RFR: 8309953: Strengthen and optimize oopDesc age methods

[Aleksey Shipilev]

See the RFE for discussion. Basically, there is little reason to do two loads of mark word, when we can do one. 

Sample generated code for `oopDesc::age` can be seen if we turn that method from `inline` to the regular method:


# Before

000000000080f440 <oopDesc::age>:
  80f440: ff 83 00 d1   sub     sp, sp, #32
  80f444: fd 7b 01 a9   stp     x29, x30, [sp, #16]
  80f448: fd 43 00 91   add     x29, sp, #16
  80f44c: 08 00 40 f9   ldr     x8, [x0]          ; <-- first mark load
  80f450: 89 27 00 d0   adrp    x9, 0xd01000 
  80f454: 1f 20 03 d5   nop     
  80f458: 29 95 4a b9   ldr     w9, [x9, #2708]
  80f45c: 0a 05 40 92   and     x10, x8, #0x3
  80f460: 5f 09 00 f1   cmp     x10, #2
  80f464: ea 17 9f 1a   cset    w10, eq
  80f468: 1f 01 40 f2   tst     x8, #0x1
  80f46c: e8 17 9f 1a   cset    w8, eq
  80f470: 3f 09 00 71   cmp     w9, #2
  80f474: 48 01 88 1a   csel    w8, w10, w8, eq
  80f478: 1f 05 00 71   cmp     w8, #1
  80f47c: 21 01 00 54   b.ne    0x80f4a0
  80f480: 08 00 40 f9   ldr     x8, [x0]          ; <-- second mark load
  80f484: e8 07 00 f9   str     x8, [sp, #8]
  80f488: e0 23 00 91   add     x0, sp, #8
  80f48c: c4 ed fd 97   bl      0x78ab9c
  80f490: 00 18 03 53   ubfx    w0, w0, #3, #4
  80f494: fd 7b 41 a9   ldp     x29, x30, [sp, #16]
  80f498: ff 83 00 91   add     sp, sp, #32
  80f49c: c0 03 5f d6   ret     
  80f4a0: 00 00 40 f9   ldr     x0, [x0]
  80f4a4: 00 18 03 53   ubfx    w0, w0, #3, #4
  80f4a8: fd 7b 41 a9   ldp     x29, x30, [sp, #16]
  80f4ac: ff 83 00 91   add     sp, sp, #32
  80f4b0: c0 03 5f d6   ret    

# After

000000000080f480 <oopDesc::age>:
  80f480: ff 83 00 d1   sub     sp, sp, #32
  80f484: fd 7b 01 a9   stp     x29, x30, [sp, #16]
  80f488: fd 43 00 91   add     x29, sp, #16
  80f48c: 00 00 40 f9   ldr     x0, [x0]          ; <-- load mark once
  80f490: e0 07 00 f9   str     x0, [sp, #8]
  80f494: 88 27 00 d0   adrp    x8, 0xd01000  
  80f498: 1f 20 03 d5   nop     
  80f49c: 08 95 4a b9   ldr     w8, [x8, #2708]
  80f4a0: 09 04 40 92   and     x9, x0, #0x3
  80f4a4: 3f 09 00 f1   cmp     x9, #2
  80f4a8: e9 17 9f 1a   cset    w9, eq
  80f4ac: 1f 00 40 f2   tst     x0, #0x1
  80f4b0: ea 17 9f 1a   cset    w10, eq
  80f4b4: 1f 09 00 71   cmp     w8, #2
  80f4b8: 28 01 8a 1a   csel    w8, w9, w10, eq
  80f4bc: 1f 05 00 71   cmp     w8, #1
  80f4c0: 61 00 00 54   b.ne    0x80f4cc 
  80f4c4: e0 23 00 91   add     x0, sp, #8
  80f4c8: c5 ed fd 97   bl      0x78abdc
  80f4cc: 00 18 03 53   ubfx    w0, w0, #3, #4
  80f4d0: fd 7b 41 a9   ldp     x29, x30, [sp, #16]
  80f4d4: ff 83 00 91   add     sp, sp, #32 
  80f4d8: c0 03 5f d6   ret     


Additional testing:
 - [x] Eyeballing generated code
 - [ ] Linux x86_64 fastdebug `tier1 tier2 tier3`
 - [ ] Linux AArch64 fastdebug `tier1 tier2 tier3

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

Commit messages:
 - Revert helper method removals
 - Fix

Changes: https://git.openjdk.org/jdk/pull/14456/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309953
  Stats: 10 lines in 1 file changed: 2 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14456/head:pull/14456

PR: https://git.openjdk.org/jdk/pull/14456