Tuesday, January 04, 2005

Byte Alignment in .NET and Structure Layouts :: JIT and .NET 2.0

What is the difference between the following structures?

struct BadValueType
char c1;
int i;
char c2;

struct GoodValueType
int i;
char c1;
char c2;

Nothing much, except the jumbled type declarations Huh?

Fine,Now let's look at the size of these structures,

The size of BadValueType Structure in:
.NET Framework 1.x : Managed sizeof= 8 Bytes, Marshal.Sizeof = 12 Bytes
.NET Framework 2.0 : Managed sizeof= 12 Bytes, Marshal.Sizeof = 12 Bytes

The size of GoodValueType Structure in:
.NET Framework 1.x : Managed sizeof= 8 Bytes, Marshal.Sizeof = 8 Bytes
.NET Framework 2.0 : Managed sizeof= 8 Bytes, Marshal.Sizeof = 8 Bytes

[Note: Size of int=4, char=2]

The Reason behind these differences is "BYTE ALIGNMENT", As with the default packing in unmanaged C++, integers are laid out on four-byte boundaries, so while the first
character uses two bytes (a char in managed code is a Unicode character, thus occupying two bytes), the integer moves up to the next 4-byte boundary, and the second character uses the subsequent 2 bytes. The resulting structure is 12 bytes when measured with Marshal.SizeOf.

32 bit microprocessors typically organize memory as shown below.

Byte 0 Byte 1 Byte 2 Byte 3
0x1004 A0 A1 A2 A3
0x100C B0 B1 B2
0x1010 B3

Most of the processer architectures cannot read data from odd addresses.
Processor Architectures are inefficient in reading the data if it starts at an address not divisible by four.

Memory is accessed by performing 32 bit bus cycles. 32 bit bus cycles can however be performed at addresses that are divisible by 4. So for efficiency purposes, compilers add the so-called pad bytes.

The reasons for not permitting misaligned long word reads and writes are not difficult to see. For example, an aligned long word A would be written as A0, A1, A2 and A3.

Thus the microprocessor can read the complete long word in a single bus cycle. If the same microprocessor now attempts to access a long word at address 0x100D, it will have to read bytes B0, B1, B2 and B3. Notice that this read cannot be performed in a single 32 bit bus cycle. The microprocessor will have to issue two different reads at address 0x100C and 0x1010 to read the complete long word. Thus it takes twice the time to read a misaligned long word.

The following byte padding rules will generally work with most 32 bit processor.

a. single byte numbers can be aligned at any address
b. Two byte numbers should be aligned to a two byte boundary
c. Four byte numbers should be aligned to a four byte boundary

This is the cause of the difference.

Fine…. But, why does this differ from .NET 1.x and .NET 2.0 ?

The Answer is that by default, the .NET compilers all apply a StructLayoutAttribute to structures, specifying a Sequential layout. This means that the fields are laid out in the type according to their order in the source file. However, in the .NET Framework 1.x, a request for a Sequential layout is not respected by the just-in-time compiler.

Here is the IL.

.class nested private sequential ansi sealed beforefieldinit BadValueType
extends [mscorlib]System.ValueType
.field public char c1

.field public char c2

.field public int32 i


In the .NET Framework 2.0, the JIT does enforce a Sequential
layout (if specified) for the managed layout of value types
though only if there are no reference type field members. Thus, the sizing of types is more important in the next version[2.0] of the Framework.

Bottom-line: in simple English, .NET 2.0 JIT does its job flawlessly on structures.

Some time back I had a bit of argument with a friend of mine, to prove the above 2 structures actually differs in implementation, and I remembered this when I read a article

on the same lines today from MSDN, A pretty neat article. Infact this article is an elaboration of the point given as one-liner in the article.

Watchout for structures when you create them next time, and think about playing around with ‘m’ structures with ‘n’ size…. m x n = !!!

posted by Logu Krishnan : 2:14 AM

Monday, January 03, 2005

MVP - Reawarded for Year 2005

Microsoft Corporation has awarded the MVP(C#) Title for the Year 2005.

My MVP Profile @ Microsoft Site is Here [^http://tinyurl.com/65mk5 ]

I Think This Year I would be able to spend more time on the community activity, and my new job does not have many burdens on my shoulders, unlike the previous one. Let's see...

If the word "MVP" Puzzles you, or have questions, you should visit http://www.microsoft.com/communities/MVP/MVP.mspx

For the List of other 130 C# MVP's worldwide... Click Here

Thanks Microsoft :-)

posted by Logu Krishnan : 10:07 AM

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