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YIELD

YIELD is a hint instruction. Software with a multithreading capability can use a YIELD instruction to indicate to the PE that it is performing a task, for example a spin-lock, that could be swapped out to improve overall system performance. The PE can use this hint to suspend and resume multiple software threads if it supports the capability.

For more information about the recommended use of this instruction, see The YIELD instruction.

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11010101000000110010000000111111
CRmop2

System

YIELD

SystemHintOp op;

case CRm:op2 of
    when '0000 000' op = SystemHintOp_NOP;
    when '0000 001' op = SystemHintOp_YIELD;
    when '0000 010' op = SystemHintOp_WFE;
    when '0000 011' op = SystemHintOp_WFI;
    when '0000 100' op = SystemHintOp_SEV;
    when '0000 101' op = SystemHintOp_SEVL;
    when '0000 110'
        if !HaveDGHExt() then EndOfInstruction();                  // Instruction executes as NOP
        op = SystemHintOp_DGH;
    when '0000 111' SEE "XPACLRI";
    when '0001 xxx'
        case op2 of
            when '000' SEE "PACIA1716";
            when '010' SEE "PACIB1716";
            when '100' SEE "AUTIA1716";
            when '110' SEE "AUTIB1716";
            otherwise EndOfInstruction();                          // Instruction executes as NOP
    when '0010 000'
        if !HaveRASExt() then EndOfInstruction();                  // Instruction executes as NOP
        op = SystemHintOp_ESB;
    when '0010 001'
        if !HaveStatisticalProfiling() then EndOfInstruction();    // Instruction executes as NOP
        op = SystemHintOp_PSB;
    when '0010 010'
        if !HaveSelfHostedTrace() then EndOfInstruction();         // Instruction executes as NOP
        op = SystemHintOp_TSB;
    when '0010 100'
        op = SystemHintOp_CSDB;
    when '0011 xxx'
        case op2 of
            when '000' SEE "PACIAZ";
            when '001' SEE "PACIASP";
            when '010' SEE "PACIBZ";
            when '011' SEE "PACIBSP";
            when '100' SEE "AUTIAZ";
            when '101' SEE "AUTHASP";
            when '110' SEE "AUTIBZ";
            when '111' SEE "AUTIBSP";
    when '0100 xx0'
        op = SystemHintOp_BTI;
        // Check branch target compatibility between BTI instruction and PSTATE.BTYPE
        SetBTypeCompatible(BTypeCompatible_BTI(op2<2:1>));
    otherwise EndOfInstruction();                                 // Instruction executes as NOP

Operation

case op of
    when SystemHintOp_YIELD
        Hint_Yield();

    when SystemHintOp_DGH
        Hint_DGH();

    when SystemHintOp_WFE
        if IsEventRegisterSet() then
            ClearEventRegister();
        else
            if PSTATE.EL == EL0 then
                // Check for traps described by the OS which may be EL1 or EL2.
                AArch64.CheckForWFxTrap(EL1, TRUE);
            if PSTATE.EL IN {EL0, EL1} && EL2Enabled() && !IsInHost() then
                // Check for traps described by the Hypervisor.
                AArch64.CheckForWFxTrap(EL2, TRUE);
            if HaveEL(EL3) && PSTATE.EL != EL3 then
                // Check for traps described by the Secure Monitor.
                AArch64.CheckForWFxTrap(EL3, TRUE);
            WaitForEvent();

    when SystemHintOp_WFI
        if !InterruptPending() then
            if PSTATE.EL == EL0 then
                // Check for traps described by the OS which may be EL1 or EL2.
                AArch64.CheckForWFxTrap(EL1, FALSE);
            if PSTATE.EL IN {EL0, EL1} && EL2Enabled() && !IsInHost() then
                // Check for traps described by the Hypervisor.
                AArch64.CheckForWFxTrap(EL2, FALSE);
            if HaveEL(EL3) && PSTATE.EL != EL3 then
                // Check for traps described by the Secure Monitor.
                AArch64.CheckForWFxTrap(EL3, FALSE);
            WaitForInterrupt();

    when SystemHintOp_SEV
        SendEvent();

    when SystemHintOp_SEVL
        SendEventLocal();

    when SystemHintOp_ESB
        SynchronizeErrors();
        AArch64.ESBOperation();
        if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then AArch64.vESBOperation();
        TakeUnmaskedSErrorInterrupts();

    when SystemHintOp_PSB
        ProfilingSynchronizationBarrier();

    when SystemHintOp_TSB
        TraceSynchronizationBarrier();

    when SystemHintOp_CSDB
        ConsumptionOfSpeculativeDataBarrier();

    when SystemHintOp_BTI
        SetBTypeNext('00');

    otherwise // do nothing