RFR: JDK-8314890: Reduce number of loads for Klass decoding in static code [v2]

[Thomas Stuefe]

> Small change that reduces the number of loads generated by the C++ compiler for a narrow Klass decoding operation (`CompressedKlassPointers::decode_xxx()`.
> 
> Stock: three loads (with two probably sharing a cache line) - UseCompressedClassPointers, encoding base and shift.
> 
> 
>   8b7b62:   48 8d 05 7f 1b c3 00    lea    0xc31b7f(%rip),%rax        # 14e96e8 <UseCompressedClassPointers>
>   8b7b69:   0f b6 00                movzbl (%rax),%eax
>   8b7b6c:   84 c0                   test   %al,%al
>   8b7b6e:   0f 84 9c 00 00 00       je     8b7c10 <_ZN10HeapRegion14object_iterateEP13ObjectClosure+0x260>
>   8b7b74:   48 8d 15 05 62 c6 00    lea    0xc66205(%rip),%rdx        # 151dd80 <_ZN23CompressedKlassPointers6_shiftE>
>   8b7b7b:   8b 7b 08                mov    0x8(%rbx),%edi
>   8b7b7e:   8b 0a                   mov    (%rdx),%ecx
>   8b7b80:   48 8d 15 01 62 c6 00    lea    0xc66201(%rip),%rdx        # 151dd88 <_ZN23CompressedKlassPointers5_baseE>
>   8b7b87:   48 d3 e7                shl    %cl,%rdi
>   8b7b8a:   48 03 3a                add    (%rdx),%rdi
> 
> 
>   Patched: one load loads all three. Since shift occupies the lowest 8 bits, compiled code uses 8bit register; ditto the UseCompressedOops flag.
> 
> 
>   8ba302:   48 8d 05 97 9c c2 00    lea    0xc29c97(%rip),%rax        # 14e3fa0 <_ZN23CompressedKlassPointers6_comboE>
>   8ba309:   48 8b 08                mov    (%rax),%rcx
>   8ba30c:   f6 c5 01                test   $0x1,%ch        # use compressed klass pointers?
>   8ba30f:   0f 84 9b 00 00 00       je     8ba3b0 <_ZN10HeapRegion14object_iterateEP13ObjectClosure+0x260>
>   8ba315:   8b 7b 08                mov    0x8(%rbx),%edi
>   8ba318:   48 d3 e7                shl    %cl,%rdi        # shift 
>   8ba31b:   66 31 c9                xor    %cx,%cx         # zero out lower 16 bits of base
>   8ba31e:   48 01 cf                add    %rcx,%rdi       # add base 
>   8ba321:   8b 4f 08                mov    0x8(%rdi),%ecx
> 
> ---
> 
> Performance measurements: 
> 
> G1, doing a full GC over a heap filled with 256 mio life j.l.Object instances.
> 
> I see a reduction of Full Pause times between 1.2% and 5%. I am unsure how reliable these numbers are since, despite my efforts (running tests on isolated CPUs etc.), the standard deviation was quite high at ˜4%. Still, in general, numbers seemed to go down rather than up.
> 
> ---
> 
> Future extensions: 
> 
> This patch uses the fact that the encoding base is aligned to metaspace reserve alignment (16 Mb). We only use 16 of those 24 bits of alignment shadow and could us...

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  fix -UseCCP case

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15389/files
  - new: https://git.openjdk.org/jdk/pull/15389/files/bc0bd12b..12d19a06

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15389&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15389&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15389.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15389/head:pull/15389

PR: https://git.openjdk.org/jdk/pull/15389