[llvm] [LoopVectorizer] Add support for partial reductions (PR #92418)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

Thanks David, I've adjusted some of the tests to account for this incorrect logic since they weren't provide the target features needed.
________________________________
From: David Sherwood ***@***.***>
Sent: 16 December 2024 12:04
To: llvm/llvm-project ***@***.***>
Cc: Samuel Tebbs ***@***.***>; Mention ***@***.***>
Subject: Re: [llvm/llvm-project] [LoopVectorizer] Add support for partial reductions (PR #92418)


@david-arm commented on this pull request.

________________________________

In llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h<https://github.com/llvm/llvm-project/pull/92418#discussion_r1886702186>:

> +                          TTI::PartialReductionExtendKind OpAExtend,
+                          TTI::PartialReductionExtendKind OpBExtend,
+                          std::optional<unsigned> BinOp) const {
+
+    InstructionCost Invalid = InstructionCost::getInvalid();
+    InstructionCost Cost(TTI::TCC_Basic);
+
+    if (Opcode != Instruction::Add)
+      return Invalid;
+
+    EVT InputEVT = EVT::getEVT(InputType);
+    EVT AccumEVT = EVT::getEVT(AccumType);
+
+    if (VF.isScalable() && !ST->isSVEorStreamingSVEAvailable())
+      return Invalid;
+    if (VF.isFixed() && !ST->isNeonAvailable() && !ST->hasDotProd())


I think this logic is wrong, and it should be

  if (VF.isFixed() && (!ST->isNeonAvailable() || !ST->hasDotProd()))
    return Invalid;


i.e. if the VF is fixed-width then return Invalid if either of these is true:

  1.  We're in streaming mode and NEON is not available, or
  2.  We're using an older architecture that doesn't have dot product.

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https://github.com/llvm/llvm-project/pull/92418