RFR: 8263891: Changes for 8076985 missed the fix.

[Vladimir Kozlov]

The fix for JDK-8076985 was supposed to be this: 
https://cr.openjdk.java.net/~kvn/8076985/webrev.00/src/hotspot/cpu/x86/x86_64.ad.udiff.html

But the final push had only comment change: 
https://hg.openjdk.java.net/jdk/jdk/rev/4d1c4400c75d

Here is explanation of the fix from 8076985 RFR:

First, this is about how C2 generates code for *constant* class pointers.

A little history here. When we implemented compressed oops and class pointers we had PermGen and classes were Java objects. We used the same decoding/encoding code for oops and classes - we used the same register containing Heap Base address. It was profitable to decode constant class and reuse it [1]. Also we greatly benefited on SPARC since decoding 32-bit constant required 4 instructions instead of up to 7 instructions to load 64-bit constant.

Now compressed class decoding is different and always takes 2 instructions on x86 [2] if either base or shift is not 0.

As result we generated 3 instructions to get full class pointer from compressed 32-bit constant (example for base = 0, shift = 3):

movl $0x200001d5,%r11d
movabs $0x0,%r10
lea (%r10,%r11,8),%r10

Also when we store compressed class pointer into new object header we don't use register anymore on x86 - keeping it in register does not help now:

movl $0x200001d5,0x8(%rax)

Aleksey suggested to have only one instruction to load full 64-bit class pointer:

movq $0x100000EA8,%r10

It frees one register and uses 10 bytes instead of up to 20 bytes of code on x86.

In JDK 9 SAP contributed nice change [3] to have choice when to use 'compressed class pointer + decoding' or full '64-bit constant class pointer'. It significantly simplified changes for this RFE.


I ran performance testing but did not see difference - we don't use biased locking now and as result we don't need to load prototype header from class. But there are other places where we need load from class. 

Tested tier1-3

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Commit messages:
 - 8263891: Changes for 8076985 missed the fix.

Changes: https://git.openjdk.java.net/jdk/pull/3102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263891
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3102.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3102/head:pull/3102

PR: https://git.openjdk.java.net/jdk/pull/3102