1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
|
[[zihintpause]]
== "Zihintpause" Extension for Pause Hint, Version 2.0
The PAUSE instruction is a HINT that indicates the current hart's rate
of instruction retirement should be temporarily reduced or paused. The
duration of its effect must be bounded and may be zero.
(((PAUSE, HINT)))
(((HINT, PAUSE)))
[NOTE]
====
Software can use the PAUSE instruction to reduce energy consumption
while executing spin-wait code sequences. Multithreaded cores might
temporarily relinquish execution resources to other harts when PAUSE is
executed. It is recommended that a PAUSE instruction generally be
included in the code sequence for a spin-wait loop.
(((PAUSE, energy consumption)))
A future extension might add primitives similar to the x86 MONITOR/MWAIT
instructions, which provide a more efficient mechanism to wait on writes
to a specific memory location. However, these instructions would not
supplant PAUSE. PAUSE is more appropriate when polling for non-memory
events, when polling for multiple events, or when software does not know
precisely what events it is polling for.
The duration of a PAUSE instruction's effect may vary significantly
within and among implementations. In typical implementations this
duration should be much less than the time to perform a context switch,
probably more on the rough order of an on-chip cache miss latency or a
cacheless access to main memory.
(((PAUSE, duration)))
A series of PAUSE instructions can be used to create a cumulative delay
loosely proportional to the number of PAUSE instructions. In spin-wait
loops in portable code, however, only one PAUSE instruction should be
used before re-evaluating loop conditions, else the hart might stall
longer than optimal on some implementations, degrading system
performance.
====
PAUSE is encoded as a FENCE instruction with _pred_=`W`, _succ_=`0`, _fm_=`0`,
_rd_=`x0`, and _rs1_=`x0`.
//include::images/wavedrom/zihintpause-hint.adoc[]
//[zihintpause-hint]
//.Zihintpause fence instructions
[NOTE]
====
PAUSE is encoded as a hint within the FENCE opcode because some
implementations are expected to deliberately stall the PAUSE instruction
until outstanding memory transactions have completed. Because the
successor set is null, however, PAUSE does not _mandate_ any particular
memory ordering—hence, it truly is a HINT.
(((PAUSE, encoding)))
Like other FENCE instructions, PAUSE cannot be used within LR/SC
sequences without voiding the forward-progress guarantee.
(((PAUSE, LR/RC sequences)))
The choice of a predecessor set of W is arbitrary, since the successor
set is null. Other HINTs similar to PAUSE might be encoded with other
predecessor sets.
====
|
