These are all important considerations.
However, what they have in common when considering Allen's
question:
This discussion is
bringing up an issue that needs wider discussion about
extensions in general.
is that they are all
tactical considerations are in the context of our current
framework of instruction space allocation. What we will find is
that these trade-off considerations will reinforce the dilemma
that Allen raises. How do we manage these conflicting
"necessities/requirements" of different target environments.
I have hinted at it
already, we need not only tactical analysis of feature tradeoff
in different domains but a strategic approach to support them.
The concern is nothing
new. It has been raised, if only obliquely, many times prior on
the [google]
groups.riscv.org
(-dev, -sw especially) and
lists.riscv.org TG threads.
The vector group,
especially, has grappled with it in the context of current V
encoding being a subset of a [hypothetical] 64 bit encoding.
Specific proposals have
been mentioned, but there was then no political will or perhaps
more fairly, no common perception that there was a compelling
reason to work systematically to address it. The [then] common
thinking was that 48 and 64 bit instruction spaces will be used
as 32 and 16 bit are exhausted, and everyone will be happy.
Well, that naive hope has not materialized and many are
envisioning clashes that will hurt RISCV progress, either
fragmentation or stagnation, as tactical approaches and
considerations are implement or debated.
Previously two major
strategic approaches were hinted at, even if they were not
outright proposed.
Hardware Support - this
has been explicitly proposed in many flavours: and is currently
in the minds of many.
The idea is a mode
shift analogous to
arm's
transition to thumb and
back and
intel's myriad
of operating modes: real, protected, virtual, long and their
disparate instantiations.
I agree that
implementations should have considerable freedom on how to
provide hardware select-able functionality.
However, a
proposed framework to support that should be provided by
RISV.org.
Recent discussion
and document tweaks about misa (Machine ISA register) suggest
that this mechanism,
though
valuable, is inadequate as robust support for the explosion of
features.
An expanded
framework will be necessary, perhaps along the lines of the two
level performance counters definitions.
The conflict with
overlapping mappings of groups of instructions to the same
encoding space is not easily addressed by this mechanism.
which leads us to
Software Support:
The Generalized
Proposal:
All future extensions
are not mapped to a fixed exclusive universal encoding,
but rather to
appropriately sized [based initially off 32 isize] minor
[22-bit], major[25-bit] or quadrant [30-bit] encoding,
that is allocated to
the appropriate instruction encoding at link/load time to match
the hardware [or hardware dynamic configuration, as above].
This handles the green
field encodings.
Each feature could have
a default minor/major/quadrant encoding designation.
Brown field can also
managed, simply if the related co-encoded feature is present,
with more complexity, and perhaps extensive opcode mapping if
blended into other feature's encodings.
An implementation
method would be to have a fixed exclusive universal prefix for
each feature.
Each instruction would
then be emitted by the compiler as a [prefix]:[instruction with
default encoding] pair.
If the initial prefixes
are also nops [most of which are currently designated as hints],
then the code would be
executable on machines that use the default mapping
without any link/load
intervention [at lower performance granted].
This approach is
backward compatible for the other established extensions:
most
notably F which consumes 7 major opcodes spaces [and *only* 5
with Zfinx (Zifloat?)] and
then
AMO which also consumes the majority of a major opcode.
This
strategic change has a number of immediate and significant
benefits:
1)
custom reserved major op codes effectively become unreserved
as "standard" extensions can be mapped there also.
The custom reserved nature will then only be the designated
default allocation, "standard extensions" will not default to
them.
2) as
mentioned above, if the prefix is a nop then link/load support
is not needed for direct execution support [only efficiency].
3)
the transition to higher bit encodings can be simplified. As
easily as the compiler emmitting the designated prefix for
that feature that encodes for 64 bit instructions.
So, two
assigned fixed exclusive encodings per feature may be useful,
one a 64bit encoding and one a nop.
I do not intent to stifle any of the tactical
discussions of co-usefulness of features and profile domains.
These are meaningful and useful considerations.
Rather, I hope that by having a framework for
coexistance of features, that those discussions can proceed in a
more guided way;
that discovers can be incorporated into a framework
centric corpus of understanding of trade-offs and cooperative
benefits of features/profiles.
On 2020-10-23 11:45 p.m., Robert Chyla
wrote:
I agree with Greg's statements. For
me 'code-size' is very important for small, deeply
embedded/IoT-class small systems.
Work in other groups (bitmanip) will
also benefit code size, but it is not primary focus I think as
these will also improve code-speed.
Linux-like big processors usually
have DDR RAM and code size is 'unlimited'.
It should not hurt as code-size
advances will benefit such big systems, but we should not forget
about 'cheap to implement'='logic size' factors.
IMO 'code-size' and 'code-speed' will
be pulling same rug (ISA-space) into opposite directions. We
must balance it properly - having a rug in one piece is IMO most
important.
Regards,
/Robert
On 10/23/2020 5:11 PM, Greg Favor
wrote:
It seems like a TG, probably through the
statement of its charter, should clearly define what types
or classes of systems it is focused on optimizing for (if
there is an intended focus) and what types or classes of
systems it does not expect to be appropriate for. More
concretely, it seems like there are a few TG's developing
extensions oriented towards embedded real time systems
and/or low-cost embedded systems. These are extensions that
would probably not be implemented in full-blown Linux-class
systems. Those extensions don't need to worry about being
acceptable to such system designs, and can optimize
for the requirements and constraints of their target
class(es) of systems.
Unless I'm mistaken, this TG falls in that category.
And as long as the charter captures this, then the
extension it produces can be properly evaluated against
its goals and target system applications (and not be
judged wrt other classes of systems). And key trade-off
considerations - like certain types of implementation
approaches being acceptable or unacceptable for the target
system applications - should probably be agreed upon early
on.
Greg
This discussion is bringing
up an issue that needs wider discussion about
extensions in general.
Risc-V is intended to be an
architecture that supports an extremely wide range of
implementations,
ranging from very low gate
count microcontrollers, to high end
superscalar out-of-order processors.
How do we evaluate an
extension that only makes sense at one end or the
other?
I don't expect a vector, or
even hypervisor extensions in a low gate count system.
There are other extensions
that are primarily aimed at specific applications
areas as well.
A micro sequenced (e.g.
push/pop[int]) op might be fairly trivial to implement
in a low gate count system
(e.g. without VM, but with PMPs) and have significant
savings in code size, power, and increased
performance.
They may have none of
those, or less significant, advantages in a high end
implementation --
and/or might be very
difficult or costly to implement in them, (e.g. for
TLB miss, interrupt, & exception handling )
(I am not claiming that
these specific ops do, but just pretend there is one
like that)
Should we avoid defining
instructions and extensions like that?
Or just allow that some
extensions just don't make sense for some class of
implementation?
Are there guidelines we can
put in place to help make those decisions?
This same (not precisely
the same) kind of issue is rearing its head in other
places, e.g. range based CMOs.
--
Regards,
Robert Chyla, Lead Engineer, Debug and Trace Probe Software
IAR Systems
1211 Flynn Rd, Unit 104
Camarillo, CA 93012 USA
Office: +1 805 383 3682 x104
E-mail: Robert.Chyla@...
Website: www.iar.com
