[PATCH v2] x86: Use asm-goto to implement mutex fast path on x86-64
From: Wedson Almeida Filho
Date: Fri Jun 28 2013 - 07:51:09 EST
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@xxxxxxxxx>
---
arch/x86/include/asm/mutex_64.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/arch/x86/include/asm/mutex_64.h b/arch/x86/include/asm/mutex_64.h
index 68a87b0..c030bee 100644
--- a/arch/x86/include/asm/mutex_64.h
+++ b/arch/x86/include/asm/mutex_64.h
@@ -16,6 +16,20 @@
*
* Atomically decrements @v and calls <fail_fn> if the result is negative.
*/
+#ifdef CC_HAVE_ASM_GOTO
+static inline void __mutex_fastpath_lock(atomic_t *v,
+ void (*fail_fn)(atomic_t *))
+{
+ asm volatile goto(LOCK_PREFIX " decl %0\n"
+ " jns %l[exit]\n"
+ : : "m" (v->counter)
+ : "memory", "cc"
+ : exit);
+ fail_fn(v);
+exit:
+ return;
+}
+#else
#define __mutex_fastpath_lock(v, fail_fn) \
do { \
unsigned long dummy; \
@@ -32,6 +46,7 @@ do { \
: "rax", "rsi", "rdx", "rcx", \
"r8", "r9", "r10", "r11", "memory"); \
} while (0)
+#endif
/**
* __mutex_fastpath_lock_retval - try to take the lock by moving the count
@@ -59,6 +74,20 @@ static inline int __mutex_fastpath_lock_retval(atomic_t *count,
*
* Atomically increments @v and calls <fail_fn> if the result is nonpositive.
*/
+#ifdef CC_HAVE_ASM_GOTO
+static inline void __mutex_fastpath_unlock(atomic_t *v,
+ void (*fail_fn)(atomic_t *))
+{
+ asm volatile goto(LOCK_PREFIX " incl %0\n"
+ " jg %l[exit]\n"
+ : : "m" (v->counter)
+ : "memory", "cc"
+ : exit);
+ fail_fn(v);
+exit:
+ return;
+}
+#else
#define __mutex_fastpath_unlock(v, fail_fn) \
do { \
unsigned long dummy; \
@@ -75,6 +104,7 @@ do { \
: "rax", "rsi", "rdx", "rcx", \
"r8", "r9", "r10", "r11", "memory"); \
} while (0)
+#endif
#define __mutex_slowpath_needs_to_unlock() 1
--
1.7.9.5
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