x86: Use asm-goto to implement mutex fast path on x86-64
The new implementation allows the compiler to better optimize the code; the
original implementation is still used when the kernel is compiled with older
versions of gcc that don't support asm-goto.
Compiling with gcc 4.7.3, the original mutex_lock() is 60 bytes with the fast
path taking 16 instructions; the new mutex_lock() is 42 bytes, with the fast
path taking 12 instructions.
The original mutex_unlock() is 24 bytes with the fast path taking 7
instructions; the new mutex_unlock() is 25 bytes (because the compiler used
a 2-byte ret) with the fast path taking 4 instructions.
The two versions of the functions are included below for reference.
Old:
ffffffff817742a0 <mutex_lock>:
ffffffff817742a0: 55 push %rbp
ffffffff817742a1: 48 89 e5 mov %rsp,%rbp
ffffffff817742a4: 48 83 ec 10 sub $0x10,%rsp
ffffffff817742a8: 48 89 5d f0 mov %rbx,-0x10(%rbp)
ffffffff817742ac: 48 89 fb mov %rdi,%rbx
ffffffff817742af: 4c 89 65 f8 mov %r12,-0x8(%rbp)
ffffffff817742b3: e8 28 15 00 00 callq ffffffff817757e0 <_cond_resched>
ffffffff817742b8: 48 89 df mov %rbx,%rdi
ffffffff817742bb: f0 ff 0f lock decl (%rdi)
ffffffff817742be: 79 05 jns ffffffff817742c5 <mutex_lock+0x25>
ffffffff817742c0: e8 cb 04 00 00 callq ffffffff81774790 <__mutex_lock_slowpath>
ffffffff817742c5: 65 48 8b 04 25 c0 b7 mov %gs:0xb7c0,%rax
ffffffff817742cc: 00 00
ffffffff817742ce: 4c 8b 65 f8 mov -0x8(%rbp),%r12
ffffffff817742d2: 48 89 43 18 mov %rax,0x18(%rbx)
ffffffff817742d6: 48 8b 5d f0 mov -0x10(%rbp),%rbx
ffffffff817742da: c9 leaveq
ffffffff817742db: c3 retq
ffffffff81774250 <mutex_unlock>:
ffffffff81774250: 55 push %rbp
ffffffff81774251: 48 c7 47 18 00 00 00 movq $0x0,0x18(%rdi)
ffffffff81774258: 00
ffffffff81774259: 48 89 e5 mov %rsp,%rbp
ffffffff8177425c: f0 ff 07 lock incl (%rdi)
ffffffff8177425f: 7f 05 jg ffffffff81774266 <mutex_unlock+0x16>
ffffffff81774261: e8 ea 04 00 00 callq ffffffff81774750 <__mutex_unlock_slowpath>
ffffffff81774266: 5d pop %rbp
ffffffff81774267: c3 retq
New:
ffffffff81774920 <mutex_lock>:
ffffffff81774920: 55 push %rbp
ffffffff81774921: 48 89 e5 mov %rsp,%rbp
ffffffff81774924: 53 push %rbx
ffffffff81774925: 48 89 fb mov %rdi,%rbx
ffffffff81774928: e8 a3 0e 00 00 callq ffffffff817757d0 <_cond_resched>
ffffffff8177492d: f0 ff 0b lock decl (%rbx)
ffffffff81774930: 79 08 jns ffffffff8177493a <mutex_lock+0x1a>
ffffffff81774932: 48 89 df mov %rbx,%rdi
ffffffff81774935: e8 16 fe ff ff callq ffffffff81774750 <__mutex_lock_slowpath>
ffffffff8177493a: 65 48 8b 04 25 c0 b7 mov %gs:0xb7c0,%rax
ffffffff81774941: 00 00
ffffffff81774943: 48 89 43 18 mov %rax,0x18(%rbx)
ffffffff81774947: 5b pop %rbx
ffffffff81774948: 5d pop %rbp
ffffffff81774949: c3 retq
ffffffff81774730 <mutex_unlock>:
ffffffff81774730: 48 c7 47 18 00 00 00 movq $0x0,0x18(%rdi)
ffffffff81774737: 00
ffffffff81774738: f0 ff 07 lock incl (%rdi)
ffffffff8177473b: 7f 0a jg ffffffff81774747 <mutex_unlock+0x17>
ffffffff8177473d: 55 push %rbp
ffffffff8177473e: 48 89 e5 mov %rsp,%rbp
ffffffff81774741: e8 aa ff ff ff callq ffffffff817746f0 <__mutex_unlock_slowpath>
ffffffff81774746: 5d pop %rbp
ffffffff81774747: f3 c3 repz retq
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Link: http://lkml.kernel.org/r/1372420245-60021-1-git-send-email-wedsonaf@gmail.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
1 file changed
