Fix System.String over-allocation (#17876)

BaseSize for System.String was not set correctly. It caused unnecessary extra 8 bytes to be allocated at the end of strings that had `Length % 4 < 2` on 64-bit platforms.

This change makes affected strings proportionally cheaper. For example, `new string('a', 1)` in a long-running loop is 7% faster.

src/debug/daccess/request.cpp

@@ -1475,7 +1475,7 @@ ClrDataAccess::GetObjectStringData(CLRDATA_ADDRESS obj, unsigned int count, __ou
             if (count > needed)
                 count = needed;
 
-            TADDR pszStr = TO_TADDR(obj)+offsetof(StringObject, m_Characters);
+            TADDR pszStr = TO_TADDR(obj)+offsetof(StringObject, m_FirstChar);
             hr = m_pTarget->ReadVirtual(pszStr, (PBYTE)stringData, count * sizeof(wchar_t), &needed);
 
             if (SUCCEEDED(hr))

src/vm/amd64/JitHelpers_InlineGetThread.asm

@@ -134,12 +134,10 @@ LEAF_ENTRY AllocateStringFastMP_InlineGetThread, _TEXT
         cmp     ecx, (ASM_LARGE_OBJECT_SIZE - 256)/2
         jae     OversizedString
 
-        mov     edx, [r9 + OFFSET__MethodTable__m_BaseSize]
-
         ; Calculate the final size to allocate.
         ; We need to calculate baseSize + cnt*2, then round that up by adding 7 and anding ~7.
 
-        lea     edx, [edx + ecx*2 + 7]
+        lea     edx, [STRING_BASE_SIZE + ecx*2 + 7]
         and     edx, -8
 
         INLINE_GETTHREAD r11

src/vm/amd64/JitHelpers_Slow.asm

@@ -276,12 +276,10 @@ LEAF_ENTRY AllocateStringFastUP, _TEXT
         cmp     ecx, (ASM_LARGE_OBJECT_SIZE - 256)/2
         jae     FramedAllocateString
 
-        mov     r8d, [r11 + OFFSET__MethodTable__m_BaseSize]
-
         ; Calculate the final size to allocate.
         ; We need to calculate baseSize + cnt*2, then round that up by adding 7 and anding ~7.
 
-        lea     r8d, [r8d + ecx*2 + 7]
+        lea     r8d, [STRING_BASE_SIZE + ecx*2 + 7]
         and     r8d, -8
 
         inc     [g_global_alloc_lock]

src/vm/amd64/asmconstants.h

@@ -545,6 +545,9 @@ ASMCONSTANTS_C_ASSERT(ASM_LARGE_OBJECT_SIZE == LARGE_OBJECT_SIZE);
 ASMCONSTANTS_C_ASSERT(OFFSETOF__ArrayBase__m_NumComponents
                     == offsetof(ArrayBase, m_NumComponents));
 
+#define                     STRING_BASE_SIZE 0x16
+ASMCONSTANTS_RUNTIME_ASSERT(STRING_BASE_SIZE == StringObject::GetBaseSize());
+
 #define               OFFSETOF__StringObject__m_StringLength 0x8
 ASMCONSTANTS_C_ASSERT(OFFSETOF__StringObject__m_StringLength
                     == offsetof(StringObject, m_StringLength));

src/vm/appdomain.cpp

@@ -2759,8 +2759,6 @@ void SystemDomain::LoadBaseSystemClasses()
 
     // Load String
     g_pStringClass = MscorlibBinder::LoadPrimitiveType(ELEMENT_TYPE_STRING);
-    _ASSERTE(g_pStringClass->GetBaseSize() == ObjSizeOf(StringObject)+sizeof(WCHAR));
-    _ASSERTE(g_pStringClass->GetComponentSize() == 2);
 
