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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>
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@ -16,6 +16,20 @@
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*
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* Atomically decrements @v and calls <fail_fn> if the result is negative.
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*/
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#ifdef CC_HAVE_ASM_GOTO
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static inline void __mutex_fastpath_lock(atomic_t *v,
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void (*fail_fn)(atomic_t *))
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{
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asm volatile goto(LOCK_PREFIX " decl %0\n"
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" jns %l[exit]\n"
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: : "m" (v->counter)
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: "memory", "cc"
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: exit);
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fail_fn(v);
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exit:
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return;
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}
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#else
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#define __mutex_fastpath_lock(v, fail_fn) \
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do { \
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unsigned long dummy; \
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@ -32,6 +46,7 @@ do { \
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: "rax", "rsi", "rdx", "rcx", \
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"r8", "r9", "r10", "r11", "memory"); \
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} while (0)
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#endif
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/**
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* __mutex_fastpath_lock_retval - try to take the lock by moving the count
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@ -59,6 +74,20 @@ static inline int __mutex_fastpath_lock_retval(atomic_t *count,
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*
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* Atomically increments @v and calls <fail_fn> if the result is nonpositive.
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*/
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#ifdef CC_HAVE_ASM_GOTO
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static inline void __mutex_fastpath_unlock(atomic_t *v,
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void (*fail_fn)(atomic_t *))
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{
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asm volatile goto(LOCK_PREFIX " incl %0\n"
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" jg %l[exit]\n"
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: : "m" (v->counter)
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: "memory", "cc"
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: exit);
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fail_fn(v);
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exit:
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return;
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}
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#else
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#define __mutex_fastpath_unlock(v, fail_fn) \
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do { \
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unsigned long dummy; \
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@ -75,6 +104,7 @@ do { \
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: "rax", "rsi", "rdx", "rcx", \
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"r8", "r9", "r10", "r11", "memory"); \
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} while (0)
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#endif
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#define __mutex_slowpath_needs_to_unlock() 1
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