[RFC, PATCH]: Introduction of callgraph annotation class

[Martin Liška]

Hello.

Following patch introduces a new class called callgraph_annotation. Idea behind the patch is to provide a generic interface one can use to register custom info related to a cgraph_node. As you know, symbol_table provides hooks for creation, deletion and duplication of a cgraph_node. If you have a pass, you need to handle all these hooks and store custom data in your data structure.

As an example, after discussion with Martin, I chose usage in ipa-prop.h:

data structure:
vec<ipa_node_params> ipa_node_params_vector

if the pass handles an event, following chunk is executed:
if (ipa_node_params_vector.length () <= (unsigned) symtab->cgraph_max_uid)
     ipa_node_params_vector.safe_grow_cleared (symtab->cgraph_max_uid + 1);

The problem is that you can have sparse UIDs of cgraph_nodes and every time you have to allocate a vector of size equal to cgraph_max_uid.

As a replacement, I implemented first version of cgraph_annotation that internally uses hash_map<cgraph_unique_identifier, T>.
Every time a node is deleted, we remove corresponding data associated to the node.

What do you think about it?

Thank you,
Martin
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