Questions about LTO, clones and ipa_ref

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Hello,

I am working on an interprocedural analysis (ipa) and transformation for GCC. The optimizations is called ipa-initcall-cp because it copies and propagates values assign to constants (with static lifetime) as in the case of calling
an initialization function.

I am new to gcc compiler development, but I already have a working
implementation for ipa-initcall-cp and can be found here [0]. ipa-initcall-cp is currently implemented as a SIMPLE_IPA_PASS and is able to compile code for
some benchmarks. The current limitation is that when compiling with
ipa-initcall-cp, one needs to specify the flag `-flto-partition=none`. I would
like to ask a few questions as a sanity check:

The GCC internal documentation says the following [0]

To facilitate [...] communication [between the different LTO stages], the GCC
  callgraph infrastructure implements virtual clones [...]

A virtual clone in the callgraph is a function that has no associated body, just a description of how to create its body based on a different function.

  The description of function modifications includes adjustments to the
function's signature [...], substitutions to perform on the function body

Looking at the code, I was able to find several examples of clones (virtual and
other types). The examples that I have been able to find, modify a
function's body but it is all done through the parameters:
* vec<ipa_replace_map *, gc> *tree_map
* ipa_param_adjustments *param_adjustments
(These are found in several functions defined on gcc/cgraphclones.c)

Looking at the definition for ipa_replace_map
(defined on gcc/cgraph.h:713)
```
struct GTY(()) ipa_replace_map
{
  /* The new (replacing) tree. */
  tree new_tree;
  /* Parameter number to replace, when old_tree is NULL. */
  int parm_num;
}
```
suggests that the only replacements possible are those that replace a parameter with a tree. Furthermore, the comments suggest that previously the capability
to replace any arbitrary old_tree with new_tree was possible.

Question #1: Is there a way to replace arbitrary old_trees for new_trees?
Question #2: If there is a way, what is the way to do so?

This is necessary for ipa-initcall-cp because functions might access global
variables with static life-time and therefore have no impact on the
method's parameters.



So far, instead of using ipa_replace map and parameter adjustments, what
ipa-initcall-cp does is it modifies the original function, and at the end of ipa-initcall-cp it creates a clone (with the modified function) and removes the original function. I would like to keep around the original function, as it seems that it might be necessary for linking without -flto-partition=none.
It might also be a source of linking errors in some other programs.

As a result, another alternative I have explored is to create a clone and only
modify the clone.

Question #3: How does creating a clone affect an IPA_REF?

More concretely: If I have the following code

```
static void
propagate_constant_to_read(tree write_val, struct ipa_ref* ref) {

  gcc_assert(ref->use == IPA_REF_LOAD);
  symtab_node *f_node = ref->referring;
  cgraph_node *f_cnode = dyn_cast<cgraph_node *> (f_node);
  f_cnode->get_untransformed_body();

  cgraph_node *clone = f_cnode->create_clone(/* ... */);

  gimple *read_stmt = ref->stmt;
  gcc_assert(gimple_code(read_stmt) == GIMPLE_ASSIGN);
  gcc_assert(gimple_num_ops(read_stmt) == 2);
  tree old_lhs = gimple_op(read_stmt, 0);

  push_cfun(clone);
  gimple *use_stmt;
  imm_use_iterator use_iter;
  FOR_EACH_IMM_USE_STMT (use_stmt, use_iter, old_lhs)
     {
     // replace old use with write...
     }
  pop_cfun();

}
```

Can I still use ipa_ref/read_stmt but now in the context of the clone?
Will I be iterating over the uses in clone or the uses in f_cnode?

Question #4: Is there a way to map the ipa_ref in one function to the ipa_ref
in its clones?

Thanks! Any help will be greatly appreciated!

[0] https://gcc.gnu.org/onlinedocs/gccint/IPA.html#IPA



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