[llvm] [VPlan] Extend getSCEVForVPV, use to compute VPReplicateRecipe cost. (PR #161276)

[Florian Hahn via llvm-commits]

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@@ -86,6 +87,85 @@ const SCEV *vputils::getSCEVExprForVPValue(VPValue *V, ScalarEvolution &SE) {
   return TypeSwitch<const VPRecipeBase *, const SCEV *>(V->getDefiningRecipe())
       .Case<VPExpandSCEVRecipe>(
           [](const VPExpandSCEVRecipe *R) { return R->getSCEV(); })
+      .Case<VPCanonicalIVPHIRecipe>([&SE, L](const VPCanonicalIVPHIRecipe *R) {
+        if (!L)
+          return SE.getCouldNotCompute();
+        const SCEV *Start = getSCEVExprForVPValue(R->getOperand(0), SE, L);
+        return SE.getAddRecExpr(Start, SE.getOne(Start->getType()), L,
+                                SCEV::FlagAnyWrap);
+      })
+      .Case<VPDerivedIVRecipe>([&SE, L](const VPDerivedIVRecipe *R) {
+        const SCEV *Start = getSCEVExprForVPValue(R->getOperand(0), SE, L);
+        const SCEV *IV = getSCEVExprForVPValue(R->getOperand(1), SE, L);
+        const SCEV *Scale = getSCEVExprForVPValue(R->getOperand(2), SE, L);
+        if (any_of(ArrayRef({Start, IV, Scale}), IsaPred<SCEVCouldNotCompute>))
+          return SE.getCouldNotCompute();
+
+        return SE.getAddExpr(SE.getTruncateOrSignExtend(Start, IV->getType()),
+                             SE.getMulExpr(IV, SE.getTruncateOrSignExtend(
+                                                   Scale, IV->getType())));
+      })
+      .Case<VPScalarIVStepsRecipe>([&SE, L](const VPScalarIVStepsRecipe *R) {
+        return getSCEVExprForVPValue(R->getOperand(0), SE, L);
+      })
+      .Case<VPReplicateRecipe>([&SE, L](const VPReplicateRecipe *R) {
+        if (R->getOpcode() != Instruction::GetElementPtr)
+          return SE.getCouldNotCompute();
+
+        const SCEV *Base = getSCEVExprForVPValue(R->getOperand(0), SE, L);
+        if (isa<SCEVCouldNotCompute>(Base))
+          return SE.getCouldNotCompute();
+
+        Type *IntIdxTy = SE.getEffectiveSCEVType(Base->getType());
+        Type *CurTy = IntIdxTy;
+        bool FirstIter = true;
+        SmallVector<const SCEV *, 4> Offsets;
+        for (VPValue *Index : drop_begin(R->operands())) {
+          const SCEV *IndexExpr = getSCEVExprForVPValue(Index, SE, L);
+          if (isa<SCEVCouldNotCompute>(IndexExpr))
+            return SE.getCouldNotCompute();
+          // Compute the (potentially symbolic) offset in bytes for this index.
+          if (StructType *STy = dyn_cast<StructType>(CurTy)) {
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fhahn wrote:

For most cases, it just uses the same type as the GEPs in LLVM IR. In some places we create new GEPs, which are usually plain `GEP i8/ptradd`

https://github.com/llvm/llvm-project/pull/161276