Intel® Architecture Instruction Set Extensions Programming Reference

INTEL® TRANSACTIONAL SYNCHRONIZATION EXTENSIONS
• An XRELEASE prefixed instruction must restore the value of the elided lock to the value it had before the lock
acquisition. This allows hardware to safely elide locks by not adding them to the write-set. The data size and
data address of the lock release (XRELEASE prefixed) instruction must match that of the lock acquire
(XACQUIRE prefixed) and the lock must not cross a cache line boundary.
• Software should not write to the elided lock inside a transactional HLE region with any instruction other than an
XRELEASE prefixed instruction, otherwise it may cause a transactional abort. In addition, recursive locks
(where a thread acquires the same lock multiple times without first releasing the lock) may also cause a transactional
abort. Note that software can observe the result of the elided lock acquire inside the critical section.
Such a read operation will return the value of the write to the lock.
The processor automatically detects violations to these guidelines, and safely transitions to a non-transactional
execution without elision. Since Intel TSX detects conflicts at the granularity of a cache line, writes to data collocated
on the same cache line as the elided lock may be detected as data conflicts by other logical processors eliding
the same lock.
8.3.4 Transactional Nesting
Both HLE and RTM support nested transactional regions. However, a transactional abort restores state to the operation
that started transactional execution: either the outermost XACQUIRE prefixed HLE eligible instruction or the
outermost XBEGIN instruction. The processor treats all nested transactions as one monolithic transaction.
8.3.4.1 HLE Nesting and Elision
Programmers can nest HLE regions up to an implementation specific depth of MAX_HLE_NEST_COUNT. Each logical
processor tracks the nesting count internally but this count is not available to software. An XACQUIRE prefixed HLEeligible
instruction increments the nesting count, and an XRELEASE prefixed HLE-eligible instruction decrements it.
The logical processor enters transactional execution when the nesting count goes from zero to one. The logical
processor attempts to commit only when the nesting count becomes zero. A transactional abort may occur if the
nesting count exceeds MAX_HLE_NEST_COUNT.
In addition to supporting nested HLE regions, the processor can also elide multiple nested locks. The processor
tracks a lock for elision beginning with the XACQUIRE prefixed HLE eligible instruction for that lock and ending with
the XRELEASE prefixed HLE eligible instruction for that same lock. The processor can, at any one time, track up to
a MAX_HLE_ELIDED_LOCKS number of locks. For example, if the implementation supports a
MAX_HLE_ELIDED_LOCKS value of two and if the programmer nests three HLE identified critical sections (by
performing XACQUIRE prefixed HLE eligible instructions on three distinct locks without performing an intervening
XRELEASE prefixed HLE eligible instruction on any one of the locks), then the first two locks will be elided, but the
third won't be elided (but will be added to the transaction’s write-set). However, the execution will still continue
transactionally. Once an XRELEASE for one of the two elided locks is encountered, a subsequent lock acquired
through the XACQUIRE prefixed HLE eligible instruction will be elided.
The processor attempts to commit the HLE execution when all elided XACQUIRE and XRELEASE pairs have been
matched, the nesting count goes to zero, and the locks have satisfied the requirements described earlier. If execution
cannot commit atomically, then execution transitions to a non-transactional execution without elision as if the
first instruction did not have an XACQUIRE prefix.
8.3.4.2 RTM Nesting
Programmers can nest RTM regions up to an implementation specific MAX_RTM_NEST_COUNT. The logical
processor tracks the nesting count internally but this count is not available to software. An XBEGIN instruction
increments the nesting count, and an XEND instruction decrements it. The logical processor attempts to commit
only if the nesting count becomes zero. A transactional abort occurs if the nesting count exceeds
MAX_RTM_NEST_COUNT.
8.3.4.3 Nesting HLE and RTM
HLE and RTM provide two alternative software interfaces to a common transactional execution capability. The
behavior when HLE and RTM are nested together—HLE inside RTM or RTM inside HLE—is implementation specific.
However, in all cases, the implementation will maintain HLE and RTM semantics. An implementation may choose to
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