* Wedson Almeida Filho <wedso...@gmail.com> wrote:
> 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
Very nice!
One detail:
> +#if defined(CC_HAVE_ASM_GOTO)
> +#if defined(CC_HAVE_ASM_GOTO)
Please change these to #ifdef.
Thanks,
Ingo
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