Rework JIT<->EE communication for struct promotion

Rework the communication so that the JIT asks the EE for struct field
related information relevant to promotion in a single JIT-EE call of a
new function called 'flattenType'.

As a side effect, removes CORINFO_FLG_CUSTOMLAYOUT and
CORINFO_FLG_INDEXABLE_FIELDS (the latter by adding support to inline
arrays in this new function). Also, last but not least, this adds
support for general recursive struct promotion provided the field limit
of 4 is not hit (for now, let's see if there are any more fundamental
issues with this).

Fix dotnet#85511
Fix dotnet#71711

src/coreclr/inc/corinfo.h

@@ -857,10 +857,8 @@ enum CorInfoFlag
     CORINFO_FLG_OVERLAPPING_FIELDS    = 0x00100000, // struct or class has fields that overlap (aka union)
     CORINFO_FLG_INTERFACE             = 0x00200000, // it is an interface
     CORINFO_FLG_DONT_DIG_FIELDS       = 0x00400000, // don't ask field info (used for types outside of AOT compilation version bubble)
-    CORINFO_FLG_CUSTOMLAYOUT          = 0x00800000, // does this struct have custom layout?
     CORINFO_FLG_CONTAINS_GC_PTR       = 0x01000000, // does the class contain a gc ptr ?
     CORINFO_FLG_DELEGATE              = 0x02000000, // is this a subclass of delegate or multicast delegate ?
-    CORINFO_FLG_INDEXABLE_FIELDS      = 0x04000000, // struct fields may be accessed via indexing (used for inline arrays)
     CORINFO_FLG_BYREF_LIKE            = 0x08000000, // it is byref-like value type
     CORINFO_FLG_VARIANCE              = 0x10000000, // MethodTable::HasVariance (sealed does *not* mean uncast-able)
     CORINFO_FLG_BEFOREFIELDINIT       = 0x20000000, // Additional flexibility for when to run .cctor (see code:#ClassConstructionFlags)
@@ -1946,6 +1944,25 @@ struct CORINFO_VarArgInfo
                                             // (The CORINFO_VARARGS_HANDLE counts as an arg)
 };
 
+struct CORINFO_FLATTENED_TYPE_FIELD
+{
+    // Primitive type, or CORINFO_TYPE_VALUECLASS for value classes marked as intrinsic.
+    CorInfoType type;
+    // For an intrinsic value class this is the handle for it.
+    CORINFO_CLASS_HANDLE intrinsicValueClassHnd;
+    // Offset of the field into the root struct.
+    unsigned offset;
+    // Field handle (only used for diagnostic purposes)
+    CORINFO_FIELD_HANDLE fieldHandle;
+};
+
+enum class FlattenTypeResult
+{
+    Success,
+    Partial,
+    Failure,
+};
+
 #define SIZEOF__CORINFO_Object                            TARGET_POINTER_SIZE /* methTable */
 
 #define CORINFO_Array_MaxLength                           0x7FFFFFC7
@@ -2448,6 +2465,29 @@ class ICorStaticInfo
             int32_t num
             ) = 0;
 
+    //------------------------------------------------------------------------------
+    // flattenType: Flatten a type into its primitive/intrinsic constituent
+    // fields.
+    //
+    // Parameters:
+    //    clsHnd             - Handle of the type.
+    //    fields             - [out] Pointer to entries to write.
+    //    numFields          - [in, out] Size of 'fields'. Updated to contain
+    //                         the number of entries written in 'fields'.
+    //    significantPadding - [out] Whether data not covered by fields should
+    //                         be considered as significant or whether the JIT
+    //                         is allowed to discard it on copies.
+    //
+    // Returns:
+    //    A result indicating whether the type was successfully flattened and
+    //    whether the result is partial or not.
+    //
+    virtual FlattenTypeResult flattenType(
+            CORINFO_CLASS_HANDLE clsHnd,
+            CORINFO_FLATTENED_TYPE_FIELD* fields,
+            size_t* numFields,
+            bool* significantPadding) = 0;
+
     virtual bool checkMethodModifier(
             CORINFO_METHOD_HANDLE hMethod,
             const char * modifier,

src/coreclr/inc/icorjitinfoimpl_generated.h

@@ -263,6 +263,12 @@ CORINFO_FIELD_HANDLE getFieldInClass(
           CORINFO_CLASS_HANDLE clsHnd,
           int32_t num) override;
 
