Re: tree decl stored during LGEN does not map to a symtab_node during WPA

[Erick Ochoa <eochoa@gcc.gnu.org>]

Hello Richard, I need a little bit more help. In our previous messages
you mentioned "<function-encoder>"

> >
> > > I guess the way to encode SSA trees would be to use sth like a
> > > <function-encoder>, SSA-version tuple much like PTA internally
> > > uses the varinfo array index as identifier for the variables in the
> > > constraints.  For local decls (as opposed to SSA names) it's a bit
> > > more difficult - you'd have to devise your own encoding here.
> > >

There was a little confusion on my part about what "encoder" meant

>
> > You mention that I need to devise my own "encoder", but I am not sure
> > if we are conflating two notions:
> >
> > 1. encoding tree variables to constraint variables (i.e., a mapping of
> > some tuple (cgraph_node x symtab_node x ssa-version) to an integer
> > that represents the constraint variable)
> > 2. encoding as an implementation of a data structure used during LTO
> > to stream in and stream out trees/symbols to and from partitions.
> > (e.g., lto_symtab_encoder_t).
>

And you proceed with the following answer

> I meant 1) and streaming using the LTO cgraph encoder for the cgraph
> part and simply using the SSA version for the second part.
>

From this exchange I understood that I should develop my own mapping
for ssa variables and local declarations. However, when dealing with
encoding a node which is available in the symbol table, I could use
the function lto_symtab_encoder_encode to map symbols to an integer
which would later make the symbol available at WPA time. Implicitly,
for me, this meant that this integer is the same for every distinct
symbol in different LGEN partitions. For example, if we encode symbol
X from partitions Y and we get the number N, then encoding symbol X in
partition Z should also yield N.

I believe this is not the case, during WPA time I am printing:
1. pid of lgen process that generated the encoding
2. index returned by lto_symtab_encoder_encode
3. varpool_node->name ()
4. the pointer address being pointed by varpool node

I think we had a previous discussion where it was mentioned that the
only way to distinguish between these cases is to look at varpool_node
cgraph_node:

(From a different email edited for brevity)

On Wed, 30 Jun 2021 at 19:38, Richard Biener <richard.guenther@gmail.com> wrote:
>
> On June 30, 2021 6:28:29 PM GMT+02:00, Erick Ochoa <eochoa@gcc.gnu.org> wrote:
> >So how would I be able to say that two different declarations are the
> >same variable?
>
> By looking at the associated varpool node.
>
> Richard.
>

If this is the case, I can indeed get the varpool node's at WPA time
(as shown above), but comparing their pointer addresses will be
distinct. How can one find out that two varpool nodes/cgraph nodes are
the same at WPA time? Is just looking at the assembler name enough? I
of course want this to be safe.

Another question, how is this currently handled in other IPA passes?
Alternatively, do you have suggestions for encoding functions and
global variables in a similar way to how you suggested encoding ssa
variables and local declarations?