RE: [PATCH] Move reload_completed and other rtl.h globals to crtl structure.

["Roger Sayle" <roger@nextmovesoftware.com>]

On 11 July 2022 08:20, Richard Biener wrote:
> On Sun, 10 Jul 2022, Roger Sayle wrote:
> 
> > This patch builds upon Richard Biener's suggestion of avoiding global
> > variables to track state/identify which passes have already been run.
> > In the early middle-end, the tree-ssa passes use the curr_properties
> > field in cfun to track this.  This patch uses a new rtl_pass_progress
> > int field in crtl to do something similar.
> 
> Why not simply add PROP_rtl_... and use the existing curr_properties for
this?
> RTL passes are also passes and this has the advantage you can add things
like
> reload_completed to the passes properties_set field hand have the flag
setting
> handled by the pass manager as it was intended?
> 

Great question, and I did initially consider simply adding more RTL fields
to
curr_properties.  My hesitation was from the comments/documentation that
the curr_properties field is used by the pass manager as a way to track
(and verify) the properties/invariants that are required, provided and
destroyed
by each pass.  This semantically makes sense for properties such as accurate
data flow, ssa form, cfg_layout, nonzero_bits etc, where hypothetically the
pass manager can dynamically schedule a pass/analysis to ensure the next
pass
has the pre-requisite information it needs.

This seems semantically slightly different from tracking time/progress,
where
we really want something more like DEBUG_COUNTER that simply provides
the "tick-tock" of a pass clock.  Alas, the "pass number", as used in the
suffix
of dump-files (where 302 currently means reload) isn't particularly useful
as
these change/evolve continually.

Perhaps the most obvious indication of this (subtle) difference is the
curr_properties field (PROP_rtl_split_insns) which tracks whether
instructions
have been split, where at a finer granularity rtl_pass_progress may wish to
distinguish split1 (after combine before reload), split2 (after reload
before
peephole2) and split3 (after peephole2).  It's conceptually not a simple
property, whether all insns have been split or not, as in practice this is
more subtle with backends choosing which instructions get split at which
"times".

There's also the concern that we've a large number of passes (currently
62 RTL passes), and only a finite number of bits (in curr_properties), so
having two integers reduces the risk of running out of bits and needing
to use a "wider" data structure.

To be honest, I just didn't want to hijack curr_properties to abuse it for a

use that didn't quite match the original intention, without checking with
you and the other maintainers first.  If the above reasoning isn't
convincing,
I can try spinning an alternate patch using curr_properties (but I'd expect
even more churn as backend source files would now need to #include
tree-passes.h and function.h to get reload_completed).  And of course,
a volunteer is welcome to contribute that re-refactoring after this one.

I've no strong feelings either way.  It was an almost arbitrary engineering
decision (but hopefully the above explains the balance of my reasoning).

Roger
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