Arm® A32/T32 Instruction Set Architecture: Armv8, for Armv8-A architecture profile Documentation

CSDB

Consumption of Speculative Data Barrier is a memory barrier that controls speculative execution and data value prediction.

No instruction other than branch instructions and instructions that write to the PC appearing in program order after the CSDB can be speculatively executed using the results of any:

• Data value predictions of any instructions.
• PSTATE.{N,Z,C,V} predictions of any instructions other than conditional branch instructions and conditional instructions that write to the PC appearing in program order before the CSDB that have not been architecturally resolved.

For purposes of the definition of CSDB, PSTATE.{N,Z,C,V} is not considered a data value. This definition permits:

• Control flow speculation before and after the CSDB.
• Speculative execution of conditional data processing instructions after the CSDB, unless they use the results of data value or PSTATE.{N,Z,C,V} predictions of instructions appearing in program order before the CSDB that have not been architecturally resolved.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

A1

if cond != '1110' then UNPREDICTABLE;      // CSDB must be encoded with AL condition

CONSTRAINED UNPREDICTABLE behavior

If cond != '1110', then one of the following behaviors must occur:

• The instruction is undefined.
• The instruction executes as NOP.
• The instruction executes unconditionally.
• The instruction executes conditionally.

T1

if InITBlock() then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If InITBlock(), then one of the following behaviors must occur:

• The instruction is undefined.
• The instruction executes as NOP.
• The instruction executes unconditionally.
• The instruction executes conditionally.

For more information about the constrained unpredictable behavior, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

if ConditionPassed() then
    EncodingSpecificOperations();

    ConsumptionOfSpeculativeDataBarrier();