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

[David Sherwood via llvm-commits]

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@@ -568,6 +568,32 @@ AArch64TTIImpl::getIntrinsicInstrCost(const IntrinsicCostAttributes &ICA,
     }
     return Cost;
   }
+  case Intrinsic::vector_extract: {
+    // If both the vector argument and the return type are legal types, then
+    // this should be a no-op or simple operation; return a relatively low cost.
+    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);
+    if (RLK.first == TargetLoweringBase::TypeLegal &&
+        PLK.first == TargetLoweringBase::TypeLegal)
+      return InstructionCost(1);
+    break;
+  }
+  case Intrinsic::vector_insert: {
+    // If both the vector and subvector arguments are legal types, then this
+    // should be a no-op or simple operation; return a relatively low cost.
+    LLVMContext &C = RetTy->getContext();
+    EVT MTy0 = getTLI()->getValueType(DL, ICA.getArgTypes()[0]);
+    EVT MTy1 = getTLI()->getValueType(DL, ICA.getArgTypes()[1]);
+    TargetLoweringBase::LegalizeKind LK0 = getTLI()->getTypeConversion(C, MTy0);
+    TargetLoweringBase::LegalizeKind LK1 = getTLI()->getTypeConversion(C, MTy1);
+    if (LK0.first == TargetLoweringBase::TypeLegal &&
+        LK1.first == TargetLoweringBase::TypeLegal)
+      return InstructionCost(1);
----------------
david-arm wrote:

Same comment as the extract - I think this model is too simple and makes the cost of vector predicate insert/extracts too cheap, as well as mixing legal fixed-width and scalable vector inserts. In particular, we should worry about insert a fixed-width vector into a region of a scalable vector beyond the known minimum size.

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