+FlattenTypeResult flattenType(
+          CORINFO_CLASS_HANDLE clsHnd,
+          CORINFO_FLATTENED_TYPE_FIELD* fields,
+          size_t* numFields,
+          bool* significantPadding) override;
+
 bool checkMethodModifier(
           CORINFO_METHOD_HANDLE hMethod,
           const char* modifier,

src/coreclr/inc/jiteeversionguid.h

@@ -43,11 +43,11 @@ typedef const GUID *LPCGUID;
 #define GUID_DEFINED
 #endif // !GUID_DEFINED
 
-constexpr GUID JITEEVersionIdentifier = { /* 387bcec3-9a71-4422-a11c-e7ce3b73c592 */
-    0x387bcec3,
-    0x9a71,
-    0x4422,
-    {0xa1, 0x1c, 0xe7, 0xce, 0x3b, 0x73, 0xc5, 0x92}
+constexpr GUID JITEEVersionIdentifier = { /* 878d63a7-ffc9-421f-81f7-db4729f0ed5c */
+    0x878d63a7,
+    0xffc9,
+    0x421f,
+    {0x81, 0xf7, 0xdb, 0x47, 0x29, 0xf0, 0xed, 0x5c}
   };
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////////

src/coreclr/jit/ICorJitInfo_names_generated.h

@@ -64,6 +64,7 @@ DEF_CLR_API(getClassAlignmentRequirement)
 DEF_CLR_API(getClassGClayout)
 DEF_CLR_API(getClassNumInstanceFields)
 DEF_CLR_API(getFieldInClass)
+DEF_CLR_API(flattenType)
 DEF_CLR_API(checkMethodModifier)
 DEF_CLR_API(getNewHelper)
 DEF_CLR_API(getNewArrHelper)

src/coreclr/jit/ICorJitInfo_wrapper_generated.hpp

@@ -599,6 +599,18 @@ CORINFO_FIELD_HANDLE WrapICorJitInfo::getFieldInClass(
     return temp;
 }
 
+FlattenTypeResult WrapICorJitInfo::flattenType(
+          CORINFO_CLASS_HANDLE clsHnd,
+          CORINFO_FLATTENED_TYPE_FIELD* fields,
+          size_t* numFields,
+          bool* significantPadding)
+{
+    API_ENTER(flattenType);
+    FlattenTypeResult temp = wrapHnd->flattenType(clsHnd, fields, numFields, significantPadding);
+    API_LEAVE(flattenType);
+    return temp;
+}
+
 bool WrapICorJitInfo::checkMethodModifier(
           CORINFO_METHOD_HANDLE hMethod,
           const char* modifier,

src/coreclr/jit/compiler.h

@@ -592,9 +592,12 @@ class LclVarDsc
                                   // 32-bit target.  For implicit byref parameters, this gets hijacked between
                                   // fgRetypeImplicitByRefArgs and fgMarkDemotedImplicitByRefArgs to indicate whether
                                   // references to the arg are being rewritten as references to a promoted shadow local.
-    unsigned char lvIsStructField : 1; // Is this local var a field of a promoted struct local?
-    unsigned char lvContainsHoles : 1; // True when we have a promoted struct that contains holes
-    unsigned char lvCustomLayout : 1;  // True when this struct has "CustomLayout"
+    unsigned char lvIsStructField : 1;         // Is this local var a field of a promoted struct local?
+    unsigned char lvContainsHoles : 1;         // Is this a promoted struct whose fields do not cover the struct local?
+    unsigned char lvAnySignificantPadding : 1; // True when we have a promoted struct that has significant padding in it
+                                               // Significant padding is any data in the struct that is not covered by a
+    // promoted field and that the EE told us we need to preserve on block copies
+    // inits.
 
     unsigned char lvIsMultiRegArg : 1; // true if this is a multireg LclVar struct used in an argument context
     unsigned char lvIsMultiRegRet : 1; // true if this is a multireg LclVar struct assigned from a multireg call
@@ -3539,8 +3542,8 @@ class Compiler
     struct lvaStructFieldInfo
     {
         CORINFO_FIELD_HANDLE fldHnd;
-        unsigned char        fldOffset;
-        unsigned char        fldOrdinal;
+        uint8_t              fldOffset;
+        uint8_t              fldOrdinal;
         var_types            fldType;
         unsigned             fldSize;
         CORINFO_CLASS_HANDLE fldTypeHnd;
@@ -3557,27 +3560,19 @@ class Compiler
         CORINFO_CLASS_HANDLE typeHnd;
         bool                 canPromote;
         bool                 containsHoles;
-        bool                 customLayout;
-        bool                 fieldsSorted;
+        bool                 anySignificantPadding;
         unsigned char        fieldCnt;
         lvaStructFieldInfo   fields[MAX_NumOfFieldsInPromotableStruct];
 
         lvaStructPromotionInfo(CORINFO_CLASS_HANDLE typeHnd = nullptr)
             : typeHnd(typeHnd)
             , canPromote(false)
-            , containsHoles(false)
-            , customLayout(false)
-            , fieldsSorted(false)
+            , anySignificantPadding(false)
             , fieldCnt(0)
         {
         }
     };
 
