Add support for "whole" stores

dotnet · Dec 16, 2022 · db187d2 · db187d2
1 parent 719146b
commit db187d2
Showing 1 changed file with 43 additions and 8 deletions.
diff --git a/src/coreclr/jit/optimizer.cpp b/src/coreclr/jit/optimizer.cpp
@@ -10361,7 +10361,7 @@ PhaseStatus Compiler::optVNBasedDeadStoreRemoval()
 
         LclVarDsc* varDsc   = lvaGetDesc(lclNum);
         unsigned   defCount = varDsc->lvPerSsaData.GetCount();
-        if (defCount <= 2)
+        if (defCount <= 1)
         {
             continue;
         }
@@ -10378,22 +10378,53 @@ PhaseStatus Compiler::optVNBasedDeadStoreRemoval()
                 JITDUMP("Considering [%06u] for removal...\n", dspTreeID(store));
 
                 GenTree* lhs = store->gtGetOp1();
-                if (!lhs->OperIs(GT_LCL_FLD) || ((lhs->gtFlags & GTF_VAR_USEASG) == 0) ||
-                    (lhs->AsLclFld()->GetLclNum() != lclNum))
+                if (lhs->AsLclVarCommon()->GetLclNum() != lclNum)
                 {
+                    JITDUMP(" -- no; composite definition\n");
                     continue;
                 }
 
-                ValueNum oldLclValue = varDsc->GetPerSsaData(defDsc->GetUseDefSsaNum())->m_vnPair.GetConservative();
-                ValueNum oldStoreValue =
-                    vnStore->VNForLoad(VNK_Conservative, oldLclValue, lvaLclExactSize(lclNum), lhs->TypeGet(),
-                                       lhs->AsLclFld()->GetLclOffs(), lhs->AsLclFld()->GetSize());
+                ValueNum oldStoreValue;
+                if ((lhs->gtFlags & GTF_VAR_USEASG) == 0)
+                {
+                    LclSsaVarDsc* lastDefDsc = varDsc->lvPerSsaData.GetSsaDefByIndex(defIndex - 1);
+                    if (lastDefDsc->GetBlock() != defDsc->GetBlock())
+                    {
+                        JITDUMP(" -- no; last def not in the same block\n");
+                        continue;
+                    }
+
+                    if ((lhs->gtFlags & GTF_VAR_EXPLICIT_INIT) != 0)
+                    {
+                        // Removing explicit inits is not profitable for primitives and not safe for structs.
+                        JITDUMP(" -- no; 'explicit init'\n");
+                        continue;
+                    }
+
+                    // CQ heuristic: avoid removing defs of enregisterable locals where this is likely to
+                    // make them "must-init", extending live ranges. Here we assume the first SSA def was
+                    // the implicit "live-in" one, which is not guaranteed, but very likely.
+                    if ((defIndex == 1) && (varDsc->TypeGet() != TYP_STRUCT))
+                    {
+                        JITDUMP(" -- no; first explicit def of a non-STRUCT local\n", lclNum);
+                        continue;
+                    }
+
+                    oldStoreValue = lastDefDsc->m_vnPair.GetConservative();
+                }
+                else
+                {
+                    ValueNum oldLclValue = varDsc->GetPerSsaData(defDsc->GetUseDefSsaNum())->m_vnPair.GetConservative();
+                    oldStoreValue =
+                        vnStore->VNForLoad(VNK_Conservative, oldLclValue, lvaLclExactSize(lclNum), lhs->TypeGet(),
+                                           lhs->AsLclFld()->GetLclOffs(), lhs->AsLclFld()->GetSize());
+                }
 
                 GenTree* rhs = store->gtGetOp2();
                 ValueNum storeValue;
                 if (lhs->TypeIs(TYP_STRUCT) && rhs->IsIntegralConst(0))
                 {
-                    storeValue = vnStore->VNForZeroObj(lhs->AsLclFld()->GetLayout());
+                    storeValue = vnStore->VNForZeroObj(lhs->AsLclVarCommon()->GetLayout(this));
                 }
                 else
                 {
@@ -10419,6 +10450,10 @@ PhaseStatus Compiler::optVNBasedDeadStoreRemoval()
 
                     madeChanges = true;
                 }
+                else
+                {
+                    JITDUMP(" -- no; not redundant\n");
+                }
             }
         }
     }