XEND

XEND ends an RTM transaction that XBEGIN started. At the outermost level it commits: every store of the transaction becomes visible to other processors at once, and a commit orders memory as a LOCK-prefixed instruction does, even for a transaction without memory operations. If the commit fails, the transaction aborts like any other: registers and memory return to their state at XBEGIN, and execution goes to XBEGIN’s fallback address with the status in EAX. An inner XEND only lowers the nesting count, which software cannot read; nothing commits until the count is back at zero.

Outside a transaction#

XEND raises #GP(0) when no transaction is running. The fallback path is such a place: after an abort the code runs normally, so XEND belongs on the transactional path alone. In the usual shape, _xend() ends the branch taken when _xbegin() returned _XBEGIN_STARTED, and the other branch does the same work under a lock; GCC 14 and Clang 23 at -O2 with -mrtm keep that shape, with XEND right after the transactional work. A transaction that checks the lock is free, and calls _xabort if not, has the lock in its read set, so a thread that takes the lock on the fallback path aborts every transaction running at that moment.

XEND inside a region of XSUSLDTRK, which suspends the tracking of loads, aborts the transaction instead of committing it.

Availability#

XEND needs RTM, CPUID.(EAX=7,ECX=0):EBX[11], like XBEGIN, whose page says where Intel disables it; without that bit it raises #UD. Where Intel’s microcode makes RTM always abort, XEND is never reached on the transactional path, since every XBEGIN goes straight to the fallback. AMD’s manual defines no XEND, and both of Zen 5’s cases raised #UD.

Forms

Latency and throughput

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

if not cpuid_leaf(7, 0)[1] >> 11 & 1:
    fault(UD)                    # CPUID: no RTM
if not rtm_active:
    fault(GP, 0)                 # no transaction to end
if ldtrk_suspended:
    tx_abort(UNDEFINED)          # inside a region of XSUSLDTRK
elif rtm_nest > 1:
    rtm_nest = rtm_nest - 1      # an inner one: nothing commits
elif tx_commit():                # the outermost: every store of the
    rtm_nest = 0                 # transaction visible at once,
    rtm_active = 0               # ordered as a LOCK instruction is
else:
    tx_abort(UNDEFINED)          # it could not commit

Operands

None: XED lists no operands for this form.

Flags

None: XED records no flag that the forms test or change.

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to XEND, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

More RTM instructions

• XABORT
• XBEGIN
• XTEST