Re: Eliminate write-only variables

[Sandra Loosemore <sandra at codesourcery dot com>]

On 05/16/2014 11:25 AM, Jan Hubicka wrote:
> Hi,
> this patch adds code to remove write only static variables.  While analyzing
> effectivity of LTO on firefox, I noticed that surprisingly large part of
> binary's data segment is occupied by these.  Fixed thus.
> (this is quite trivial transformation, I just never considered it important
> enough to work on it).
>
> The patch goes by marking write only variables in ipa.c (at same time we
> discover addressable flag) and also fixes handling of the flags for
> aliases. References to variables are then removed by fixup_cfg.
> As first cut, I only remove stores without side effects, so copies from
> volatile variables are preserved. I also kill LHS of function calls.
> I do not attempt to remove asm statements.  This means that some references
> may be left in the code and therefore the IPA code does not eliminate the
> referneces after discovering write only variable and instead it relies
> on dead variable elimination to do the job later.  Consequently not all write
> only variables are removed with WHOPR in the case the references ends up
> in different partitions. Something I can address incrementally.

This patch seems quite similar in purpose to the remove_local_statics 
optimization that Mentor has proposed, although the implementation is 
quite different.  Here is the last version of our patch, prepared by 
Bernd Schmidt last year:

https://gcc.gnu.org/ml/gcc-patches/2013-06/msg00317.html

I think we can drop our patch from our local tree now, but it includes a 
large number of test cases which I think are worth keeping on mainline. 
 A few of them fail with your implementation, though -- which might be 
genuine bugs, or just different limitations of the two approaches.  Can 
you take a look?

The failing tests are remove-local-statics-{4,5,7,12,14b}.c.

-Sandra