RFR: JDK-8274795: AArch64: avoid spilling and restoring r18 in macro assembler

[Bernhard Urban-Forster]

r18 should not be used as it is reserved as platform register. Linux is fine with userspace using it, but Windows and also recently macOS (https://github.com/openjdk/jdk11u-dev/pull/301#issuecomment-911998917 ) are actually using it on the kernel side.

The macro assembler uses the bit pattern `0x7fff_ffff` (== `r0-r30`) to specify which registers to spill; fortunately this helper is only used here:

https://github.com/openjdk/jdk/blob/c05dc268acaf87236f30cf700ea3ac778e3b20e5/src/hotspot/cpu/aarch64/templateInterpreterGenerator_aarch64.cpp#L1400-L1404

I haven't seen causing this particular instance any issues in practice _yet_, presumably because it looks hard to align the stars in order to trigger a problem (between `stp` and `ldp` of `r18` a transition to kernel space must happen *and* the kernel needs to do something with `r18`). But jdk11u-dev has more usages of the `::pusha`/`::popa` macro and that causes troubles as explained in the link above.

Output of `-XX:+PrintInterpreter` before this change:

----------------------------------------------------------------------
method entry point (kind = native)  [0x0000000138809b00, 0x000000013880a280]  1920 bytes
--------------------------------------------------------------------------------
  0x0000000138809b00:   ldr x2, [x12, #16]
  0x0000000138809b04:   ldrh    w2, [x2, #44]
  0x0000000138809b08:   add x24, x20, x2, uxtx #3
  0x0000000138809b0c:   sub x24, x24, #0x8
[...]
  0x0000000138809fa4:   stp x16, x17, [sp, #128]
  0x0000000138809fa8:   stp x18, x19, [sp, #144]
  0x0000000138809fac:   stp x20, x21, [sp, #160]
[...]
  0x0000000138809fc0:   stp x30, xzr, [sp, #240]
  0x0000000138809fc4:   mov x0, x28
 ;; 0x10864ACCC
  0x0000000138809fc8:   mov x9, #0xaccc                 // #44236
  0x0000000138809fcc:   movk    x9, #0x864, lsl #16
  0x0000000138809fd0:   movk    x9, #0x1, lsl #32
  0x0000000138809fd4:   blr x9
  0x0000000138809fd8:   ldp x2, x3, [sp, #16]
[...]
  0x0000000138809ff4:   ldp x16, x17, [sp, #128]
  0x0000000138809ff8:   ldp x18, x19, [sp, #144]
  0x0000000138809ffc:   ldp x20, x21, [sp, #160]


After:

----------------------------------------------------------------------
method entry point (kind = native)  [0x0000000108e4db00, 0x0000000108e4e280]  1920 bytes

--------------------------------------------------------------------------------
  0x0000000108e4db00:   ldr x2, [x12, #16]
  0x0000000108e4db04:   ldrh    w2, [x2, #44]
  0x0000000108e4db08:   add x24, x20, x2, uxtx #3
  0x0000000108e4db0c:   sub x24, x24, #0x8
[...]
  0x0000000108e4dfa4:   stp x16, x17, [sp, #128]
  0x0000000108e4dfa8:   stp x19, x20, [sp, #144]
  0x0000000108e4dfac:   stp x21, x22, [sp, #160]
[...]
  0x0000000108e4dfbc:   stp x29, x30, [sp, #224]
  0x0000000108e4dfc0:   mov x0, x28
 ;; 0x107E4A06C
  0x0000000108e4dfc4:   mov x9, #0xa06c                 // #41068
  0x0000000108e4dfc8:   movk    x9, #0x7e4, lsl #16
  0x0000000108e4dfcc:   movk    x9, #0x1, lsl #32
  0x0000000108e4dfd0:   blr x9
  0x0000000108e4dfd4:   ldp x2, x3, [sp, #16]
[...]
  0x0000000108e4dff0:   ldp x16, x17, [sp, #128]
  0x0000000108e4dff4:   ldp x19, x20, [sp, #144]
  0x0000000108e4dff8:   ldp x21, x22, [sp, #160]
[...]

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

Commit messages:
 - JDK-8274795: avoid spilling and restoring r18 in macro assembler

Changes: https://git.openjdk.java.net/jdk/pull/5828/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5828&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274795
  Stats: 10 lines in 2 files changed: 8 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5828.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5828/head:pull/5828

PR: https://git.openjdk.java.net/jdk/pull/5828