     // Used by Buffer::BlockCopy
     g_pByteArrayMT = ClassLoader::LoadArrayTypeThrowing(

src/vm/arm/asmconstants.h

@@ -95,18 +95,6 @@ ASMCONSTANTS_C_ASSERT(MethodTableWriteableData__m_dwFlags == offsetof(MethodTabl
 #define MethodTableWriteableData__enum_flag_Unrestored 0x04
 ASMCONSTANTS_C_ASSERT(MethodTableWriteableData__enum_flag_Unrestored == MethodTableWriteableData::enum_flag_Unrestored);
 
-#define StringObject__m_StringLength    0x04
-ASMCONSTANTS_C_ASSERT(StringObject__m_StringLength == offsetof(StringObject, m_StringLength));
-
-#define SIZEOF__BaseStringObject       0xe
-ASMCONSTANTS_C_ASSERT(SIZEOF__BaseStringObject == (ObjSizeOf(StringObject) + sizeof(WCHAR)));
-
-#define SIZEOF__ArrayOfObjectRef       0xc
-ASMCONSTANTS_C_ASSERT(SIZEOF__ArrayOfObjectRef == ObjSizeOf(ArrayBase));
-
-#define SIZEOF__ArrayOfValueType       0xc
-ASMCONSTANTS_C_ASSERT(SIZEOF__ArrayOfValueType == ObjSizeOf(ArrayBase));
-
 #define ArrayBase__m_NumComponents     0x4
 ASMCONSTANTS_C_ASSERT(ArrayBase__m_NumComponents == offsetof(ArrayBase, m_NumComponents));
 
@@ -116,33 +104,8 @@ ASMCONSTANTS_C_ASSERT(ArrayTypeDesc__m_Arg == offsetof(ArrayTypeDesc, m_Arg));
 #define PtrArray__m_Array              0x8
 ASMCONSTANTS_C_ASSERT(PtrArray__m_Array == offsetof(PtrArray, m_Array));
 
-#define SYSTEM_INFO__dwNumberOfProcessors 0x14
-ASMCONSTANTS_C_ASSERT(SYSTEM_INFO__dwNumberOfProcessors == offsetof(SYSTEM_INFO, dwNumberOfProcessors));
-
 #define TypeHandle_CanCast 0x1 // TypeHandle::CanCast
 
-// Maximum number of characters to be allocated for a string in AllocateStringFast*. Chosen so that we'll
-// never have to check for overflow and will never try to allocate a string on regular heap that should have
-// gone on the large object heap. Additionally the constant has been chosen such that it can be encoded in a
-// single Thumb2 CMP instruction.
-#define MAX_FAST_ALLOCATE_STRING_SIZE   42240
-ASMCONSTANTS_C_ASSERT(MAX_FAST_ALLOCATE_STRING_SIZE < ((LARGE_OBJECT_SIZE - SIZEOF__BaseStringObject) / 2));
-
-
-// Array of objectRef of this Maximum number of elements can be allocated in JIT_NewArr1OBJ_MP*. Chosen so that we'll
-// never have to check for overflow and will never try to allocate the array on regular heap that should have
-// gone on the large object heap. Additionally the constant has been chosen such that it can be encoded in a
-// single Thumb2 CMP instruction.
-#define MAX_FAST_ALLOCATE_ARRAY_OBJECTREF_SIZE   21120
-ASMCONSTANTS_C_ASSERT(MAX_FAST_ALLOCATE_ARRAY_OBJECTREF_SIZE < ((LARGE_OBJECT_SIZE - SIZEOF__ArrayOfObjectRef) / sizeof(void*)));
-
-// Array of valueClass of this Maximum number of characters can be allocated JIT_NewArr1VC_MP*. Chosen so that we'll
-// never have to check for overflow and will never try to allocate the array on regular heap that should have
-// gone on the large object heap. Additionally the constant has been chosen such that it can be encoded in a
-// single Thumb2 CMP instruction.
-#define MAX_FAST_ALLOCATE_ARRAY_VC_SIZE   65280
-ASMCONSTANTS_C_ASSERT(MAX_FAST_ALLOCATE_ARRAY_VC_SIZE < ((4294967296 - 1 - SIZEOF__ArrayOfValueType) / 65536));
-
 #define SIZEOF__GSCookie              0x4
 ASMCONSTANTS_C_ASSERT(SIZEOF__GSCookie == sizeof(GSCookie));
 
