8145948: C2: Initializing volatile fields to default values should be optimized

[shipilev]

WIP.

New benchmark shows expected improvements. Actually, I suppose we can eliminate the barriers for all volatile writes in initializers, and instead rely on final-field-like semantics, but that requires a more comprehensive arguments, so I'll leave that for future work.

Benchmark                                        Mode  Cnt   Score   Error  Units

# Baseline
ConstructorDefaultInitBarriers.plain_init_0      avgt    3   5.820 ± 0.161  ns/op
ConstructorDefaultInitBarriers.plain_init_42     avgt    3   5.504 ± 0.108  ns/op
ConstructorDefaultInitBarriers.volatile_init_0   avgt    3  11.969 ± 0.223  ns/op
ConstructorDefaultInitBarriers.volatile_init_42  avgt    3  12.028 ± 1.671  ns/op

# Patched
ConstructorDefaultInitBarriers.plain_init_0      avgt    3   5.815 ± 0.187  ns/op
ConstructorDefaultInitBarriers.plain_init_42     avgt    3   5.511 ± 0.053  ns/op
ConstructorDefaultInitBarriers.volatile_init_0   avgt    3   5.737 ± 0.079  ns/op   ; <----
ConstructorDefaultInitBarriers.volatile_init_42  avgt    3  11.885 ± 0.950  ns/op

Additional testing:

• New IR tests pass
• New benchmark shows expected improvement
• Linux x86_64 server fastdebug, all
• Linux AArch64 server fastdebug, all

---

Progress

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Issue

• JDK-8145948: C2: Initializing volatile fields to default values should be optimized (Enhancement - P4)

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[apangin]

only if it's the very first put, right?

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[shipilev]

Not now, bot.