[lworld] RFR: 8367785: [lworld] Compiler adherence to new memory model rules for strict instance fields

[Chen Liang]

On Fri, 3 Oct 2025 14:12:34 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> The new memory model rules for strict instance fields (see [JDK-8366372](https://bugs.openjdk.org/browse/JDK-8366372)) require a memory barrier at the beginning of `java.lang.Object::<init>`. The JITs will try to omit it if the receiver type is statically known (potentially via CHA + a dependency) and the class does not contain any strict fields.
> 
> Potentially, there's some more tricks that we can play. Most importantly, I think we can omit barriers for final fields at the end of the constructors if the field is also strict. I filed [JDK-8369166](https://bugs.openjdk.org/browse/JDK-8369166) for this.
> 
> I added a test that already caught [JDK-8369044](https://bugs.openjdk.org/browse/JDK-8369044) and will fail immediately without the JIT changes (on AArch64).
> 
> Thanks,
> Tobias

src/hotspot/share/c1/c1_GraphBuilder.cpp line 4304:

> 4302:   // Check if we need a membar at the beginning of the java.lang.Object
> 4303:   // constructor to satisfy the memory model for strict fields.
> 4304:   if (EnableValhalla && method()->intrinsic_id() == vmIntrinsics::_Object_init) {

Suggestion:

  if (EnableValhalla && callee->intrinsic_id() == vmIntrinsics::_Object_init) {

Fits better in the context of this function.

src/hotspot/share/c1/c1_LIRGenerator.cpp line 3019:

> 3017:   // Check if we need a membar at the beginning of the java.lang.Object
> 3018:   // constructor to satisfy the memory model for strict fields.
> 3019:   if (EnableValhalla && method()->intrinsic_id() == vmIntrinsics::_Object_init) {

Should we perform checks to try skip this fence here?

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1656#discussion_r2403746159
PR Review Comment: https://git.openjdk.org/valhalla/pull/1656#discussion_r2403748696