[Bug rtl-optimization/44194] struct returned by value generates useless stores

["rguenther at suse dot de" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44194

--- Comment #26 from rguenther at suse dot de <rguenther at suse dot de> 2011-06-16 08:36:21 UTC ---
On Wed, 15 Jun 2011, xinliangli at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44194
> 
> davidxl <xinliangli at gmail dot com> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                  CC|                            |xinliangli at gmail dot com
> 
> --- Comment #23 from davidxl <xinliangli at gmail dot com> 2011-06-15 23:14:50 UTC ---
> (In reply to comment #22)
> > > The DSE patch still leaves 2 redundant stores.
> > 
> > OK, I missed this, reopening...
> > 
> > > The following patch will enable DSE to remove those two stores. Does this
> > > look ok?
> > 
> > Calling into the gimplifier from the RTL expander doesn't look appropriate.

It also should use create_tmp_var, not create_tmp_reg.  But I wonder why
memory allocated via assign_temp isn't marked in a way to let dse
do its job (I guess dse thinks that memory escapes?).

> > More fundamentally, it's a little unfortunate to spill to memory a value
> > returned in registers.  Can we try to use emit_group_move_into_temps here
> > instead (under the appropriate circumstances)?
> 
> It would be nice if the expander does not spill the return into memory in the
> first place if possible.  On other hand tagging compiler created memory
> location with temp decls so that aliaser has the symbolic information seems a
> useful mechanism. 

Sure - but I wonder why assign_temp doesn't do something equivalent
that doesn't require a automatic VAR_DECL to be created.

Where does the aliaser catch things with the VAR_DECL around that
it doesn't without it?

Richard.