the beginning of the sequence of
> bit-fields, especially given store_split_bit_field. That is,

This is what I was trying to explain to you on irc.  And I obviously 
muffed up the whole explanation :).

> struct A
> {
> int i;
> int j: 32;
> int k: 8;
> char c[2];
> };
>
> Here maxbits would be 40, so we decide that it's OK to use SImode to
> access the word starting with k, and clobber c in the process. Am I wrong?

You are correct.  I have redesigned the patch to pass around starting 
and ending bit positions, so get_best_mode() can make a more informed 
decision.

I also started using DECL_BIT_FIELD_TYPE instead of DECL_BIT_FIELD to 
determine if a DECL is a bit field.  It turns out DECL_BIT_FIELD is not 
set for bit fields with mode sized number of bits (32-bits, 16-bits, etc).

Furthermore, I added another test to check the above scenario.

Bootstrapped and tested on x86-64 with --param=allow-store-data-races=0.

How do you like these apples?

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