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

[Andrew Haley]

On Mon, 21 Jul 2025 16:16:44 GMT, Andrew Haley <aph at openjdk.org> wrote:

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

> I agree with you. I proposed to use the switch when JDK-8321371 was being reviewed: [#16994 (comment)](https://github.com/openjdk/jdk/pull/16994#issuecomment-1865147655)
> 
> Frederick (@fbredber) wanted to avoid branches: [#16994 (comment)](https://github.com/openjdk/jdk/pull/16994#issuecomment-1865955740)

You're getting branches, one way or the other.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26387#discussion_r2219661891