Ingo Molnar a écrit :
add the x86_64 version of mutex.h, optimized in assembly.
+/**
+ * __mutex_fastpath_lock - decrement and call function if negative
+ * @v: pointer of type atomic_t
+ * @fn: function to call if the result is negative
+ *
+ * Atomically decrements @v and calls <fn> if the result is negative.
+ */
+#define __mutex_fastpath_lock(v, fn_name) \
+do { \
+ /* type-check the function too: */ \
+ fastcall void (*__tmp)(atomic_t *) = fn_name; \
+ unsigned long dummy; \
+ \
+ (void)__tmp; \
+ typecheck(atomic_t *, v); \
+ \
+ __asm__ __volatile__( \
+ LOCK " decl (%%rdi) \n" \
+ " js 2f \n" \
+ "1: \n" \
+ \
+ LOCK_SECTION_START("") \
+ "2: call "#fn_name" \n" \
+ " jmp 1b \n" \
+ LOCK_SECTION_END \
+ \
+ :"=D" (dummy) \
+ : "D" (v) \
+ : "rax", "rsi", "rdx", "rcx", \
+ "r8", "r9", "r10", "r11", "memory"); \
+} while (0)
Hi Ingo
I do think this assembly is not very fair.
It has an *insane* register pressure for the compiler :
The fast path is thus not so fast.
Compare with the include/asm-x86_64/semaphore.h
__asm__ __volatile__(
"# atomic down operation\n\t"
LOCK "decl %0\n\t" /* --sem->count */
"js 2f\n"
"1:\n"
LOCK_SECTION_START("")
"2:\tcall __down_failed\n\t"
"jmp 1b\n"
LOCK_SECTION_END
:"=m" (sem->count)
:"D" (sem)
:"memory");
Only one register is mandatory (%rdi), instead of nine.
Two solutions :
(This one has no register constraint, but the slowpath is 'long')
It also requires the mutex is not on the stack.
#define PUSH_SCRATCH "push %%rdi; push %%rax; push %%rsi;" \
"push %%rdx; push %%rcx; push %%r8;" \
"push %%r9; push %%r10; push %%r11\n"
#define POP_SCRATCH "pop %%r11; pop %%r10; pop %%r9;" \
"pop %%r8; pop %%rcx; pop %%rdx;" \
"pop %%rsi; pop %%rax"; pop %%rdi\n"
__asm__ __volatile__( \
LOCK " decl %0 \n" \
" js 2f \n" \
"1: \n" \
\
LOCK_SECTION_START("") \
"2:" PUSH_SCRATCH \
"lea %0,%%rdi \n" \
"call "#fn_name" \n" \
POP_SCRATCH \
" jmp 1b \n" \
LOCK_SECTION_END \
\
:"=m" (v->count) \
: "m" (v->count) \
: "memory"); \
Or call a wrapper that does the PUSH/POP thing.
Thank you
Eric
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