Serge Rogatch Serge Rogatch - 2 years ago 333
C++ Question

What are _mm_prefetch() locality hints?

The intrinsics guide says only this much about

void _mm_prefetch (char const* p, int i)
:


Fetch the line of data from memory that contains address p to a
location in the cache heirarchy specified by the locality hint i.


Could you list the possible values for
int i
parameter and explain their meanings?

I've found
_MM_HINT_T0
,
_MM_HINT_T1
,
_MM_HINT_T2
,
_MM_HINT_NTA
and
_MM_HINT_ENTA
, but I don't know whether this is an exhaustive list and what they mean.

If processor-specific, I would like to know what they do on Ryzen and latest Intel Core processors.

Answer Source

Sometimes intrinsics are better understood in terms of the instruction they represent rather than as the abstract semantic given in their descriptions.


The full set of the locality constants, as today, is

#define _MM_HINT_T0 1
#define _MM_HINT_T1 2
#define _MM_HINT_T2 3
#define _MM_HINT_NTA 0
#define _MM_HINT_ENTA 4
#define _MM_HINT_ET0 5
#define _MM_HINT_ET1 6
#define _MM_HINT_ET2 7

as described in this paper about Intel Xeon Phi coprocessor prefetching capabilities.

For IA32/AMD processors, the set is reduced to

#define _MM_HINT_T0 1
#define _MM_HINT_T1 2
#define _MM_HINT_T2 3
#define _MM_HINT_NTA 0
#define _MM_HINT_ET1 6

_mm_prefetch is compiled into different instructions based on the architecture and the locality hint

    Hint              IA32/AMD          iMC
_MM_HINT_T0           prefetcht0     vprefetch0
_MM_HINT_T1           prefetcht1     vprefetch1
_MM_HINT_T2           prefetcht2     vprefetch2
_MM_HINT_NTA          prefetchtnta   vprefetchnta
_MM_HINT_ENTA              -         vprefetchenta
_MM_HINT_ET0               -         vprefetchet0
_MM_HINT_ET1          prefetchtwt1   vprefetchet1
_MM_HINT_ET2               -         vprefetchet2

What the (v)prefetch instructions do, if all the requirements are satisfied, is to bring a cache line worth of data into the cache level specified by the locality hint.
The instruction is just a hint, it may be ignored.

When a line is prefetched into level X, the manuals (both Intel and AMD) say that it also fetched into all the other higher level (but for the case X=3).
I'm not sure if this is actually true, I believe that the line is prefetched with-respect-to cache level X and depending on the caching strategies of the higher levels (inclusive vs non-inclusive) it may or may not be present there too.

Another attribute of the (v)prefetch instructions is the non-temporal attribute.
A non-temporal data is unlikely to be reused soon.
In my understanding, NT data is stored in the "streaming load buffers" for the IA32 architecture1 while for the iMC architecture it is stored in the normal cache (using as the way the hardware thread id) but with Most Recent Use replacement policy (so that it will be the next evicted line if needed).
For AMD the manual read that the actual location is implementation dependent, ranging from a software invisible buffer to a dedicated non-temporal cache.

The last attribute of the (v)prefetch instructions is the "intent" attribute or the "eviction" attribute.
Due to the MESI-and-variant protocols, a Request-for-ownership must be made to bring a line into an exclusive state (in order to modify it).
An RFO is just a special read, so prefetching it with an RFO will bring it into the Exclusive state directly (otherwise the first store to it will cancel the benefits of prefetching due to the "delayed" RFO needed), granted we know we will write to it later.

The IA32 and AMD architectures don't support and exclusive non-temporal hint (yet) since the way the non-temporal cache level is implementation-defined.
The iMC architecture allows for it with the locality code _MM_HINT_ENTA.

1 Which I understand to be the WC buffers.


For reference here is the description of the instructions involved

PREFETCHh

Fetches the line of data 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.

PREFETCHWT1

Fetches the line of data from memory that contains the byte specified with the source operand to a location in the cache hierarchy specified by an intent to write hint (so that data is brought into ‘Exclusive’ state via a request for ownership) and a locality hint:

• T1 (temporal data with respect to first level cache)—prefetch data into the second level cache.

VPREFETCHh

                Cache  Temporal    Exclusive state
                 Level
VPREFETCH0       L1     NO          NO
VPREFETCHNTA     L1     YES         NO
VPREFETCH1       L2     NO          NO
VPREFETCH2       L2     YES         NO
VPREFETCHE0      L1     NO          YES
VPREFETCHENTA    L1     YES         YES
VPREFETCHE1      L2     NO          YES
VPREFETCHE2      L2     YES         YES
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