[PATCH] fold-const: Don't access bit fields with too big mode (PR71310)

[Jeff Law]

On 06/10/2016 12:48 PM, Segher Boessenkool wrote:
> Currently, optimize_bit_field_compare reads the bitfield in word_mode
> if it can.  If the bit field is normally accessed in a smaller mode,
> this might be a violation of the memory model, although the "extra"
> part of the read is not used.  But also, previous stores to the bit
> field will have been done in the smaller mode, and then bigger loads
> from it cause a LHS problem.
>
> Bootstrapped and regchecked on powerpc64-linux.  Is this okay for
> trunk?
>
>
> Segher
>
>
> 2016-06-10  Segher Boessenkool  <segher@kernel.crashing.org>
>
> 	PR middle-end/71310
> 	* fold-const.c (optimize_bit_field_compare): Don't try to use
> 	word_mode unconditionally for reading the bit field, look at
> 	DECL_BIT_FIELD_REPRESENTATIVE instead.
Testcase?  It would seem to me that you could make a ppc testcase 
without major difficulties from either of the tests in the BZ.

The change itself is fine, and it's approved with a testcase or at least 
an explanation of why you can't turn either of the tests from the BZ 
into a testcase in our framework.

I do note there's two more calls to get_best_mode that unconditionally 
uses word_mode in fold_truth_andor_1.  I think in that case we're 
looking to try and loads of adjacent fields that are being compared.  I 
haven't thought much about whether or not it's safe WRT the C++11 memory 
model.


Jeff