hint

Hint

HINT #<imm>

Provides a hint to the processor (e.g., NOP, YIELD).

Details

Hint provides a processor hint via an immediate value; the processor may optimize behavior based on the hint type but is not required to act on it. Common hints include NOP (0x0), YIELD (0x1), WFE (0x2), and WFI (0x3). Condition flags (N, Z, C, V) are unaffected. The instruction executes in all exception levels without privilege requirements.

Pseudocode Operation

case imm of
  0: NOP
  1: Yield()
  2: WaitForEvent()
  3: WaitForInterrupt()
  otherwise: NOP

Example

HINT #16

Encoding

Binary Layout

11010101000000110010

CRm

op2

11111

Format System

Opcode 0xD503201F

Extension Base

Operands

imm
Hint ID

Reference (Arm A64 ISA)

Instruction Forms

Encoding	Instruction		ISA		Bit pattern
0xD503201F	HINT #<imm>		A64		11010101000000110010 \| CRm \| op2 \| 11111

Description

Hint instruction is for the instruction set space that is reserved for architectural hint instructions. Some encodings described here are not allocated in this revision of the architecture, and behave as NOPs. These encodings might be allocated to other hint functionality in future revisions of the architecture and therefore must not be used by software.

Operation

case op of
    when SystemHintOp_YIELD
        Hint_Yield();

    when SystemHintOp_DGH
        Hint_DGH();

    when SystemHintOp_WFE
        integer localtimeout = 1 << 64;    // No local timeout event is generated
        Hint_WFE(localtimeout, WFxType_WFE);

    when SystemHintOp_WFI
        integer localtimeout = 1 << 64;    // No local timeout event is generated
        Hint_WFI(localtimeout, WFxType_WFI);

    when SystemHintOp_SEV
        SendEvent();

    when SystemHintOp_SEVL
        SendEventLocal();

    when SystemHintOp_ESB
        if IsFeatureImplemented(FEAT_TME) && TSTATE.depth > 0 then
            FailTransaction(TMFailure_ERR, FALSE);
        SynchronizeErrors();
        AArch64.ESBOperation();
        if PSTATE.EL IN {EL0, EL1} && EL2Enabled() then AArch64.vESBOperation();
        TakeUnmaskedSErrorInterrupts();

    when SystemHintOp_PSB
        ProfilingSynchronizationBarrier();

    when SystemHintOp_TSB
        TraceSynchronizationBarrier();

    when SystemHintOp_GCSB
        GCSSynchronizationBarrier();

    when SystemHintOp_CHKFEAT
        X[16, 64] = AArch64.ChkFeat(X[16, 64]);

    when SystemHintOp_CSDB
        ConsumptionOfSpeculativeDataBarrier();

    when SystemHintOp_CLRBHB
        Hint_CLRBHB();

    when SystemHintOp_BTI
        SetBTypeNext('00');

    when SystemHintOp_NOP
        return;    // do nothing

    otherwise
        Unreachable();