RFR: 8327109: Refactor data graph cloning used in create_new_if_for_predicate() into separate class

Fri Mar 1 14:14:55 UTC 2024

On Fri, 1 Mar 2024 13:27:38 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> In the review process of https://github.com/openjdk/jdk/pull/16877, we identified an existing issue in `create_bool_from_template_assertion_predicate()` which is also still present in the refactoring of https://github.com/openjdk/jdk/pull/16877: In rare cases, we could endlessly re-process nodes in the DFS walk since a visited set is missing. This needs to be addressed.
> 
> #### Redo refactoring of `create_bool_from_template_assertion_predicate()`
> On top of that bug, the refactored version of https://github.com/openjdk/jdk/pull/16877 is still quite complicated to understand since it tries to do multiple steps simultaneously. We've decided to redo the refactoring and better separate the steps to simplify the algorithm. By doing so, we also want to fix the existing bug. This work is split into three separate RFEs (JDK-8327109, JDK-8327110, and JDK-8327111).
> 
> #### Share data graph cloning code - start from existing code 
> This first PR starts with the existing code found in `clone_nodes_with_same_ctrl()` which is called by `create_new_if_for_predicate()`. `clone_nodes_with_same_ctrl()` already does the data graph cloning in 3 separate steps which can be used as foundation:
> 
> 1. Collect data nodes to clone by using a node filter
> 2. Clone the collected nodes (their data and control inputs still point to the old nodes)
> 3. Fix the cloned data node inputs pointing to the old nodes to the cloned inputs by using an old->new mapping. In this pass, also fix the control inputs of any pinned data node from the old uncommon projection to the new one.
> 
> #### Shared data graph cloning class
> Some of these steps above are shared with the data graph cloning done in `create_bool_from_template_assertion_predicate()` (refactored in JDK-8327110 and JDK-8327111). We therefore extract them in this patch such that we can reuse it in the refactoring for `create_bool_from_template_assertion_predicate()` later. We create a new `DataNodeGraph` class which does the following (to be shared) cloning of a data graph:
> 
> 1. Take a collection of data nodes (the collection step is different in `clone_nodes_with_same_ctrl()` compared to `create_bool_from_template_assertion_predicate()` and thus cannot be shared) and clone them. [Same as step 2 above]
>  2. Fix the cloned data node inputs pointing to the old nodes to the cloned inputs by using an old->new mapping. [Same as first part of step 3 above but drop the second part of rewiring control inputs which is specific to `clone_nodes_with_same_ctrl()`]
> 
> `...

Looks like a nice refactoring! I left a few comments and questions :)

src/hotspot/share/opto/loopPredicate.cpp line 254:

> 252:   const Unique_Node_List nodes_with_same_ctrl = find_nodes_with_same_ctrl(start_node, old_uncommon_proj);
> 253:   DataNodeGraph data_node_graph(nodes_with_same_ctrl, this);
> 254:   auto& orig_to_new = data_node_graph.clone(new_uncommon_proj);

This was a bit confusing. At first I thought you are cloning the `data_node_graph`, since the `auto` did not tell me that here we are getting a hash-table back.
I wonder if this cloning should be done in the constructor of `DataNodeGraph`.

src/hotspot/share/opto/loopPredicate.cpp line 255:

> 253:   DataNodeGraph data_node_graph(nodes_with_same_ctrl, this);
> 254:   auto& orig_to_new = data_node_graph.clone(new_uncommon_proj);
> 255:   fix_cloned_data_node_controls(old_uncommon_proj, new_uncommon_proj, orig_to_new);

And is there a reason why `fix_cloned_data_node_controls` is not part of the `DataNodeGraph` class? Is there any use of the class where we don't have to call `fix_cloned_data_node_controls`?

src/hotspot/share/opto/loopPredicate.cpp line 256:

> 254:   auto& orig_to_new = data_node_graph.clone(new_uncommon_proj);
> 255:   fix_cloned_data_node_controls(old_uncommon_proj, new_uncommon_proj, orig_to_new);
> 256:   Node** cloned_node_ptr = orig_to_new.get(start_node);

Boah, this `**` is a bit nasty. Would have been nicer if there was a reference pass instead, which checks already that the element exists.

src/hotspot/share/opto/loopPredicate.cpp line 265:

> 263: void PhaseIdealLoop::fix_cloned_data_node_controls(
> 264:     const ProjNode* old_uncommon_proj, Node* new_uncommon_proj,
> 265:     const ResizeableResourceHashtable<Node*, Node*, AnyObj::RESOURCE_AREA, mtCompiler>& orig_to_new) {

Suggestion:

    const ResizeableResourceHashtable<Node*, Node*, AnyObj::RESOURCE_AREA, mtCompiler>& orig_to_new)
{

This might also help with understanding the indentation. But this is a taste question for sure.

src/hotspot/share/opto/loopPredicate.cpp line 271:

> 269:       set_ctrl(clone, new_uncommon_proj);
> 270:     }
> 271:   });

Indentation is suboptimal here. I found it difficult to read.
Style guide:

Indentation for multi-line lambda:

c.do_entries([&] (const X& x) {
               do_something(x, a);
               do_something1(x, b);
               do_something2(x, c);
             });

src/hotspot/share/opto/loopPredicate.cpp line 291:

> 289:   for (uint i = 1; i < next->req(); i++) {
> 290:     Node* in = next->in(i);
> 291:     if (!in->is_Phi()) {

What happened with the `is_Phi`? Is it not needed anymore?

src/hotspot/share/opto/loopnode.hpp line 1889:

> 1887: //   1. Clone the data nodes
> 1888: //   2. Fix the cloned data inputs pointing to the old nodes to the cloned inputs by using an old->new mapping.
> 1889: class DataNodeGraph : public StackObj {

You could have a typedef for `ResizeableResourceHashtable<Node*, Node*, AnyObj::RESOURCE_AREA, mtCompiler>`. Then you don't need to use `auto` for it elsewhere, and it is clear what it is.
Suggestion: `OrigToNewHashtable`.

src/hotspot/share/opto/loopnode.hpp line 1921:

> 1919:     rewire_clones_to_cloned_inputs();
> 1920:     return _orig_to_new;
> 1921:   }

Currently, it looks like one could call `clone` multiple times. But I think that would be wrong, right?
That is why I'd put all the active logic in the constructor, and only the passive stuff is publicly accessible, with `const` to indicate that these don't have any effect.

src/hotspot/share/opto/loopopts.cpp line 4519:

> 4517:   _orig_to_new.iterate_all([&](Node* node, Node* clone) {
> 4518:     for (uint i = 1; i < node->req(); i++) {
> 4519:       Node** cloned_input = _orig_to_new.get(node->in(i));

You don't need to check for `is_Phi` on `node->in(i)` anymore?

-------------

Changes requested by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18080#pullrequestreview-1911220168
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509038222
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509065385
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509040263
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509045154
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509044654
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509060128
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509047305
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509057459
PR Review Comment: https://git.openjdk.org/jdk/pull/18080#discussion_r1509060906