[llvm-dev] [RFC] Changes to llvm.experimental.vector.reduce intrinsics
Sander De Smalen via llvm-dev
llvm-dev at lists.llvm.org
Thu May 16 05:53:38 PDT 2019
I've been meaning to follow up on this thread for the last couple of weeks, my apologies for the delay.
To summarise the feedback on the proposal for vector.reduce.fadd/fmul:
There seems to be consensus to keep the explicit start value to better accommodate chained reductions (as opposed to generating IR that performs the reduction of the first element using extract/fadd/insert pattern). An important use-case for these reductions is to work inside vectorized loops, where chaining happens through the reduction value's PHI node (i.e. the scalar reduction value from one iteration will be the input to the next iteration). This intrinsic would also naturally match reduction instructions of ARM SVE and NEC SX-aurora.
For Option A (https://reviews.llvm.org/D60261), there is an argument that code creating or operating on these intrinsics can treat ordered and unordered reductions the same (in that they have the same arguments). Fast-math flags determine whether or not the intrinsic needs to be evaluated in strict order. Codegen for non-strict reductions should be able to fold away the identity-value.
For Option B (https://reviews.llvm.org/D60262), David made the argument that making the reduction-order explicit (as opposed to deducing this from fast-math flags) would ensure the ordering is always as expected, even when FMF on the call sites are dropped for some reason.
Is it correct that I sensed a slight preference for Option A? i.e. Renaming the intrinsics and keeping the same signature, but dropping the special-cased behaviour for the identity-value with non-strict reductions. For David's argument, I think that although the extra expressiveness would be nice to have, LLVM normally depends on the FMF being propagated correctly to produce faster code so this should also be sufficient for reductions.
If we go for Option A, I suggest we drop the 'experimental' prefix from experimental.vector.reduce.fadd/fmul to avoid having to add an awkward '.v2' suffix to the new intrinsic. When we implement all the suggestions from this proposal (possibly including the one mentioned below), I wouldn't really know what other features we could add other than predication (which would be covered by the LLVM-VP proposal and thus require another renaming), or possibly adding 'constrained' variants which I assume would have separate intrinsics. So we might as well drop the 'experimental' prefix.
Finally, do we want to remove the restriction that the result type must always match the vector-element type? A wider result type would then allow the reduction to be performed in the wider type.
> On 10 Apr 2019, at 18:56, Amara Emerson <aemerson at apple.com> wrote:
> I’m fine with the direction this is going, but let’s keep renaming to a minimum. They’ve been experimental long enough now that we should be able to now jump to a final form after all the feedback.
>> On Apr 10, 2019, at 5:59 AM, Sander De Smalen via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> On 8 Apr 2019, at 11:37, Simon Moll <moll at cs.uni-saarland.de> wrote:
>>> On 4/5/19 10:47 AM, Simon Pilgrim via llvm-dev wrote:
>>>> On 05/04/2019 09:37, Simon Pilgrim via llvm-dev wrote:
>>>>> On 04/04/2019 14:11, Sander De Smalen wrote:
>>>>>> Proposed change:
>>>>>> In this RFC I propose changing the intrinsics for llvm.experimental.vector.reduce.fadd and llvm.experimental.vector.reduce.fmul (see options A and B). I also propose renaming the 'accumulator' operand to 'start value' because for fmul this is the start value of the reduction, rather than a value to which the fmul reduction is accumulated into.
>>> Note that the LLVM-VP proposal also changes the way reductions are handled in IR (https://reviews.llvm.org/D57504). This could be an opportunity to avoid the "v2" suffix issue: LLVM-VP moves the intrinsic to the "llvm.vp.*" namespace and we can fix the reduction semantics in the progress.
>> Thanks for pointing out Simon. I think for now we should keep this proposal separate from LLVM-VP as they serve different purposes and have different scope. But yes we can easily rename the intrinsics again when the VP proposal lands.