@@ -203,14 +166,6 @@ ASMCONSTANTS_C_ASSERT(UnmanagedToManagedFrame__m_pvDatum == offsetof(UnmanagedTo
 
 #endif // FEATURE_COMINTEROP
 
-#ifndef CROSSGEN_COMPILE
-#define               Thread__m_alloc_context__alloc_limit   0x44
-ASMCONSTANTS_C_ASSERT(Thread__m_alloc_context__alloc_limit == offsetof(Thread, m_alloc_context) + offsetof(gc_alloc_context, alloc_limit));
-
-#define               Thread__m_alloc_context__alloc_ptr   0x40
-ASMCONSTANTS_C_ASSERT(Thread__m_alloc_context__alloc_ptr == offsetof(Thread, m_alloc_context) + offsetof(gc_alloc_context, alloc_ptr));
-#endif // CROSSGEN_COMPILE
-
 #define               Thread__m_fPreemptiveGCDisabled   0x08
 #ifndef CROSSGEN_COMPILE
 ASMCONSTANTS_C_ASSERT(Thread__m_fPreemptiveGCDisabled == offsetof(Thread, m_fPreemptiveGCDisabled));

src/vm/i386/jitinterfacex86.cpp

@@ -1063,14 +1063,13 @@ void *JIT_TrialAlloc::GenAllocString(Flags flags)
     // jae noLock - seems tempting to jump to noAlloc, but we haven't taken the lock yet
     sl.X86EmitCondJump(noLock, X86CondCode::kJAE);
 
-    // mov edx, [ecx]MethodTable.m_BaseSize
-    sl.X86EmitIndexRegLoad(kEDX, kECX, offsetof(MethodTable,m_BaseSize));
-
     // Calculate the final size to allocate.
     // We need to calculate baseSize + cnt*2, then round that up by adding 3 and anding ~3.
 
-    // lea eax, [edx+eax*2+5]
-    sl.X86EmitOp(0x8d, kEAX, kEDX, (DATA_ALIGNMENT-1), kEAX, 2);
+    // lea eax, [basesize+(alignment-1)+eax*2]
+    sl.Emit16(0x048d);
+    sl.Emit8(0x45);
+    sl.Emit32(StringObject::GetBaseSize() + (DATA_ALIGNMENT-1));
 
     // and eax, ~3
     sl.Emit16(0xe083);

src/vm/methodtablebuilder.cpp

@@ -9737,8 +9737,8 @@ void MethodTableBuilder::CheckForSystemTypes()
         {
             // Strings are not "normal" objects, so we need to mess with their method table a bit
             // so that the GC can figure out how big each string is...
-            DWORD baseSize = ObjSizeOf(StringObject) + sizeof(WCHAR);
-            pMT->SetBaseSize(baseSize); // NULL character included
+            DWORD baseSize = StringObject::GetBaseSize();
+            pMT->SetBaseSize(baseSize);
 
             GetHalfBakedClass()->SetBaseSizePadding(baseSize - bmtFP->NumInstanceFieldBytes);
 

src/vm/object.h

@@ -914,7 +914,7 @@ typedef PTR_StringObject STRINGREF;
  * Special String implementation for performance.   
  *
  *   m_StringLength - Length of string in number of WCHARs
- *   m_Characters   - The string buffer
+ *   m_FirstChar    - The string buffer
  *
  */
 
@@ -932,9 +932,6 @@ typedef PTR_StringObject STRINGREF;
 #define STRING_STATE_FAST_OPS         0x80000000
 #define STRING_STATE_SPECIAL_SORT     0xC0000000
 
