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Re: RISC-V vector extension cauldron discussion

Sorry if this is a bit roundabout, I wrote this over a few days and ended up thinking a bunch while writing. As a result, the beginning might not match the end.

On Sat, 08 Sep 2018 03:04:38 PDT (-0700), wrote:
This attempts to capture (most of) the two hour discussion that
we had in the Entrance Hall at GNU Cauldron yesterday.  Please
correct any faulty memory on my part and forward or cc this to
the appropriate RISC-V forum.


I think Roger forwarded this to the private RISC-V vector mailing list, but I'm not in the club so I'll have to talk about it here :)

The RISC-V vector extension described something other than what is
present in the currently released 2.2 standard.  To clarify the
language within this message, based on what I remember:

Yes. The current RISC-V ISA standard contains no vector instructions, they will be added under the "V" extension as part of a future revision of the RISC-V standard. This is how we manage the standard: as new revisions of the ISA manual come out we can add new extensions, but we can never change or remove an existing extension.

vl:     the current number of elements that are active in each vector reg
maxvl:  the maximum number of elements in the current config
maxel:  the (maximum) element width within each vector reg
vsz:    (new language here, vector size) maxvl * maxel

(I) We talked about the needs of the register allocator.

No one wants their loop kernel to spill registers, but eventually
it will happen, and we have to be prepared for it.

For a series of loop nests, while there is no spilling, it is easy
to select a vconfig for each loop nest independently.  However, when
spills occur, we need to be able to allocate spill slots of sufficient
size, and locate their position within the stack frame.

Because VECSZ can change based on vconfig, the only manageable frame
allocation strategy may require selecting a single vconfig for the
entire function.

We posited new instructions, vspill and vfill, that ignore VL, ignore
predication, and operate on all MAXVL elements of MAXEL.  This allows
the compiler to save and restore the entire contents of the register
without knowing the current configuration.

While I'm not part of the vector working group, I'd anticipate these sorts of instructions don't make it into the V extension because they leak too much about the microarchitecture to software. One of the goals of the V extension is to allow for software compatibility between different implementations, and instructions with semantics like these tend to lead to incompatible software.

Additionally, I don't think this is necessary because our proposed vector ABI is to clobber the entire state of the vector unit on all function calls. As a result, the compiler should always know how to spill registers: when you enter the function you don't have to spill anything, and if you spill as part of the loop you must have set up the vector unit in some manner and therefor should know about it.

This ABI may be a bad idea, but if we stick with it then we should be able to get away without this sort of instruction. There's a few other places in the ISA where we've somewhat wed ourselves to this ABI, but with some modifications it might be possible to produce a reasonable one.

An instruction that does something similar (or even more revealing of the microarchitecture) may be necessary as part of the supervisor-mode vector extensions, but those are less scary because we can just mandate that supervisors write compatible code. We operate under the assumption that we can't mandate anything about userspace.

We posited a new instruction, akin to SVE's addvl, such as

        addvsz  rd, rs, imm     (rd = rs + imm * vsz)

which can be used to rapidly size the stack frame and form addresses
of spill slots within the stack frame, also without knowing the current
configuration.  E.g.

        addvsz  sp, fp, -7

to allocate space for 7 spill slots below the frame pointer.
When a spill needs to occur, use e.g.

        addvsz  tmp, fp, -3
        vspill  v0, tmp

In particular, this approach allows one to form the address in one
instruction instead of 3, and also providing VSZ implicitly as opposed
to requiring the register allocator to somehow re-materialize this value.

(Which is not impossible, given the example of pic_offset_register.
But also given that example, requires invasive special-casing within
the register allocator.  So, ew.)

I agree here: providing some mechanism to obtain the current VL should be provided by the ISA. While it's not strictly necessary given the proposed ABI, it does avoid a bunch of headaches trying to ensure that value sticks around. I think it will end up being useful for any ABI that avoids clobbering every vector register. I'd propose that the vector config should also be obtainable, which will also be useful for such an ABI.

(II) We talked about the needs of a "simd" abi

The normal integer abi, is already fixed, and because of that the
entire vector state must needs be call-clobbered.  However, OpenMP
has a #pragma simd that instructs the compiler to generate a set of
vectorized versions of a given function.


Something similar must be defined for RISC-V.  Such an abi must
consider how vconfig is to be managed across function boundaries
and with separate compilation.  In my opinion this should be done
before finalizing the ISA, as detailed below.

Must is a strong word, but I agree that we should at least ensure that it's possible to define a sane ABI that saves vector registers around function calls and passes arguments via vector registers. In other words: I think we'll still want to support something like "-march=rv64gcv -mabi=lp64d", but I don't think we want to preclude ourselves from "-march=rv64gcv -mabi=lp64dv" being better.

I think the best way to go about this is to figure out what features of an ABI might be worth having, and then to enumerate the mechanisms that an V-style ISA extension must provide in order to sanely implement such an ABI. Essentially we've still got time to change the ISA, so let's just design a good ABI, figure out what's necessary from the ISA to implement said ABI, and then make sure that's in the standard.

