RFR: 8254252: Generic arraycopy stub overwrites callee-save rdi register on 64-bit Windows

[Tobias Hartmann]

Since [JDK-8241825](https://bugs.openjdk.java.net/browse/JDK-8241825), MacroAssembler::load_klass requires a temporary
register to decode the klass pointer. In the generic arraycopy stub, rdi is used for that on 64-bit Windows because r9
is already used as an argument register: https://hg.openjdk.java.net/jdk/jdk/rev/0bb101fbeb10#l17.32

The problem is that rdi is callee-save [1] but not restored when returning from the stub. This leads to register
corruption and more or less random crashes in the caller.

Although JDK-8241825 is part of JDK 15, this was never a problem because we did not set the _WIN64 macro in adlc and as
a result accidentally treated rdi (and rsi) as caller-save:
https://github.com/openjdk/jdk/blob/b9873e18330b7e43ca47bc1c0655e7ab20828f7a/src/hotspot/cpu/x86/x86_64.ad#L89

Now that this got fixed as part of [JDK-8248238](https://bugs.openjdk.java.net/browse/JDK-8248238) [2], we hit the bug.

Unfortunately, we are running out of caller-save registers on Windows. Since rcx, rdx, r8 and r9 are used for
arguments, only rax, r10 and r11 remain which are already used as temporary registers and live throughout the stub
code. I've decided to free up r11 by postponing the load of the array length which is only needed on Windows anyway
(because only Windows passes the 5th argument on stack).

Thanks,
Tobias

[1] https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2019
[2] https://openjdk.github.io/cr/?repo=jdk&pr=212&range=11#sdiff-8

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Commit messages:
 - 8254252: Generic arraycopy stub overwrites callee-save rdi register on 64-bit Windows

Changes: https://git.openjdk.java.net/jdk/pull/603/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=603&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254252
  Stats: 28 lines in 1 file changed: 2 ins; 5 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/603.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/603/head:pull/603

PR: https://git.openjdk.java.net/jdk/pull/603