RFR: 8254793: [JVMCI] improve speculation encoding [v3]

[Vladimir Kozlov]

On Fri, 16 Oct 2020 16:30:43 GMT, Doug Simon <dnsimon at openjdk.org> wrote:

>> src/hotspot/share/runtime/thread.hpp line 1171:
>> 
>>> 1169:   // uniquely identify the  speculative optimization guarded by the uncommon trap.
>>> 1170:   // The id value is only 32-bits but since this field is exposed via VMStructs to
>>> 1171:   // JVMCI as a jlong, it needs to be kept as a long to maintain backwards compatibility
>> 
>> I am confusing about backword compatibility comment. It said that old Graal (link in current JDK) generate code which
>> writes 64 bits into this word. Will it use [32:32] index:length format or it will use new [0:27:5] format? I don't see
>> changes to Graal in this PR.
>
> The version of Graal in the JDK does not change. It is agnostic about the encoding format. All is does is write a value
> to `Thread::_pending_failed_speculation` where said value is provided by JVMCI. The width of the write is determined by
> the width of the value. You can follow the code that does this
> [here](https://github.com/openjdk/jdk/blob/6c3bc71079bd9f4de005d005ded5a7cc3b7e373a/src/jdk.internal.vm.compiler/share/classes/org.graalvm.compiler.hotspot.amd64/src/org/graalvm/compiler/hotspot/amd64/AMD64HotSpotLIRGenerator.java#L529).

Okay, that it what I looked for - the value encoding is provided by JVMCI. Good.

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PR: https://git.openjdk.java.net/jdk/pull/667