_xbegin

Specifies the start of a restricted transactional memory (RTM) code region and returns a value indicating status. The corresponding Intel® AVX2 instruction is XBEGIN.

Syntax

unsigned int _xbegin(void);

Arguments

None.

Description

Starts a RTM code region and returns a value indicating transaction successfully started or status from a transaction abort.

If the logical processor was not already in transactional execution, then the xbegin instruction causes the logical processor to start transactional execution. The xbegin instruction that transitions the logical processor into transactional execution is referred to as the outermost xbegin instruction.

The xbegin instruction specifies a relative offset to the fallback code path executed following a transactional abort. To promote proper program structure, this is not exposed in C++ code and the intrinsic function operates as if it invoked the following model code:

__inline unsigned int _xbegin() { 
  unsigned status;
  __asm {
    move   eax, 0xFFFFFFFF 
    xbegin _txnL1
    _txnL1:
    move   status, eax
  }
  return status;
}

When a transaction is successfully created the function will return 0xFFFFFFFF, which is never a valid status code for an aborted transaction. If the transaction aborts for any reason, the logical processor discards all architectural register and memory updates performed during the transaction execution and restores the architectural state to that corresponding to the outermost xbegin instruction. The EAX register is then updated with the status code of the aborted transaction, which can be used to transfer control to a fallback handler.

The instruction also specifies a relative offset to compute the address of the fallback code path following a transactional abort. On an RTM abort, the logical processor discards all architectural register and memory updates performed during the RTM execution and restores architectural state to that corresponding to the outermost xbegin instruction. The abort destination operand of the xbegin instruction is targeted to the following instruction so that there is no change in control flow whether the transaction aborts or not.

Returns

Returns value indicating transaction successfully started or status from a transaction abort.

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