JIT: empty array enumerator opt (dotnet#109237)

For empty arrays, `GetEnumerator` returns a static instance rather
than a new instance. This hinders the JIT's ability to optimize when
it is able to stack allocate the new instance.

Detect when a stack allocation comes from an array `GetEnumerator`
and fold the branch in the inlined enumerator method to always take
the new instance path (since it is now cheap, allocation free, and
is functionally equivalent).

Contributes to dotnet#108913.

src/coreclr/jit/gentree.h

@@ -548,6 +548,8 @@ enum GenTreeFlags : unsigned int
 
     GTF_MDARRLOWERBOUND_NONFAULTING = 0x20000000, // GT_MDARR_LOWER_BOUND -- An MD array lower bound operation that cannot fault. Same as GT_IND_NONFAULTING.
 
+    GTF_ALLOCOBJ_EMPTY_STATIC = 0x80000000, // GT_ALLOCOBJ -- allocation site is part of an empty static pattern
+
 #ifdef FEATURE_HW_INTRINSICS
     GTF_HW_EM_OP                  = 0x10000000, // GT_HWINTRINSIC -- node is used as an operand to an embedded mask
     GTF_HW_USER_CALL              = 0x20000000, // GT_HWINTRINSIC -- node is implemented via a user call

src/coreclr/jit/importer.cpp

@@ -8966,6 +8966,15 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                             return;
                         }
 
+                        // Flag if this allocation happens within a method that uses the static empty
+                        // pattern (if we stack allocate this object, we can optimize the empty side away)
+                        //
+                        if (lookupNamedIntrinsic(info.compMethodHnd) == NI_System_SZArrayHelper_GetEnumerator)
+                        {
+                            JITDUMP("Allocation is part of empty static pattern\n");
+                            op1->gtFlags |= GTF_ALLOCOBJ_EMPTY_STATIC;
+                        }
+
                         // Remember that this basic block contains 'new' of an object
                         block->SetFlags(BBF_HAS_NEWOBJ);
                         optMethodFlags |= OMF_HAS_NEWOBJ;

src/coreclr/jit/objectalloc.cpp

@@ -623,6 +623,58 @@ unsigned int ObjectAllocator::MorphAllocObjNodeIntoStackAlloc(
 
     comp->fgInsertStmtBefore(block, stmt, initStmt);
 
+    // If this allocation is part the special empty static pattern, find the controlling
+    // branch and force control to always flow to the new instance side.
+    //
+    if ((allocObj->gtFlags & GTF_ALLOCOBJ_EMPTY_STATIC) != 0)
+    {
+        BasicBlock* const predBlock = block->GetUniquePred(comp);
+        assert(predBlock != nullptr);
+        assert(predBlock->KindIs(BBJ_COND));
+
+        JITDUMP("Empty static pattern controlled by " FMT_BB ", optimizing to always use stack allocated instance\n",
+                predBlock->bbNum);
+        Statement* const controllingStmt = predBlock->lastStmt();
+        GenTree* const   controllingNode = controllingStmt->GetRootNode();
+        assert(controllingNode->OperIs(GT_JTRUE));
+
+        FlowEdge* const trueEdge    = predBlock->GetTrueEdge();
+        FlowEdge* const falseEdge   = predBlock->GetFalseEdge();
+        FlowEdge*       keptEdge    = nullptr;
+        FlowEdge*       removedEdge = nullptr;
+
+        if (trueEdge->getDestinationBlock() == block)
+        {
+            keptEdge    = trueEdge;
+            removedEdge = falseEdge;
+        }
+        else
+        {
+            assert(falseEdge->getDestinationBlock() == block);
+            keptEdge    = falseEdge;
+            removedEdge = trueEdge;
+        }
+
+        BasicBlock* removedBlock = removedEdge->getDestinationBlock();
+        comp->fgRemoveRefPred(removedEdge);
+        predBlock->SetKindAndTargetEdge(BBJ_ALWAYS, keptEdge);
+
+        if (predBlock->hasProfileWeight())
+        {
+            block->setBBProfileWeight(predBlock->bbWeight);
+        }
+
+        // Just lop off the JTRUE, the rest can clean up later
+        // (eg may have side effects)
+        //
+        controllingStmt->SetRootNode(controllingNode->AsOp()->gtOp1);
+
+        // We must remove the empty static block now too.
+        assert(removedBlock->bbRefs == 0);
+        assert(removedBlock->KindIs(BBJ_ALWAYS));
+        comp->fgRemoveBlock(removedBlock, /* unreachable */ true);
+    }
+
     return lclNum;
 }