This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: Continue strict-volatile-bitfields fixes
On Fri, Apr 27, 2012 at 10:29 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Fri, Apr 27, 2012 at 12:42:41PM +0800, Thomas Schwinge wrote:
>> > > GET_MODE_BITSIZE (lmode)« (8 bits). ?(With the current sources, lmode is
>> > > VOIDmode.)
>> > >
>> > > Is emmitting »BIT_FIELD_REF <*common, 32, 0> & 255« wrong in this case,
>> > > or should a later optimization pass be able to figure out that
>> > > »BIT_FIELD_REF <*common, 32, 0> & 255« is in fact the same as
>> > > common->code, and then be able to conflate these? ?Any suggestions
>> > > where/how to tackle this?
>> >
>> > The BIT_FIELD_REF is somewhat of a red-herring. ?It is created by fold-const.c
>> > in optimize_bit_field_compare, code that I think should be removed completely.
>> > Or it needs to be made aware of strict-volatile bitfield and C++ memory model
>> > details.
>
> I'd actually very much prefer the latter, just disable
> optimize_bit_field_compare for strict-volatile bitfield mode and when
> avoiding load data races in C++ memory model (that isn't going to be
> default, right?). ?This optimization is useful, and it is solely about
> loads, so even C++ memory model usually shouldn't care.
But won't it re-introduce bugs like PR52080, 52097 or 48124? Also the
proper place for this optimization is lowering and CSEing of bit-field loads
(not that this lowering already exists).
Well, I'm obviously biased here - fold doing this kind of transform looks
out-of-place.
Richard.
> ? ? ? ?Jakub