-#ifdef _MSC_VER
-#pragma warning(disable : 4200)     // disable zero-sized array warning
-#endif
 class StringObject : public Object
 {
 #ifdef DACCESS_COMPILE
@@ -947,11 +944,7 @@ class StringObject : public Object
 
   private:
     DWORD   m_StringLength;
-    WCHAR   m_Characters[0];
-    // GC will see a StringObject like this:
-    //   DWORD m_StringLength
-    //   WCHAR m_Characters[0]
-    //   DWORD m_OptionalPadding (this is an optional field and will appear based on need)
+    WCHAR   m_FirstChar;
 
   public:
     VOID    SetStringLength(DWORD len)                   { LIMITED_METHOD_CONTRACT; _ASSERTE(len >= 0); m_StringLength = len; }
@@ -961,12 +954,11 @@ class StringObject : public Object
    ~StringObject() {LIMITED_METHOD_CONTRACT; }
 
   public:
+    static DWORD GetBaseSize();
     static SIZE_T GetSize(DWORD stringLength);
 
     DWORD   GetStringLength()                           { LIMITED_METHOD_DAC_CONTRACT; return( m_StringLength );}
-    WCHAR*  GetBuffer()                                 { LIMITED_METHOD_CONTRACT; _ASSERTE(this != nullptr); return (WCHAR*)( dac_cast<TADDR>(this) + offsetof(StringObject, m_Characters) );  }
-    WCHAR*  GetBuffer(DWORD *pdwSize)                   { LIMITED_METHOD_CONTRACT; _ASSERTE((this != nullptr) && pdwSize); *pdwSize = GetStringLength(); return GetBuffer();  }
-    WCHAR*  GetBufferNullable()                         { LIMITED_METHOD_CONTRACT; return( (this == nullptr) ? nullptr : (WCHAR*)( dac_cast<TADDR>(this) + offsetof(StringObject, m_Characters) ) );  }
+    WCHAR*  GetBuffer()                                 { LIMITED_METHOD_CONTRACT; _ASSERTE(this != nullptr); return (WCHAR*)( dac_cast<TADDR>(this) + offsetof(StringObject, m_FirstChar) );  }
 
     DWORD GetHighCharState() {
         WRAPPER_NO_CONTRACT;
@@ -992,7 +984,7 @@ class StringObject : public Object
     static UINT GetBufferOffset()
     {
         LIMITED_METHOD_DAC_CONTRACT;
-        return (UINT)(offsetof(StringObject, m_Characters));
+        return (UINT)(offsetof(StringObject, m_FirstChar));
     }
     static UINT GetStringLengthOffset()
     {
@@ -3329,7 +3321,7 @@ class Nullable {
     static inline void *Value(void *src, MethodTable *nullableMT)
     {
         Nullable *nullable = (Nullable *)src;
-        return nullable->ValueAddr(nullableMT);        
+        return nullable->ValueAddr(nullableMT);
     }
 
 private:

src/vm/object.inl

@@ -58,12 +58,18 @@ inline SIZE_T Object::GetSize()
     return s;
 }
 
+__forceinline /*static*/ DWORD StringObject::GetBaseSize()
+{
+    LIMITED_METHOD_DAC_CONTRACT;
+
+    return ObjSizeOf(Object) + sizeof(DWORD) /* length */ + sizeof(WCHAR) /* null terminator */;
+}
+
 __forceinline /*static*/ SIZE_T StringObject::GetSize(DWORD strLen)
 {
     LIMITED_METHOD_DAC_CONTRACT;
 
-    // Extra WCHAR for null terminator
-    return ObjSizeOf(StringObject) + sizeof(WCHAR) + strLen * sizeof(WCHAR);
+    return GetBaseSize() + strLen * sizeof(WCHAR);
 }
 
 #ifdef DACCESS_COMPILE