diff options
Diffstat (limited to 'src/System.Private.CoreLib/src/System/String.CoreRT.cs')
| -rw-r--r-- | src/System.Private.CoreLib/src/System/String.CoreRT.cs | 57 |
1 files changed, 51 insertions, 6 deletions
diff --git a/src/System.Private.CoreLib/src/System/String.CoreRT.cs b/src/System.Private.CoreLib/src/System/String.CoreRT.cs index a892006d9..3aedf2c50 100644 --- a/src/System.Private.CoreLib/src/System/String.CoreRT.cs +++ b/src/System.Private.CoreLib/src/System/String.CoreRT.cs @@ -16,9 +16,52 @@ using Internal.Runtime.CompilerServices; namespace System { + // STRING LAYOUT + // ------------- + // Strings are null-terminated for easy interop with native, but the value returned by String.Length + // does NOT include this null character in its count. As a result, there's some trickiness here in the + // layout and allocation of strings that needs explanation... + // + // String is allocated like any other array, using the RhNewArray API. It is essentially a very special + // char[] object. In order to be an array, the String EEType must have an 'array element size' of 2, + // which is setup by a special case in the binder. Strings must also have a typical array instance + // layout, which means that the first field after the m_pEEType field is the 'number of array elements' + // field. However, here, it is called _stringLength because it contains the number of characters in the + // string (NOT including the terminating null element) and, thus, directly represents both the array + // length and String.Length. + // + // As with all arrays, the GC calculates the size of an object using the following formula: + // + // obj_size = align(base_size + (num_elements * element_size), sizeof(void*)) + // + // The values 'base_size' and 'element_size' are both stored in the EEType for String and 'num_elements' + // is _stringLength. + // + // Our base_size is the size of the fixed portion of the string defined below. It, therefore, contains + // the size of the _firstChar field in it. This means that, since our string data actually starts + // inside the fixed 'base_size' area, and our num_elements is equal to String.Length, we end up with one + // extra character at the end. This is how we get our extra null terminator which allows us to pass a + // pinned string out to native code as a null-terminated string. This is also why we don't increment the + // requested string length by one before passing it to RhNewArray. There is no need to allocate an extra + // array element, it is already allocated here in the fixed layout of the String. + // + // Typically, the base_size of an array type is aligned up to the nearest pointer size multiple (so that + // array elements start out aligned in case they need alignment themselves), but we don't want to do that + // with String because we are allocating String.Length components with RhNewArray and the overall object + // size will then need another alignment, resulting in wasted space. So the binder specially shrinks the + // base_size of String, leaving it unaligned in order to allow the use of that otherwise wasted space. + // + // One more note on base_size -- on 64-bit, the base_size ends up being 22 bytes, which is less than the + // min_obj_size of (3 * sizeof(void*)). This is OK because our array allocator will still align up the + // overall object size, so a 0-length string will end up with an object size of 24 bytes, which meets the + // min_obj_size requirement. + // + // NOTE: This class is marked EagerStaticClassConstruction because McgCurrentModule class being eagerly + // constructed itself depends on this class also being eagerly constructed. Plus, it's nice to have this + // eagerly constructed to avoid the cost of defered ctors. I can't imagine any app that doesn't use string + // [StructLayout(LayoutKind.Sequential)] [System.Runtime.CompilerServices.EagerStaticClassConstructionAttribute] - [System.Runtime.CompilerServices.TypeForwardedFrom("mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089")] public partial class String { #if BIT64 @@ -47,11 +90,8 @@ namespace System #pragma warning restore - public static readonly String Empty = ""; + public static readonly string Empty = ""; - // Gets the character at a specified position. - // - // Spec#: Apply the precondition here using a contract assembly. Potential perf issue. [System.Runtime.CompilerServices.IndexerName("Chars")] public unsafe char this[int index] { @@ -72,7 +112,12 @@ namespace System #endif } - internal static String FastAllocateString(int length) + public int Length + { + get { return _stringLength; } + } + + internal static string FastAllocateString(int length) { // We allocate one extra char as an interop convenience so that our strings are null- // terminated, however, we don't pass the extra +1 to the string allocation because the base |