>>> Btw, if you are at EuroLLVM. There is a BoF at 2pm today on LLVM-VP.
>>>>>> [Option A] Always using the start value operand in the reduction (https://reviews.llvm.org/D60261)
>>>>>> declare float @llvm.experimental.vector.reduce.v2.fadd.f32.v4f32(float %start_value, <4 x float> %vec)
>>>>>> This means that if the start value is 'undef', the result will be undef and all code creating such a reduction will need to ensure it has a sensible start value (e.g. 0.0 for fadd, 1.0 for fmul). When using 'fast' or ‘reassoc’ on the call it will be implemented using an unordered reduction, otherwise it will be implemented with an ordered reduction. Note that a new intrinsic is required to capture the new semantics. In this proposal the intrinsic is prefixed with a 'v2' for the time being, with the expectation this will be dropped when we remove 'experimental' from the reduction intrinsics in the future.
>>>>>> [Option B] Having separate ordered and unordered intrinsics (https://reviews.llvm.org/D60262).
>>>>>> declare float @llvm.experimental.vector.reduce.ordered.fadd.f32.v4f32(float %start_value, <4 x float> %vec)
>>>>>> declare float @llvm.experimental.vector.reduce.unordered.fadd.f32.v4f32(<4 x float> %vec)
>>>>>> This will mean that the behaviour is explicit from the intrinsic and the use of 'fast' or ‘reassoc’ on the call has no effect on how that intrinsic is lowered. The ordered reduction intrinsic will take a scalar start-value operand, where the unordered reduction intrinsic will only take a vector operand.
>>>>>> Both options auto-upgrade the IR to use the new (version of the) intrinsics. I'm personally slightly in favour of [Option B], because it better aligns with the definition of the SelectionDAG nodes and is more explicit in its semantics. We also avoid having to use an artificial 'v2' like prefix to denote the new behaviour of the intrinsic.
>>>>> Do we have any targets with instructions that can actually use the start value? TBH I'd be tempted to suggest we just make the initial extractelement/fadd/insertelement pattern a manual extra stage and avoid having having that argument entirely.
>>> NEC SX-Aurora has reduction instructions that take in a start value in a scalar register. We are hoping to upstream the backend: http://lists.llvm.org/pipermail/llvm-dev/2019-April/131580.html
>> Great, I think combined with the argument for chaining of ordered reductions (often inside vectorized loops) and two architectures (ARM SVE and SX-Aurora) taking a scalar start register, this is enough of an argument to keep the explicit operand for the ordered reductions.
>>>>>> Further efforts:
>>>>>> Here a non-exhaustive list of items I think work towards making the intrinsics non-experimental:
>>>>>> • Adding SelectionDAG legalization for the _STRICT reduction SDNodes. After some great work from Nikita in D58015, unordered reductions are now legalized/expanded in SelectionDAG, so if we add expansion in SelectionDAG for strict reductions this would make the ExpandReductionsPass redundant.
>>>>>> • Better enforcing the constraints of the intrinsics (see https://reviews.llvm.org/D60260 ).
>>>>>> • I think we'll also want to be able to overload the result operand based on the vector element type for the intrinsics having the constraint that the result type must match the vector element type. e.g. dropping the redundant 'i32' in:
>>>>>> i32 @llvm.experimental.vector.reduce.and.i32.v4i32(<4 x i32> %a) => i32 @llvm.experimental.vector.reduce.and.v4i32(<4 x i32> %a)
>>>>>> since i32 is implied by <4 x i32>. This would have the added benefit that LLVM would automatically check for the operands to match.
>>>>> Won't this cause issues with overflow? Isn't the point of an add (or mul....) reduction of say, <64 x i8> giving a larger (i32 or i64) result so we don't lose anything? I agree for bitop reductions it doesn't make sense though.
>>>> Sorry - I forgot to add: which asks the question - should we be considering signed/unsigned add/mul and possibly saturation reductions?
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