RFR: JDK-8275854: C2: assert(stride_con != 0) failed: missed some peephole opt [v3]

[王超]

> `If subsume` optimization will eliminate `LongCountedLoopEndNode` node by mistake, which will lead to `PhaseIdealLoop` optimization crash.
> 
> For example, the test of node 538 and node 553 will become the same after the first `PhaseIdealLoop` optimization. Node 555 is the back edge to the loop, and node 553 will be replaced by a `LongCountedLoopEndNode` node.
> <img width="592" alt="image" src="https://user-images.githubusercontent.com/25214855/138656330-9f825c85-0468-40a5-9eb9-b576130648db.png">
> 
> 
>  In the next `PhaseIdealLoop` optimization, node 538 find node 553 is redundant, and will subsume node 553. Then the `PhaseIdealLoop` optimization will crash, because there is no loop end node.
> <img width="603" alt="image" src="https://user-images.githubusercontent.com/25214855/138656951-53685723-92c2-42d6-91ae-445107dcb01d.png">
> 
> There are two way to fix the crash, the first is like the way in this pr, just exit `IFNode subsume` optimization when it's a `LongCountedLoopEndNode` node. The second possible fix is that exchange the dominating `IF` node with the `LongCountedLoopEndNode` node:
> 
> diff --git a/src/hotspot/share/opto/ifnode.cpp b/src/hotspot/share/opto/ifnode.cpp
> index 38b40a6..31ff172 100644
> --- a/src/hotspot/share/opto/ifnode.cpp
> +++ b/src/hotspot/share/opto/ifnode.cpp
> @@ -1674,6 +1674,21 @@ Node* IfNode::simple_subsuming(PhaseIterGVN* igvn) {
>      }
>    }
>  
> +  if (is_LongCountedLoopEnd()) {
> +    set_req(0, dom->in(0));
> +    set_req(1, dom->in(1));
> +    dom->set_req(0, pre);
> +    dom->set_req(1, igvn->intcon(is_always_true ? 1 : 0));
> +    Node* proj0 = raw_out(0);
> +    Node* proj1 = raw_out(1);
> +    Node* dom_proj0 = dom->raw_out(0);
> +    Node* dom_proj1 = dom->raw_out(1);
> +    dom_proj0->set_req(0, this);
> +    dom_proj1->set_req(0, this);
> +    proj0->set_req(0, dom);
> +    proj1->set_req(0, dom);
> +  }
> +
>    if (bol->outcnt() == 0) {
>      igvn->remove_dead_node(bol);    // Kill the BoolNode.
>    }
> diff --git a/src/hotspot/share/opto/loopnode.cpp b/src/hotspot/share/opto/loopnode.cpp
> index 6f7e34d..7955722 100644
> --- a/src/hotspot/share/opto/loopnode.cpp
> +++ b/src/hotspot/share/opto/loopnode.cpp
> @@ -802,7 +802,7 @@ bool PhaseIdealLoop::transform_long_counted_loop(IdealLoopTree* loop, Node_List
>    Node* back_control = head->in(LoopNode::LoopBackControl);
>  
>    // data nodes on back branch not supported
> -  if (back_control->outcnt() > 1) {
> +  if (back_control->outcnt() > 1 || back_control->Opcode() != Op_IfTrue) {
>      return false;
>    }

王超 has updated the pull request incrementally with one additional commit since the last revision:

  Adjust the code style

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6099/files
  - new: https://git.openjdk.java.net/jdk/pull/6099/files/a2444447..606fb7e2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6099&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6099&range=01-02

  Stats: 5 lines in 1 file changed: 4 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6099.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6099/head:pull/6099

PR: https://git.openjdk.java.net/jdk/pull/6099