RFR: 8362193: Re-work MacOS/AArch64 SpinPause to handle SB [v2]

[Andrew Haley]

On Mon, 21 Jul 2025 16:11:05 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> The switch-based version is committed: https://github.com/openjdk/jdk/pull/26387/commits/e984fdeb10cc91a04860cbdbe45aacb05c4ffd3e
>
> OK, so this inline assembly was to optimize `SpinPause`, since it sits in hot loop. Have you looked at disassembly for `SpinPause` before/after? On my M1, I see this:
> 
> 
> % lldb -o "disassemble -n SpinPause" -o quit -- build/macosx-aarch64-server-release/images/jdk/lib/server/libjvm.dylib
> 
> # Before
> libjvm.dylib`::SpinPause():
> libjvm.dylib[0x89f0d4] <+0>:  stp    x29, x30, [sp, #-0x10]!
> libjvm.dylib[0x89f0d8] <+4>:  mov    x29, sp
> libjvm.dylib[0x89f0dc] <+8>:  adrp   x8, 1409
> libjvm.dylib[0x89f0e0] <+12>: add    x8, x8, #0x80 ; VM_Version::_spin_wait
> libjvm.dylib[0x89f0e4] <+16>: ldrsw  x8, [x8]
> libjvm.dylib[0x89f0e8] <+20>: lsl    x8, x8, #3
> libjvm.dylib[0x89f0ec] <+24>: adr    x9, 0x89f100 ; <+44> at os_bsd_aarch64.cpp:545:5
> libjvm.dylib[0x89f0f0] <+28>: add    x9, x9, x8
> libjvm.dylib[0x89f0f4] <+32>: br     x9
> libjvm.dylib[0x89f0f8] <+36>: b      0x89f114       ; <+64> at os_bsd_aarch64.cpp:561:5
> libjvm.dylib[0x89f0fc] <+40>: nop    
> libjvm.dylib[0x89f100] <+44>: nop    
> libjvm.dylib[0x89f104] <+48>: b      0x89f114       ; <+64> at os_bsd_aarch64.cpp:561:5
> libjvm.dylib[0x89f108] <+52>: isb    
> libjvm.dylib[0x89f10c] <+56>: b      0x89f114       ; <+64> at os_bsd_aarch64.cpp:561:5
> libjvm.dylib[0x89f110] <+60>: yield  
> libjvm.dylib[0x89f114] <+64>: mov    w0, #0x1 ; =1 
> libjvm.dylib[0x89f118] <+68>: ldp    x29, x30, [sp], #0x10
> libjvm.dylib[0x89f11c] <+72>: ret  
> 
> # After
> libjvm.dylib`::SpinPause():
> libjvm.dylib[0x89f074] <+0>:   stp    x29, x30, [sp, #-0x10]!
> libjvm.dylib[0x89f078] <+4>:   mov    x29, sp
> libjvm.dylib[0x89f07c] <+8>:   adrp   x8, 1409
> libjvm.dylib[0x89f080] <+12>:  add    x8, x8, #0x80 ; VM_Version::_spin_wait
> libjvm.dylib[0x89f084] <+16>:  ldr    w8, [x8]
> libjvm.dylib[0x89f088] <+20>:  add    w8, w8, #0x1
> libjvm.dylib[0x89f08c] <+24>:  cmp    w8, #0x4
> libjvm.dylib[0x89f090] <+28>:  b.hi   0x89f0ec       ; <+120> at os_bsd_aarch64.cpp:551:7
> libjvm.dylib[0x89f094] <+32>:  adrp   x9, 0
> libjvm.dylib[0x89f098] <+36>:  add    x9, x9, #0xfc ; ___lldb_unnamed_symbol66913
> libjvm.dylib[0x89f09c] <+40>:  adr    x10, 0x89f09c ; <+40> at os_bsd_aarch64.cpp
> libjvm.dylib[0x89f0a0] <+44>:  ldrsw  x11, [x9, x8, lsl #2]
> libjvm.dylib[0x89f0a4] <+48>:  add    x10, x10, x11
> libjvm.dylib[0x89f0a8] <+52>:  br     x10
> libjvm.dylib[0x89f0ac] <+56>:  nop    
> libjvm.dylib[0x89f0b0] <+60>:  mov    w0, #0x1 ; =1 
> libjvm.dylib[0x89f0b4] <+64>:  ldp    x29, x30, [sp], #0x10
> libjvm.dylib[0x89f0b8] <+68>:  ret    
> libjvm.dylib[0x89f0bc] <+72>:  isb    
> libjvm.dylib[0x89f0...

Exactly. When writing inline asm, it doesn't much help to try to out-guess the compiler.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26387#discussion_r2219660187