The ABI features I can think of are:

* Passing at least one argument in a vector register.
- Presumably we'll clobber vector argument registers on calls, like we do for everything else. Thus there isn't any ISA requirement here. - How does one go about indicating at the C level that an argument is passed in a register? If we just say "any __attribute__((vector)) of length less than N bytes/elements" then N must be less than the ISA mandated minimum vector length (IIRC 4 elements?) -- that might be OK. * Saving the contents of at least one vector register across a function call. In order to do so we need: - A mechanism for determining the number of bytes used by a vector register, to reserve stack space. - A mechanism for saving a vector register to the stack. This could be a simple vector store, but if we want to maintain the entire register (as opposed to just the first vl elements) we need * Saving vl across a function call. - We need a mechanism for determining the vector length. Currently the only way to do so is destructive, we'd need a non-destructive way to do so.
* Saving vconfig across a function call.
   - There is no way to determine the config, we'd need a way to do so.

My proposed vector ABI is:

* Don't pass any vector arguments in registers.
* Save all enabled vector registers across function calls, but only the first VL elements of each vector (again, with the number of saved-over-call registers decided upon later). This allows some flexibility on the side of the caller, as it can inform the callee that it does not actually need any of the saved registers. This also avoids any of the headaches related to saving MAXVL elements.
* Save vl and vconfig across function calls.

One of the nice things here is backwards compatibility -- we can actually avoid a second vector ABI here, with a simply runtime check to see if the vector unit is enabled before saving any registers. This also allows us to avoid the my biggest fear with any proposed vector ABI: a big cliff for any function that uses the vector unit where a bunch of data is saved that isn't actually needed by the caller. My worry here is that this favors short-VL implementations in favor of long-VL implementations, which seems like the wrong way to go -- the long-VL implementations will be fast either, way, so it'd be better to penalize those. Having two ABIs for the two implementation styles would be a mess.

I have no idea if this is sane or not, so feel free to suggest something else :). I have to go play around with the OpenMP SIMD stuff to see if this makes any sense in that realm.

(II-a) The callee must know how many registers are enabled by vconfig.

The simplest solution is simply to require all 32 registers to be enabled.

Expanding on this slightly, one could require a reduced set N (e.g. 16)
and defined this as abi.  This would trade off potentially unused
registers and potentially more spilling for longer vectors in the
(presumably) common case.

One could require N registers by default and override this by an
explicit target-specific clause in the #pragma.  This would allow
programmers to tune the compiler output (bearing in mind that changing
the clause changes the function abi), while also providing a sensible
default for code that has not been explicitly tuned for a given risc-v

Makes sense -- my only worry here is that we're leaving a lot on the floor. Maybe this is just because I'm not really a vector guy, but my biggest worry with the vector unit is ensuring that memcpy() and friends are reasonably efficient. I'm a bit worried about throwing a factor of 32 in vector length on the floor here (or requiring saving a huge vector state), particularly as I think that most vectorized code won't need to worry about calling standard ABI functions.

(II-b) The callee must know MAXEL.

It seems obvious that MAXEL must be set as large as the largest vector
type that is passed in or returned from the callee.  However, the
callee may be passed a vector of bytes but internally may need to
operate on doubles as part of the computation.  One cannot simply
set a new vconfig without potentially losing data in the byte input.

I think the important distinction here is not that it's impossible to set a vconfig without clearing the vector state, but instead that it's impossible to interrogate the current state of the vector unit.

We suggested the possibility of generating up to 4 different functions
(or equivalently 4 different entry point symbols), which allows the
caller to give the callee the minimal MAXEL, N, that may be assumed.

We didn't talk specifics, but assume N would be encoded into the
normal _Z name mangling that is used for C++ and the other target
specific simd abis.  But for purposes of example here let's just
use foo_N.

A function taking a vector of doubles of course requires MAXEL = 8,
and so only one entry point foo_8 is generated.

A function taking a vector of bytes would generate all of foo_1,
foo_2, foo_4, foo_8.  Suppose further that the function requires at
maximum a uint16_t immediate.  In that case, foo_2, foo_4, foo_8
would all be aliases for the same symbol.  However, foo_1 would need
to do something different.

While the "something" of course lies firmly in the compiler prerogative,
so long as correct results are obtained, we suggested a simple approach
that may perform as well as might be expected: spill the inputs to the
stack, adjust vconfig as required, and vectorize the operation on the
data on the stack just like any other array.

While this sounds like a reasonable approach, it also seems quite complicated. This might be a bit of a pipe dream, but I was hoping to avoid this sort of complexity.

We again suggested that a target-specific clause in the #pragma might
allow tuning the compiler output by specifying the minimum MAXEL.

Unfortunately, it certainly smells like some mechanism for allowing users to override the ABI on a per-function basis will be necessary.

(II-c) The callee must be able to reset to a previous vconfig

When performing the "something" above, one needs to be able to save
the current vconfig and restore it at the end.  The specification
of the vconfig insn is not within the 2.2 document, nor was it
fully defined within the presentation.  However, the presentation
implied that the argument is always immediate.  A RMW version is
required, and with input from a register.

I agree.

(II-d) The callee should be able to call-save registers

The simd abi may well want to provide some call-saved vector registers.
This is where vspill, vfill, and addvsz being able to operate without
the callee knowing the exact vconfig is imperative.

I agree.

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