CPU指令重排/内存乱序

为什么80%的码农都做不了架构师？>>>   

LOCKED ATOMIC OPERATIONS

• Guaranteed atomic operations
• Bus locking, using the LOCK# signal and the LOCK instruction prefix
• Cache coherency protocols that ensure that atomic operations can be carried out on cached data structures (cache lock); this mechanism is present in the Pentium 4, Intel Xeon, and P6 family processors

MEMORY ORDERING

single processor

• Reads are not reordered with other reads.
• Writes are not reordered with older reads.
• Writes to memory are not reordered with other writes, with the following exceptions:
  • streaming stores (writes) executed with the non-temporal move instructions (MOVNTI, MOVNTQ, MOVNTDQ, MOVNTPS, and MOVNTPD); and
  • string operations (see Section 8.2.4.1).
• No write to memory may be reordered with an execution of the CLFLUSH instruction; a write may be reordered with an execution of the CLFLUSHOPT instruction that flushes a cache line other than the one being written.Executions of the CLFLUSH instruction are not reordered with each other. Executions of CLFLUSHOPT that access different cache lines may be reordered with each other. An execution of CLFLUSHOPT may be reordered with an execution of CLFLUSH that accesses a different cache line.
• Reads may be reordered with older writes to different locations but not with older writes to the same location.
• Reads or writes cannot be reordered with I/O instructions, locked instructions, or serializing instructions.
• Reads cannot pass earlier LFENCE and MFENCE instructions.
• Writes and executions of CLFLUSH and CLFLUSHOPT cannot pass earlier LFENCE, SFENCE, and MFENCE instructions.
• LFENCE instructions cannot pass earlier reads.
• SFENCE instructions cannot pass earlier writes or executions of CLFLUSH and CLFLUSHOPT.
• MFENCE instructions cannot pass earlier reads, writes, or executions of CLFLUSH and CLFLUSHOPT.

multipule processor

• Individual processors use the same ordering principles as in a single-processor system.
• Writes by a single processor are observed in the same order by all processors.
• Writes from an individual processor are NOT ordered with respect to the writes from other processors.
• Memory ordering obeys causality (memory ordering respects transitive visibility).
• Any two stores are seen in a consistent order by processors other than those performing the stores
• Locked instructions have a total order

examples

• Neither Loads Nor Stores Are Reordered with Like Operations
• Stores Are Not Reordered With Earlier Loads
• Loads May Be Reordered with Earlier Stores to Different Locations
• Loads Are not Reordered with Older Stores to the Same Location

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