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SB

Speculation Barrier is a barrier that controls speculation.

The semantics of the Speculation Barrier are that the execution, until the barrier completes, of any instruction that appears later in the program order than the barrier:

  • Cannot be performed speculatively to the extent that such speculation can be observed through side-channels as a result of control flow speculation or data value speculation.
  • Can be speculatively executed as a result of predicting that a potentially exception generating instruction has not generated an exception.

In particular, any instruction that appears later in the program order than the barrier cannot cause a speculative allocation into any caching structure where the allocation of that entry could be indicative of any data value present in memory or in the registers.

The SB instruction:

  • Cannot be speculatively executed as a result of control flow speculation or data value speculation.
  • Can be speculatively executed as a result of predicting that a potentially exception generating instruction has not generated an exception. The potentially exception generating instruction can complete once it is known not to be speculative, and all data values generated by instructions appearing in program order before the SB instruction have their predicted values confirmed.

When the prediction of the instruction stream is not informed by data taken from the register outputs of the speculative execution of instructions appearing in program order after an uncompleted SB instruction, the SB instruction has no effect on the use of prediction resources to predict the instruction stream that is being fetched.

313029282726252423222120191817161514131211109876543210
11010101000000110011(0)(0)(0)(0)11111111
CRmopc

System

SB

MemBarrierOp op;
MBReqDomain domain;
MBReqTypes types;

if HaveSBExt() && CRm<3:0> == '0000' then
    op = MemBarrierOp_SB;
else
    UNDEFINED;

case CRm<3:2> of
    when '00' domain = MBReqDomain_OuterShareable;
    when '01' domain = MBReqDomain_Nonshareable;
    when '10' domain = MBReqDomain_InnerShareable;
    when '11' domain = MBReqDomain_FullSystem;

case CRm<1:0> of
    when '01' types = MBReqTypes_Reads;
    when '10' types = MBReqTypes_Writes;
    when '11' types = MBReqTypes_All;
    otherwise
        if CRm<3:2> == '01' then
            op = MemBarrierOp_PSSBB;
        elsif HaveSBExt() && CRm<3:2> == '00' then
            op = MemBarrierOp_SB;
        else
            types = MBReqTypes_All;
            domain = MBReqDomain_FullSystem;

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