From f7e2ce04e12f56946cea15b519bfc4204d6d630f Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Mon, 8 Oct 2018 13:15:36 -0700
Subject: [PATCH] JIT: retrieve element class for arrays of ref types
 (dotnet/coreclr#20292)

Allows element-typed based optimizations for some array cases, say
virtual calls through a `string[]`.

Commit migrated from https://github.com/dotnet/coreclr/commit/bcc4cec7646f0df99c5a83e3dd72ec2d39e0ee01
---
 src/coreclr/src/jit/compiler.h  |  2 ++
 src/coreclr/src/jit/gentree.cpp | 66 +++++++++++++++++++++++++++++++++++++++--
 2 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/src/coreclr/src/jit/compiler.h b/src/coreclr/src/jit/compiler.h
index 77ed2cc..9525745 100644
--- a/src/coreclr/src/jit/compiler.h
+++ b/src/coreclr/src/jit/compiler.h
@@ -2491,6 +2491,8 @@ public:
     CORINFO_CLASS_HANDLE gtGetStructHandle(GenTree* tree);
     // Get the handle for a ref type.
     CORINFO_CLASS_HANDLE gtGetClassHandle(GenTree* tree, bool* isExact, bool* isNonNull);
+    // Get the element handle for an array of ref type.
+    CORINFO_CLASS_HANDLE gtGetArrayElementClassHandle(GenTree* array);
 
 //-------------------------------------------------------------------------
 // Functions to display the trees
diff --git a/src/coreclr/src/jit/gentree.cpp b/src/coreclr/src/jit/gentree.cpp
index a3fcd88..92e8c27 100644
--- a/src/coreclr/src/jit/gentree.cpp
+++ b/src/coreclr/src/jit/gentree.cpp
@@ -16578,12 +16578,14 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTree* tree, bool* isExact, bo
 
         case GT_IND:
         {
-            // indir(addr(lcl)) --> lcl
-            //
-            // This comes up during constrained callvirt on ref types.
             GenTreeIndir* indir = obj->AsIndir();
+
             if (indir->HasBase() && !indir->HasIndex())
             {
+                // indir(addr(lcl)) --> lcl
+                //
+                // This comes up during constrained callvirt on ref types.
+
                 GenTree*             base = indir->Base();
                 GenTreeLclVarCommon* lcl  = base->IsLocalAddrExpr();
 
@@ -16593,6 +16595,16 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTree* tree, bool* isExact, bo
                     objClass              = lvaTable[objLcl].lvClassHnd;
                     *isExact              = lvaTable[objLcl].lvClassIsExact;
                 }
+                else if (base->OperGet() == GT_ARR_ELEM)
+                {
+                    // indir(arr_elem(...)) -> array element type
+
+                    GenTree* array = base->AsArrElem()->gtArrObj;
+
+                    objClass   = gtGetArrayElementClassHandle(array);
+                    *isExact   = false;
+                    *isNonNull = false;
+                }
             }
             break;
         }
@@ -16612,6 +16624,16 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTree* tree, bool* isExact, bo
             break;
         }
 
+        case GT_INDEX:
+        {
+            GenTree* array = obj->AsIndex()->Arr();
+
+            objClass   = gtGetArrayElementClassHandle(array);
+            *isExact   = false;
+            *isNonNull = false;
+            break;
+        }
+
         default:
         {
             break;
@@ -16621,6 +16643,44 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTree* tree, bool* isExact, bo
     return objClass;
 }
 
+//------------------------------------------------------------------------
+// gtGetArrayElementClassHandle: find class handle for elements of an array
+// of ref types
+//
+// Arguments:
+//    array -- array to find handle for
+//
+// Return Value:
+//    nullptr if element class handle is unknown, otherwise the class handle.
+
+CORINFO_CLASS_HANDLE Compiler::gtGetArrayElementClassHandle(GenTree* array)
+{
+    bool                 isArrayExact   = false;
+    bool                 isArrayNonNull = false;
+    CORINFO_CLASS_HANDLE arrayClassHnd  = gtGetClassHandle(array, &isArrayExact, &isArrayNonNull);
+
+    if (arrayClassHnd != nullptr)
+    {
+        // We know the class of the reference
+        DWORD attribs = info.compCompHnd->getClassAttribs(arrayClassHnd);
+
+        if ((attribs & CORINFO_FLG_ARRAY) != 0)
+        {
+            // We know for sure it is an array
+            CORINFO_CLASS_HANDLE elemClassHnd  = nullptr;
+            CorInfoType          arrayElemType = info.compCompHnd->getChildType(arrayClassHnd, &elemClassHnd);
+
+            if (arrayElemType == CORINFO_TYPE_CLASS)
+            {
+                // We know it is an array of ref types
+                return elemClassHnd;
+            }
+        }
+    }
+
+    return nullptr;
+}
+
 void GenTree::ParseArrayAddress(
     Compiler* comp, ArrayInfo* arrayInfo, GenTree** pArr, ValueNum* pInxVN, FieldSeqNode** pFldSeq)
 {
-- 
2.7.4