RFR: 8372634: C2: Materialize type information from instanceof checks

[Quan Anh Mai]

On Thu, 27 Nov 2025 00:53:54 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Even though `instanceof` check (and reflective `Class.isInstance` call) narrows operand's type, sharpened type information is not explicitly materialized in the IR.
> 
> There's a `SubTypeCheck` node present, but it is not a substitute for a `CheckCastPP` node with a proper type.
> 
> The difference can be illustrated with the following simple cases:
> 
>    class A { void m() {} }
>    class B extends A { void m() {} }
> 
>     void testInstanceOf(A obj) {
>         if (obj instanceof B) {
>             obj.m();
>         }
>     }
> 
> InstanceOf::testInstanceOf (12 bytes)
>   @ 8 InstanceOf$A::m (0 bytes) failed to inline: virtual call
> 
> vs
> 
>     void testInstanceOfCast(A obj) {
>         if (obj instanceof B) {
>             B b = (B)obj;
>             b.m();
>         }
>     }
> 
> InstanceOf::testInstanceOfCast (17 bytes)
>   @ 13 InstanceOf$B::m (1 bytes) inline (hot)
> 
> 
> Proposed fix annotates operands of subtype checks with proper type information which reflects the effects of subtype check. Not-yet-canonicalized IR shape poses some challenges, but I decided to match it early so information is available right away, rather than waiting for IGVN pass and delay inlining to post-parse phase.
> 
> FTR it is not a complete fix. It works for trivial cases, but for more complex conditions the IR shape becomes too complex during parsing (as illustrated by some test cases). I experimented with annotating subtype checks after initial parsing pass is over, but the crucial simplification step happens as part of split-if transformation which happens when no more inlining is possible. So, the only possible benefit (without forcing split-if optimization earlier) is virtual-to-direct call strength reduction. I plan to explore it separately.
> 
> Testing: hs-tier1 - hs-tier5

src/hotspot/share/opto/parse2.cpp line 1739:

> 1737:   }
> 1738: 
> 1739:   // Match an instanceof check.

We seem to require that the input of `SubTypeCheck` is not `null`. What do you think about allowing `SubTypeCheck` to accept `null` and return `false`?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28517#discussion_r2568870697