RFR: 8307084: C2: Vectorized drain loop is not executed for some small trip counts [v4]

[Fei Gao]

On Fri, 16 Jan 2026 09:42:57 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/loopnode.hpp line 1440:
>> 
>>> 1438:                                  // result control flow branches
>>> 1439:                                  // either to inner clone or outer
>>> 1440:                                  // strip mined loop.
>> 
>> I have trouble understanding the comments here (not your fault, it was here already).
>> I'm also wondering if this is only used for `post_loop`? If so, maybe we could rename it, and improve the comments here?
>
> At least in your code, it would read much better if it was called `InsertPost`

> I have trouble understanding the comments here (not your fault, it was here already). I'm also wondering if this is only used for `post_loop`? If so, maybe we could rename it, and improve the comments here?

`ControlAroundStripMined` is not used only for `post_loop`; it is also used by `do_peeling()` and `duplicate_loop_backedge()`. For that reason, I’m afraid renaming it to something like `InsertPost` wouldn’t be appropriate.

Based on my understanding, `ControlAroundStripMined` means that only the inner strip-mined loop is cloned, and a control-flow decision is inserted around the inner clone, allowing the exit control flow either to the inner clone or to the outer strip-mined loop.

How about refining the comments as follows:

...
  ControlAroundStripMined = 2, // Only clone the inner strip-mined loop and insert
                               // control flow around it. Exit control flow
                               // branches either to inner clone or to the outer
                               // strip-mined loop.
  InsertVectorizedDrain = 3    // Only clone  the inner strip-mined vector loop and
                               // insert control flow that branches either to the
                               // cloned inner loop or to the scalar post loop.
...

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PR Review Comment: https://git.openjdk.org/jdk/pull/22629#discussion_r2705287124