Function Prototype and Macro Definitions for RTM
The following function prototypes are included in the immintrin.h header file:
unsigned int _xbegin(void);
void _xend(void);
void _xabort(const unsigned int imm);
unsigned char _xtest(void);
The following macro definitions are included in the immintrin.h header file:
#define _XBEGIN_STARTED (~0u) #define _XABORT_EXPLICIT (1 << 0) #define _XABORT_RETRY (1 << 1) #define _XABORT_CONFLICT (1 << 2) #define _XABORT_CAPACITY (1 << 3) #define _XABORT_DEBUG (1 << 4) #define _XABORT_NESTED (1 << 5)
Note
The following function Macros are not included in immintrin.h header file. Users can consider defining and using them in applications as necessary.
For the HW with RTM support
#define __try_transaction(x) if ((x =_xbegin()) == _XBEGIN_STARTED) #define __try_else _xend() } else #define __transaction_abort(c) _xabort(c)
For the HW with no RTM support
#define __try_transaction(x) if (0) {
#define __try_else } else
#define __transaction_abort(c)
x is an unsigned integer type local variable for programmers to access RTM transaction abort code and holds the return value of _xbegin(). c is an unsigned integer compile-time constant value that is returned in the upper bits of x when _xabort(c) is executed.
A usage sample code of macros |
foo() { // user macros
int status;
__try_transaction (status) {
,,, ,,, ,,,
transaction code ….
}
__try_else {
if (status & _XABORT_CONFLICT) {
… code
}
}
}
|
Pseudo-ASM code |
foo() { or eax 0xffffffff
xbegin L1 L1: mov status, eax
cmp eax 0xffffffff jnz
L2 transaction code
// when abort happens, HW restarts from L1
xend jmp L3 L2: abort
handler code L3: ret
}
|
The compiler will convert the macros to the instruction sequence with a proper branching for speculative execution path and alternative execution path.
The above example is similar to the usage example, except __try_transaction and __try_else macros are used instead of RTM intrinsic functions.