Function Prototype and Macro Definitions

Intel® C++ Compiler Classic Developer Guide and Reference

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ID 767249

Date 3/31/2023

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Function Prototype and Macro Definitions

To use the prototypes and macro definitions shown in Group 1, include the immintrin.h file as follows:

#include <immintrin.h>

Group 1: Function Prototypes:

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:

Group 1: Macro Definitions:

#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)

Group 2: Function Macros

The following function Macros are not included in immintrin.h header file. If you want to use them, you need to define them in your applications.

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.

Parent topic: Intrinsics for Intel® Transactional Synchronization Extensions (Intel® TSX)

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Function Prototype and Macro Definitions