Re: [RFC] Teach vectorizer to deal with bitfield reads

[Richard Biener <rguenther@suse.de>]

On Fri, 29 Jul 2022, Jakub Jelinek wrote:

> On Fri, Jul 29, 2022 at 09:57:29AM +0100, Andre Vieira (lists) via Gcc-patches wrote:
> > The 'only on the vectorized code path' remains the same though as vect_recog
> > also only happens on the vectorized code path right?
> 
> if conversion (in some cases) duplicates a loop and guards one copy with
> an ifn which resolves to true if that particular loop is vectorized and
> false otherwise.  So, then changes that shouldn't be done in case of
> vectorization failure can be done on the for vectorizer only copy of the
> loop.

And just to mention, one issue with lowering of bitfield accesses
is bitfield inserts which, on some architectures (hello m68k) have
instructions operating on memory directly.  For those it's difficult
to not regress in code quality if a bitfield store becomes a
read-modify-write cycle.  That's one of the things holding this
back.  One idea would be to lower to .INSV directly for those targets
(but presence of insv isn't necessarily indicating support for
memory destinations).

Richard.