Opcode |
Instruction |
Description |
|---|---|---|
|
0F AE /5 |
LFENCE |
Serializes load operations. |
Performs a serializing operation on all load instructions that were issued prior the LFENCE instruction. This serializing operation guarantees that every load instruction that precedes in program order the LFENCE instruction is globally visible before any load instruction that follows the LFENCE instruction is globally visible. The LFENCE instruction is ordered with respect to load instructions, other LFENCE instructions, any MFENCE instructions, and any serializing instructions (such as the CPUID instruction). It is not ordered with respect to store instructions or the SFENCE instruction.
Weakly ordered memory types can enable higher performance through such techniques as out-of-order issue and speculative reads. The degree to which a consumer of data recognizes or knows that the data is weakly ordered varies among applications and may be unknown to the producer of this data. The LFENCE instruction provides a performance-efficient way of ensuring ordering between routines that produce weakly-ordered results and routines that consume this data.
It should be noted that processors are free to speculatively fetch and cache data from system memory regions that are assigned a memory-type that permits speculative reads (that is, the WB, WC, and WT memory types). The PREFETCHh instruction is considered a hint to this speculative behavior. Because this speculative fetching can occur at any time and is not tied to instruction execution, the LFENCE instruction is not ordered with respect to PREFETCHh or any of the specualtive fetching mechanisms (that is, data could be speculative loaded into the cache just before, during, or after the execution of an LFENCE instruction).
Wait_On_Following_Loads_Until(preceding_loads_globally_visible);
void_mm_lfence(void)
None.
For details, see Volume 2A and Volume 2B of the Intel(R) 64 and IA-32 Intel Architecture Software Developer's Manual. For the latest updates on the instruction set information, go to the web site.