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

[Doug Simon]

On Thu, 15 Oct 2020 18:56:38 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Doug Simon has refreshed the contents of this pull request, and previous commits have been removed. The incremental
>> views will show differences compared to the previous content of the PR.
>
> 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).

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