-    struct lvaFieldOffsetCmp
-    {
-        bool operator()(const lvaStructFieldInfo& field1, const lvaStructFieldInfo& field2);
-    };
-
     // This class is responsible for checking validity and profitability of struct promotion.
     // If it is both legal and profitable, then TryPromoteStructVar promotes the struct and initializes
     // necessary information for fgMorphStructField to use.
@@ -3597,12 +3592,6 @@ class Compiler
         bool CanPromoteStructVar(unsigned lclNum);
         bool ShouldPromoteStructVar(unsigned lclNum);
         void PromoteStructVar(unsigned lclNum);
-        void SortStructFields();
-
-        bool CanConstructAndPromoteField(lvaStructPromotionInfo* structPromotionInfo);
-
-        lvaStructFieldInfo GetFieldInfo(CORINFO_FIELD_HANDLE fieldHnd, BYTE ordinal);
-        bool TryPromoteStructField(lvaStructFieldInfo& outerFieldInfo);
 
     private:
         Compiler*              compiler;

src/coreclr/jit/compiler.hpp

@@ -4271,21 +4271,11 @@ inline static bool StructHasOverlappingFields(DWORD attribs)
     return ((attribs & CORINFO_FLG_OVERLAPPING_FIELDS) != 0);
 }
 
-inline static bool StructHasCustomLayout(DWORD attribs)
-{
-    return ((attribs & CORINFO_FLG_CUSTOMLAYOUT) != 0);
-}
-
 inline static bool StructHasDontDigFieldsFlagSet(DWORD attribs)
 {
     return ((attribs & CORINFO_FLG_DONT_DIG_FIELDS) != 0);
 }
 
-inline static bool StructHasIndexableFields(DWORD attribs)
-{
-    return ((attribs & CORINFO_FLG_INDEXABLE_FIELDS) != 0);
-}
-
 //------------------------------------------------------------------------------
 // DEBUG_DESTROY_NODE: sets value of tree to garbage to catch extra references
 //

src/coreclr/jit/gentree.cpp

@@ -11700,28 +11700,13 @@ void Compiler::gtDispLeaf(GenTree* tree, IndentStack* indentStack)
                 {
                     for (unsigned index = 0; index < varDsc->lvFieldCnt; index++)
                     {
-                        unsigned    fieldLclNum = varDsc->lvFieldLclStart + index;
-                        LclVarDsc*  fieldVarDsc = lvaGetDesc(fieldLclNum);
-                        const char* fieldName;
-#if !defined(TARGET_64BIT)
-                        if (varTypeIsLong(varDsc))
-                        {
-                            fieldName = (index == 0) ? "lo" : "hi";
-                        }
-                        else
-#endif // !defined(TARGET_64BIT)
-                        {
-                            CORINFO_CLASS_HANDLE typeHnd = varDsc->GetLayout()->GetClassHandle();
-                            CORINFO_FIELD_HANDLE fldHnd =
-                                info.compCompHnd->getFieldInClass(typeHnd, fieldVarDsc->lvFldOrdinal);
-                            fieldName = eeGetFieldName(fldHnd, true, buffer, sizeof(buffer));
-                        }
+                        unsigned   fieldLclNum = varDsc->lvFieldLclStart + index;
+                        LclVarDsc* fieldVarDsc = lvaGetDesc(fieldLclNum);
 
                         printf("\n");
                         printf("                                                            ");
                         printIndent(indentStack);
-                        printf("    %-6s V%02u.%s (offs=0x%02x) -> ", varTypeName(fieldVarDsc->TypeGet()),
-                               tree->AsLclVarCommon()->GetLclNum(), fieldName, fieldVarDsc->lvFldOffset);
+                        printf("    %-6s %s -> ", varTypeName(fieldVarDsc->TypeGet()), fieldVarDsc->lvReason);
                         gtDispLclVar(fieldLclNum);
                         gtDispSsaName(fieldLclNum, tree->AsLclVarCommon()->GetSsaNum(this, index), isDef);