# XACQUIRE/XRELEASE — Hardware Lock Elision Prefix Hints

Opcode/Instruction | 64/32bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | ---
F2 XACQUIRE | V/V | HLE1 | A hint used with an “XACQUIRE-enabled“ instruction to start lock elision on the instruction memory operand address.
F3 XRELEASE | V/V | HLE | A hint used with an “XRELEASE-enabled“ instruction to end lock elision on the instruction memory operand address.

> 1. Software is not required to check the HLE feature flag to use XACQUIRE or XRELEASE, as they are treated as regular prefix if HLE feature flag reports 0.

## Description ¶

The XACQUIRE prefix is a hint to start lock elision on the memory address specified by the instruction and the XRELEASE prefix is a hint to end lock elision on the memory address specified by the instruction.

The XACQUIRE prefix hint can only be used with the following instructions (these instructions are also referred to as XACQUIRE-enabled when used with the XACQUIRE prefix):

- Instructions with an explicit LOCK prefix (F0H) prepended to forms of the instruction where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCHG8B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG.
- The XCHG instruction either with or without the presence of the LOCK prefix.

The XRELEASE prefix hint can only be used with the following instructions (also referred to as XRELEASE-enabled when used with the XRELEASE prefix):

- Instructions with an explicit LOCK prefix (F0H) prepended to forms of the instruction where the destination operand is a memory operand: ADD, ADC, AND, BTC, BTR, BTS, CMPXCHG, CMPXCHG8B, DEC, INC, NEG, NOT, OR, SBB, SUB, XOR, XADD, and XCHG.
- The XCHG instruction either with or without the presence of the LOCK prefix.
- The “MOV mem, reg” (Opcode 88H/89H) and “MOV mem, imm” (Opcode C6H/C7H) instructions. In these cases, the XRELEASE is recognized without the presence of the LOCK prefix.

The lock variables must satisfy the guidelines described in Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, Section 16.3.3, for elision to be successful, otherwise an HLE abort may be signaled.

If an encoded byte sequence that meets XACQUIRE/XRELEASE requirements includes both prefixes, then the HLE semantic is determined by the prefix byte that is placed closest to the instruction opcode. For example, an F3F2C6 will not be treated as a XRELEASE-enabled instruction since the F2H (XACQUIRE) is closest to the instruction opcode C6. Similarly, an F2F3F0 prefixed instruction will be treated as a XRELEASE-enabled instruction since F3H (XRELEASE) is closest to the instruction opcode.

Intel 64 and IA-32 Compatibility

The effect of the XACQUIRE/XRELEASE prefix hint is the same in non-64-bit modes and in 64-bit mode.

For instructions that do not support the XACQUIRE hint, the presence of the F2H prefix behaves the same way as prior hardware, according to

- REPNE/REPNZ semantics for string instructions,
- Serve as SIMD prefix for legacy SIMD instructions operating on XMM register
- Cause #UD if prepending the VEX prefix.
- Undefined for non-string instructions or other situations.

For instructions that do not support the XRELEASE hint, the presence of the F3H prefix behaves the same way as in prior hardware, according to

- REP/REPE/REPZ semantics for string instructions,
- Serve as SIMD prefix for legacy SIMD instructions operating on XMM register
- Cause #UD if prepending the VEX prefix.
- Undefined for non-string instructions or other situations.

## Operation ¶

### XACQUIRE ¶

    IF XACQUIRE-enabled instruction
        THEN
            IF (HLE_NEST_COUNT < MAX_HLE_NEST_COUNT) THEN
                HLE_NEST_COUNT++
                IF (HLE_NEST_COUNT = 1) THEN
                    HLE_ACTIVE := 1
                    IF 64-bit mode
                        THEN
                            restartRIP := instruction pointer of the XACQUIRE-enabled instruction
                        ELSE
                            restartEIP := instruction pointer of the XACQUIRE-enabled instruction
                    FI;
                    Enter HLE Execution (* record register state, start tracking memory state *)
                FI; (* HLE_NEST_COUNT = 1*)
                IF ElisionBufferAvailable
                    THEN
                        Allocate elision buffer
                        Record address and data for forwarding and commit checking
                        Perform elision
                    ELSE
                        Perform lock acquire operation transactionally but without elision
                FI;
            ELSE (* HLE_NEST_COUNT = MAX_HLE_NEST_COUNT*)
                    GOTO HLE_ABORT_PROCESSING
            FI;
        ELSE
            Treat instruction as non-XACQUIRE F2H prefixed legacy instruction
    FI;

### XRELEASE ¶

    IF XRELEASE-enabled instruction
        THEN
            IF (HLE_NEST_COUNT > 0)
                THEN
                    HLE_NEST_COUNT--
                    IF lock address matches in elision buffer THEN
                        IF lock satisfies address and value requirements THEN
                            Deallocate elision buffer
                        ELSE
                            GOTO HLE_ABORT_PROCESSING
                        FI;
                    FI;
                    IF (HLE_NEST_COUNT = 0)
                        THEN
                            IF NoAllocatedElisionBuffer
                                THEN
                                    Try to commit transactional execution
                                    IF fail to commit transactional execution
                                        THEN
                                            GOTO HLE_ABORT_PROCESSING;
                                        ELSE (* commit success *)
                                            HLE_ACTIVE := 0
                                    FI;
                                ELSE
                                    GOTO HLE_ABORT_PROCESSING
                            FI;
                    FI;
            FI; (* HLE_NEST_COUNT > 0 *)
        ELSE
            Treat instruction as non-XRELEASE F3H prefixed legacy instruction
    FI;
    (* For any HLE abort condition encountered during HLE execution *)
    HLE_ABORT_PROCESSING:
        HLE_ACTIVE := 0
        HLE_NEST_COUNT := 0
        Restore architectural register state
        Discard memory updates performed in transaction
        Free any allocated lock elision buffers
        IF 64-bit mode
            THEN
                RIP := restartRIP
            ELSE
                EIP := restartEIP
        FI;
        Execute and retire instruction at RIP (or EIP) and ignore any HLE hint
    END

## SIMD Floating-Point Exceptions ¶

None.

## Other Exceptions ¶

#GP(0) | If the use of prefix causes instruction length to exceed 15 bytes.