[VPlan] Add legal checks in VPWidenMemoryRecipe::computeCost(). NFC

[ElvisWang123]

Add legal checks in the vplan-based cost model that align to the legacy cost model (setCostBasedWideningDecision()).

This is patch is not quite a NFC patch but currently we don't have a target that support ActiveVectorLength && ScalableVector but not support gather/scatter nor return a valid cost by TTI.getGatherScatterOpCost.

Fixing the gap can prevent potentail failure in the future.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Elvis Wang (ElvisWang123)

Changes

Add legal checks in the vplan-based cost model that align to the legacy cost model (setCostBasedWideningDecision()).

This is patch is not quite a NFC patch but currently we don't have a target that support ActiveVectorLength && ScalableVector but not support gather/scatter nor return a valid cost by TTI.getGatherScatterOpCost.

Fixing the gap can prevent potentail failure in the future.

---

Full diff: https://github.com/llvm/llvm-project/pull/110445.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp (+5)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
index 2f0ba5510b8f34..c941479e7c6589 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -2148,6 +2148,11 @@ InstructionCost VPWidenMemoryRecipe::computeCost(ElementCount VF,
     const Value *Ptr = getLoadStorePointerOperand(&Ingredient);
     assert(!Reverse &&
            "Inconsecutive memory access should not have the order.");
+    if ((isa<LoadInst>(Ingredient) &&
+         !Ctx.TTI.isLegalMaskedGather(Ty, Alignment)) ||
+        (isa<StoreInst>(Ingredient) &&
+         !Ctx.TTI.isLegalMaskedScatter(Ty, Alignment)))
+      return InstructionCost::getInvalid();
     return Ctx.TTI.getAddressComputationCost(Ty) +
            Ctx.TTI.getGatherScatterOpCost(Ingredient.getOpcode(), Ty, Ptr,
                                           IsMasked, Alignment, CostKind,

[fhahn]

shouldn't getGatherScatterOpCost return Invalid in those cases as we pass IsMasked ?

[ElvisWang123]

I am not sure who has the responsibility to do the legal check transformations or TTI.
In RISCV, getGatherScatterOpCost will do the legal check and return invalid cost when it fail.
In the default target, getGatherScatterOpCost just return the constant cost no matter it is legal or not.

I found the legacy cost model explicit check if the gather/scatter is legal in LoopVectorizationCostModel::setCostBasedWideningDecision().

So adding this check can prevent misaligned between legacy cost model and the vplan-based cost model when a target isLegalMaskedGather() = false && getGatherScatterOpCost() is valid .

[fhahn]

I see. I think the difference is that while the legacy cost model needs to consider all options for the input, no masked VPWidenMemoryRecipe should be created , if masked mem-ops aren't legal I think