PREFETCHh

Prefetch Data Into Caches

stableVMJITAOTinstruction

Encodings

Operand encoding

Each mode is a value of the Op/En column above. It says which field of the encoded instruction carries each operand, in the order they are written, and whether the instruction reads it, writes it or both.

Measured cost

Not measured: it does not run on 64-bit processors.

Description#

Fetches the line of data or code (instructions' bytes) from memory that contains the byte specified with the source operand to a location in the cache hierarchy specified by a locality hint:

• T0 (temporal data)--prefetch data into all levels of the cache hierarchy. T1 (temporal data with respect to first level cache misses)--prefetch data into level 2 cache and higher. T2 (temporal data with respect to second level cache misses)--prefetch data into level 3 cache and higher, or

an implementation-specific choice.

• NTA (non-temporal data with respect to all cache levels)--prefetch data into non-temporal cache structure and

into a location close to the processor, minimizing cache pollution.

• IT0 (temporal code)--prefetch code into all levels of the cache hierarchy. * IT1 (temporal code with respect to first level cache misses)--prefetch code into all but the first-level of the

cache hierarchy.

The source operand is a byte memory location. (The locality hints are encoded into the machine level instruction using bits 3 through 5 of the ModR/M byte.) Some locality hints may prefetch only for RIP-relative memory addresses; see additional details below. The address to prefetch is NextRIP + 32-bit displacement, where NextRIP is the first byte of the instruction that follows the prefetch instruction itself.

If the line selected is already present in the cache hierarchy at a level closer to the processor, no data movement occurs. Prefetches from uncacheable or WC memory are ignored.

The PREFETCHh instruction is merely a hint and does not affect program behavior. If executed, this instruction moves data closer to the processor in anticipation of future use.

The implementation of prefetch locality hints is implementation-dependent, and can be overloaded or ignored by a processor implementation. The amount of data or code lines prefetched is also processor implementation-dependent. It will, however, be a minimum of 32 bytes. Additional details of the implementation-dependent locality hints are described in Section 7.4 of Intel(R) 64 and IA-32 Architectures Optimization Reference Manual.

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). A

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, a PREFETCHh instruction is not ordered with respect to the fence instructions (MFENCE, SFENCE, and LFENCE) or locked memory references. A PREFETCHh instruction is also unordered with respect to CLFLUSH and CLFLUSHOPT instructions, other PREFETCHh instructions, or any other general instruction. It is ordered with respect to serializing instructions such as CPUID, WRMSR, OUT, and MOV CR.

PREFETCHIT0/1 can be used in 64-bit mode with RIP-relative addressing; they remain NOPs otherwise. For optimal performance, the addresses used with these instructions should be the starting byte of a real instruction.

PREFETCHIT0/1 instructions are enumerated by CPUID.07H.01H:EDX.PREFETCHI[14].The encodings remain NOPs in processors that do not enumerate these instructions.

Operation#

FETCH (m8);

Intel C/C++ compiler intrinsics#

void _mm_prefetch(char *p, int i) The argument "*p" gives the address of the byte (and corresponding cache line) to be prefetched. The value "i" gives a constant (_MM_HINT_T0, _MM_HINT_T1, _MM_HINT_T2, _MM_HINT_NTA, _MM_HINT_IT0, or _MM_HINT_IT1) that specifies the type of prefetch operation to be performed.;

Numeric Exceptions#

None.

Exceptions

Sources

• This instruction in the Intel manual