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SSE Cacheability Control instructions
Data referenced by a program can have temporal
(data will be used again) or spatial (data will be in adjacent locations,
such as the same cache line) locality, but some multimedia data types are
referenced once and not reused in the immediate future (called non-temporal
data). Thus, non-temporal data should not overwrite the application’s cached
code and data: the cacheability control instructions enable the programmer
to control caching so that non-temporal accesses will minimize cache pollution.
In addition, the execution engine needs
to be fed such that it does not become stalled waiting for data. SSE
allows the programmer to prefetch data long before its final use to minimize
memory latency. Prior to SSE, read miss latency and execution and
subsequent store miss latency comprised total execution in a serial fashion.
SSE lets read miss latency overlap execution via the use of prefetching,
and it allowes store miss latency to be reduced and overlap execution via
streaming stores.
The following three instructions provide
programmatic control for minimizing cache pollution when writing data to
memory from either MMX or SSE registers.
MASKMOVQ stores data from an MMX register
to the location specified by the EDI register. The most significant bit
in each byte of the second MMX mask register is used to selectively write
the data of the first register on a per-byte basis. This instruction does
not write-allocate (i.e., the processor will not fetch the corresponding
cache line into the cache hierarchy, prior to performing the store), and
so minimizes cache pollution.
MOVNTQ stores data from an MMX register
to memory; this instruction is implicitly weakly-ordered, does not write-allocate,
and minimizes cache pollution.
MOVNTPS stores data from a SIMD floating-point
register to memory. The memory address must be aligned to a 16-byte boundary;
if it is not aligned, a general protection exception will occur. The instruction
is implicitly weakly ordered, does not write-allocate, and minimizes cache
pollution.
PREFETCH loads either non-temporal data
or temporal data in the specified cache level. As this instruction merely
provides a hint to the hardware, it will not generate exceptions or faults.
SFENCE guarantees that every store instruction
that precedes the store fence instruction in program order is globally
visible before any store instruction that follows the fence. The SFENCE
instruction provides an efficient way of ensuring ordering between routines
that produce weakly-ordered results and routines that consume this data.
The use of weakly-ordered memory types can be important under certain data
sharing relation-ships, such as a producer-consumer relationship. The use
of weakly-ordered memory can make the assembling of data more efficient,
but care must be taken to ensure that the consumer obtains the data that
the producer intended it to see.
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