Stack allocate unescaped boxes (#103361)

Enable object stack allocation for ref classes and extend the support to include boxed value classes. Use a specialized unbox helper for stack allocated boxes, both to avoid apparent escape of the box by the helper, and to ensure all box field accesses are visible to the JIT. Update the local address visitor to rewrite trees involving address of stack allocated boxes in some cases to avoid address exposure. Disable old promotion for stack allocated boxes (since we have no field handles) and allow physical promotion to enregister the box method table and/or payload as appropriate. In OSR methods handle the fact that the stack allocation may actually have been a heap allocation by the Tier0 method.

The analysis TP cost is around 0.4-0.7% (notes below). Boxes are much less likely to escape than ref classes (roughly ~90% of boxes escape, ~99.8% of ref classes escape). Codegen impact is diminished somewhat because many of the boxes are dead and were already getting optimized away.
 
Fixes #4584, #9118, #10195, #11192, #53585, #58554, #85570

---------

Co-authored-by: Jakob Botsch Nielsen <[email protected]>
Co-authored-by: Jan Kotas <[email protected]>

Author: 3 people

src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -197,6 +197,7 @@
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\CastHelpers.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\ICastableHelpers.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\RuntimeHelpers.CoreCLR.cs" />
+    <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\StackAllocatedBox.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\ControlledExecution.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\DependentHandle.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)..\nativeaot\Common\src\System\Runtime\RhFailFastReason.cs" />

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs

@@ -0,0 +1,15 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+
+namespace System.Runtime.CompilerServices
+{
+    [StructLayout(LayoutKind.Sequential)]
+    internal unsafe struct StackAllocatedBox<T>
+    {
+        // These fields are only accessed from jitted code
+        private MethodTable* _pMethodTable;
+        private T _value;
+    }
+}

src/coreclr/inc/corinfo.h

@@ -445,6 +445,7 @@ enum CorInfoHelpFunc
     CORINFO_HELP_BOX,               // Fast box helper. Only possible exception is OutOfMemory
     CORINFO_HELP_BOX_NULLABLE,      // special form of boxing for Nullable<T>
     CORINFO_HELP_UNBOX,
+    CORINFO_HELP_UNBOX_TYPETEST,    // Verify unbox type, throws if incompatible
     CORINFO_HELP_UNBOX_NULLABLE,    // special form of unboxing for Nullable<T>
     CORINFO_HELP_GETREFANY,         // Extract the byref from a TypedReference, checking that it is the expected type
 
@@ -2552,6 +2553,14 @@ class ICorStaticInfo
             CORINFO_CLASS_HANDLE        cls
             ) = 0;
 
+    // Get a representation for a stack-allocated boxed value type.
+    //
+    // This differs from getTypeForBox in that it includes an explicit field
+    // for the method table pointer.
+    virtual CORINFO_CLASS_HANDLE getTypeForBoxOnStack(
+            CORINFO_CLASS_HANDLE        cls
+            ) = 0;
+
     // returns the correct box helper for a particular class.  Note
     // that if this returns CORINFO_HELP_BOX, the JIT can assume
     // 'standard' boxing (allocate object and copy), and optimize

src/coreclr/inc/icorjitinfoimpl_generated.h

@@ -275,6 +275,9 @@ CorInfoHelpFunc getSharedCCtorHelper(
 CORINFO_CLASS_HANDLE getTypeForBox(
           CORINFO_CLASS_HANDLE cls) override;
 
+CORINFO_CLASS_HANDLE getTypeForBoxOnStack(
+          CORINFO_CLASS_HANDLE cls) override;
+
 CorInfoHelpFunc getBoxHelper(
           CORINFO_CLASS_HANDLE cls) override;
 

src/coreclr/inc/jiteeversionguid.h

@@ -43,11 +43,11 @@ typedef const GUID *LPCGUID;
 #define GUID_DEFINED
 #endif // !GUID_DEFINED
 
