[llvm-branch-commits] [llvm] [AArch64] Improve cost model for legal subvec insert/extract (PR #81135)

[Sander de Smalen via llvm-branch-commits]

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@@ -568,6 +568,48 @@ AArch64TTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     }
     return Cost;
   }
+  case Intrinsic::vector_extract: {
+    // If both the vector argument and the return type are legal types and the
+    // index is 0, then this should be a no-op or simple operation; return a
+    // relatively low cost.
+
+    // If arguments aren't actually supplied, then we cannot determine the
+    // value of the index.
+    if (ICA.getArgs().size() < 2)
+      break;
+    LLVMContext &C = RetTy->getContext();
+    EVT MRTy = getTLI()->getValueType(DL, RetTy);
+    EVT MPTy = getTLI()->getValueType(DL, ICA.getArgTypes()[0]);
+    TargetLoweringBase::LegalizeKind RLK = getTLI()->getTypeConversion(C, MRTy);
+    TargetLoweringBase::LegalizeKind PLK = getTLI()->getTypeConversion(C, MPTy);
+    const ConstantInt *Idx = dyn_cast<ConstantInt>(ICA.getArgs()[1]);
+    if (RLK.first == TargetLoweringBase::TypeLegal &&
+        PLK.first == TargetLoweringBase::TypeLegal && Idx &&
+        Idx->getZExtValue() == 0)
+      return InstructionCost(1);
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sdesmalen-arm wrote:

Is there a reason this wouldn't this be zero-cost?

Also, stylistically to match the rest of this file, maybe return `TTI::TCC_Free` (if this is considered a cost of 0) or `TTI::TCC_Basic` (if this is considered a cost of 1) instead?

https://github.com/llvm/llvm-project/pull/81135