Re: [PATCH] A pass which merges constant stores to bitfields

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Nov 13, 2012 at 1:50 AM, Andrew Pinski
<andrew.pinski@caviumnetworks.com> wrote:
> Hi,
>   I know we are in stage3, I thought I would send this out now for
> review as I finally have it ready for consumption as I finally got
> around to removing the limitation of it only working on big-endian.
> This pass was originally written by Adam Nemet while he was at Cavium.
>  I modified it to work on little-endian and also update the code to
> use the aliasing oracle and some of the new VEC interface.
>
> Yes I know I forgot to add documentation for the new option and for
> the new pass.  I will add it soon.

Note that I think the patch doesn't try to honor the C++ memory model
nor arms strict volatile bitfields.

My plan was to get similar results as your patch by lowering bitfield
loads and stores using DECL_BIT_FIELD_REPRESENTATIVE and
then let DCE/DSE and tree combine merge things.

That is, you can replace a bit-field a.b.c.d with
BIT_FIELD_REF <a.b.c.DECL_BIT_FIELD_REPRESENTATIVE, ...>
and perform stores via read-modify-write.  The DECL_BIT_FIELD_REPRESENTATIVE
loads can be CSEd leaving a combining opportunity, stores will end up
being redundant.

The advantage is that using DECL_BIT_FIELD_REPRESENTATIVE will
make you honor the memory model issues automatically - you basically
perform what expand would do.

Instead of generating the component-ref with the representative you can of
course also lower the whole thing to a MEM_REF (if the ref does not
contain variable indexes).

Now, you have a pass that might be able to figure out when this lowering
would be profitable - that's good, because that is what I was missing with
the very simple approach of performing the above lowering from insinde
gimplification.

For reference I attached the BITFIELD_COMPOSE tree expression
patch.

Richard.

> Thanks,
> Andrew Pinski
>
> ChangeLog:
> * tree-merge-const-bfstores.c: New file.
> * tree-pass.h (pass_merge_const_bfstores): Add pass.
> * opts.c (default_options_table): Add OPT_fmerge_const_bfstores at -O2
> and above.
> * timevar.def (TV_MERGE_CONST_BFSTORES): New timevar.
> * common.opt (fmerge-const-bfstores): New option.
> * Makefile.in (OBJS): Add tree-merge-const-bfstores.o.
> (tree-merge-const-bfstores.o): New target.
> * passes.c (init_optimization_passes): Add pass_merge_const_bfstores
> right after the last pass_phiopt.

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