-constexpr GUID JITEEVersionIdentifier = { /* e428e66d-5e0e-4320-ad8a-fa5a50f6da07 */
-    0xe428e66d,
-    0x5e0e,
-    0x4320,
-    {0xad, 0x8a, 0xfa, 0x5a, 0x50, 0xf6, 0xda, 0x07}
+constexpr GUID JITEEVersionIdentifier = { /* 748cd07e-c686-4085-8f5f-54c1b9cff44c */
+    0x748cd07e,
+    0xc686,
+    0x4085,
+    {0x8f, 0x5f, 0x54, 0xc1, 0xb9, 0xcf, 0xf4, 0x4c}
   };
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////////

src/coreclr/inc/jithelpers.h

@@ -106,6 +106,7 @@
     DYNAMICJITHELPER(CORINFO_HELP_BOX,          JIT_Box,            CORINFO_HELP_SIG_REG_ONLY)
     JITHELPER(CORINFO_HELP_BOX_NULLABLE,        JIT_Box,            CORINFO_HELP_SIG_REG_ONLY)
     DYNAMICJITHELPER(CORINFO_HELP_UNBOX,        NULL,               CORINFO_HELP_SIG_REG_ONLY)
+    JITHELPER(CORINFO_HELP_UNBOX_TYPETEST,      JIT_Unbox_TypeTest, CORINFO_HELP_SIG_REG_ONLY)
     JITHELPER(CORINFO_HELP_UNBOX_NULLABLE,      JIT_Unbox_Nullable, CORINFO_HELP_SIG_4_STACK)
 
     JITHELPER(CORINFO_HELP_GETREFANY,           JIT_GetRefAny,      CORINFO_HELP_SIG_8_STACK)

src/coreclr/jit/ICorJitInfo_names_generated.h

@@ -68,6 +68,7 @@ DEF_CLR_API(getNewArrHelper)
 DEF_CLR_API(getCastingHelper)
 DEF_CLR_API(getSharedCCtorHelper)
 DEF_CLR_API(getTypeForBox)
+DEF_CLR_API(getTypeForBoxOnStack)
 DEF_CLR_API(getBoxHelper)
 DEF_CLR_API(getUnBoxHelper)
 DEF_CLR_API(getRuntimeTypePointer)

src/coreclr/jit/ICorJitInfo_wrapper_generated.hpp

@@ -635,6 +635,15 @@ CORINFO_CLASS_HANDLE WrapICorJitInfo::getTypeForBox(
     return temp;
 }
 
+CORINFO_CLASS_HANDLE WrapICorJitInfo::getTypeForBoxOnStack(
+          CORINFO_CLASS_HANDLE cls)
+{
+    API_ENTER(getTypeForBoxOnStack);
+    CORINFO_CLASS_HANDLE temp = wrapHnd->getTypeForBoxOnStack(cls);
+    API_LEAVE(getTypeForBoxOnStack);
+    return temp;
+}
+
 CorInfoHelpFunc WrapICorJitInfo::getBoxHelper(
           CORINFO_CLASS_HANDLE cls)
 {

src/coreclr/jit/compiler.h

@@ -690,7 +690,8 @@ class LclVarDsc
     unsigned char lvSingleDefDisqualifyReason = 'H';
 #endif
 
-    unsigned char lvAllDefsAreNoGc : 1; // For pinned locals: true if all defs of this local are no-gc
+    unsigned char lvAllDefsAreNoGc    : 1; // For pinned locals: true if all defs of this local are no-gc
+    unsigned char lvStackAllocatedBox : 1; // Local is a stack allocated box
 
 #if FEATURE_MULTIREG_ARGS
     regNumber lvRegNumForSlot(unsigned slotNum)
@@ -805,6 +806,11 @@ class LclVarDsc
         return lvIsMultiRegArg || lvIsMultiRegRet;
     }
 
+    bool IsStackAllocatedBox() const
+    {
+        return lvStackAllocatedBox;
+    }
+
 #if defined(DEBUG)
 private:
     DoNotEnregisterReason m_doNotEnregReason;
@@ -3523,7 +3529,7 @@ class Compiler
 
     GenTree* gtNewRuntimeLookup(CORINFO_GENERIC_HANDLE hnd, CorInfoGenericHandleType hndTyp, GenTree* lookupTree);
 
-    GenTreeIndir* gtNewMethodTableLookup(GenTree* obj);
+    GenTreeIndir* gtNewMethodTableLookup(GenTree* obj, bool onStack = false);
 
     //------------------------------------------------------------------------
     // Other GenTree functions

src/coreclr/jit/compiler.hpp

@@ -1872,9 +1872,9 @@ inline GenTreeCast* Compiler::gtNewCastNodeL(var_types typ, GenTree* op1, bool f
     return cast;
 }
 
-inline GenTreeIndir* Compiler::gtNewMethodTableLookup(GenTree* object)
+inline GenTreeIndir* Compiler::gtNewMethodTableLookup(GenTree* object, bool onStack)
 {
-    assert(object->TypeIs(TYP_REF));
+    assert(onStack || object->TypeIs(TYP_REF));
     GenTreeIndir* result = gtNewIndir(TYP_I_IMPL, object, GTF_IND_INVARIANT);
     return result;
 }