[llvm-dev] [RFC] Changes to llvm.experimental.vector.reduce intrinsics

Sander De Smalen via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 4 06:11:11 PDT 2019

Hi all,

While working on a patch to improve codegen for fadd reductions on AArch64, I stumbled upon an outstanding issue with the experimental.vector.reduce.fadd/fmul intrinsics where the accumulator argument is ignored when fast-math is set on the intrinsic call. This behaviour is described in the LangRef (https://www.llvm.org/docs/LangRef.html#id1905) and is mentioned in https://bugs.llvm.org/show_bug.cgi?id=36734 and further discussed in D45336 and D59356.

This means that for example:
    %res = call fast float @llvm.experimental.vector.reduce.fadd.f32.v4f32(float undef, <4 x float> %v)

does not result in %res being 'undef', but rather a reduction of <4 x float> %v. The definition of these intrinsics are different from their corresponding SelectionDAG nodes which explicitly split out a non-strict VECREDUCE_FADD that explicitly does not take a start-value operand, and a VECREDUCE_STRICT_FADD which does.

With the vector reduction intrinsics still experimental, I would like to propose to change this behaviour. I would also like to take this opportunity to ask what other changes are required, so that we can make an effort towards dropping the 'experimental' prefix at some point.

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.

[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.

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.

  *   Dropping the 'experimental' from the reduction intrinsics and auto-upgrading to the new intrinsics once there is agreement on stability of the definitions.

Please let me know if anything obvious is missing here.


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