Re: [PATCH][RFC] Gimplify "into SSA"

[Jeff Law <law at redhat dot com>]

On 04/21/2016 06:55 AM, Richard Biener wrote:
> The following patch makes us not allocate decls but SSA names for
> temporaries required during gimplification.  This is basically the
> same thing as we do when calling the gimplifier on GENERIC expressions
> from optimization passes (when we are already in SSA).
>
> There are two benefits of doing this.
>
> 1) SSA names are smaller (72 bytes) than VAR_DECLs (144 bytes) and we
> rewrite them into anonymous SSA names later anyway, leaving up the
> VAR_DECLs for GC reclaim (but not their UID)
>
> 2) We keep expressions "connected" by having the use->def link via
> SSA_NAME_DEF_STMT for example allowing match-and-simplify of
> larger expressions on early GIMPLE
I like it -- I can't see any significant reason to keep the _DECL nodes 
for these temporaries.  They're not useful for end-user debugging or 
debugging GCC itself.  In fact, I would claim that these temporary _DECL 
nodes just add noise when diffing debugging dumps.

While GC would reclaim the _DECL nodes, I'm all for avoiding placing 
work on the GC system when it can be easily avoided.

> Complications arise from the fact that there is no CFG built and thus
> we have to make sure to not use SSA names where we'd need PHIs.  Or
> when CFG build may end up separating SSA def and use in a way current
> into-SSA doesn't fix up (adding of abnormal edges, save-expr placement,
> gimplification of type sizes, etc.).
:(

> As-is the patch has the downside of effectively disabling the
> lookup_tmp_var () CSE for register typed temporaries and not
> preserving the "fancy" names we derive from val in
> create_tmp_from_val (that can be recovered easily though if
> deemed worthwhile).
I don't think it's worthwhile.

ISTM this will affect something like the gimple front-end project which 
would need to see the anonymous name and map it back to a suitable type, 
but I don't think that should stop this from moving forward.

Jeff