The Lost Art of C Structure Packing. This page is about a technique for reducing the memory footprint of C. C structure declarations for reduced. Reasons for using assembly code Assembly coding is not used as much today as previously. However, there are still reasons for learning and using assembly code. Proposed Syllabus by C. S. J. M. University,Kanpur. Bachelors of Computer Application Semester wise breakup of course SemesterIst Course Code Course Name L T P C. Application Report SLAA294AMarch 2006Revised August 2006 MSP430 Software Coding Techniques KeithQuiring. OptimizeGeneratedCodeByDevelopingAndUsingCRLsExample_01.png' alt='C Code Optimization Techniques Pdf' title='C Code Optimization Techniques Pdf' />To read it, you will require basic knowledge of the C. You need to know this technique if you intend to write code for. It. is useful if you are working with application data sets so large that. It is good to know in any. Finally, knowing this technique is a gateway to other esoteric C. You are not an advanced C programmer until you have grasped. You are not a master of C until you could have written this. This webpage exists because in late 2. I found myself heavily. C optimization technique that I had learned more than two. I needed to reduce the memory footprint of a program that used thousands. C struct instances. The program. was cvs fast export and. There are ways to reduce memory usage significantly in situations like. This can lead to dramatic gains in my case I was able to cut the. But as I worked, and thought about what I was doing, it began to dawn. I was using has been more than half forgotten. A little web research confirmed that C. A couple of Wikipedia entries. I found nobody who covered it comprehensively. There are actually reasons for this that arent stupid. CS courses. rightly steer people away from micro optimization towards finding better. The plunging price of machine resources has made squeezing. And the way hackers used to learn how to. But the technique still has value in important situations, and will. This document is intended to save C. Is Outlook 2010 Compatible With Vista more. Alignment requirements. A179NF_JvwYku4e6o8HOejYWv46u0tfBXqxKqG-ocRL0u8FSJwCfq7ulV_FkHycjqBU=h900' alt='C Code Optimization Techniques Pdf' title='C Code Optimization Techniques Pdf' />The first thing to understand is that, on modern processors, the way. C compiler lays out basic C datatypes in memory is constrained. Storage for the basic C datatypes on an x. ARM processor doesnt. Rather, each. type except char has an alignment requirement chars can start on. Signed or unsigned makes no difference. The jargon for this is that basic C types on x. A to Z of C is the nonprofit project to bring cheapest and best book for CDOS programming. ARM are. self aligned. Pointers, whether 3. C Code Optimization Techniques Pdf Merge' title='C Code Optimization Techniques Pdf Merge' />C Code Optimization Techniques Pdf FreeSelf alignment makes access faster because it facilitates generating. Without. alignment constraints, on the other hand, the code might end up having. Bol Raz Jeden Zivot. Characters are a special case theyre equally expensive from anywhere. Thats why they dont have a. I said on modern processors because on some older ones forcing your. C program to violate alignment rules say, by casting an odd address. This was the behavior, for. Sun SPARC chips. In fact, with sufficient determination. Also, self alignment is not the only possible rule. Historically, some. If you do embedded systems, you might trip over one. Be aware this is possible. From when it was written at the beginning of 2. During that period Ive learned. NTP. It does packet analysis by reading. The interesting news is that NTP has apparently being getting away. Unixes but under Windows. This suggests that platforms with padding rules. NTP servers or. clients. Now well look at a simple example of variable layout in. Consider the following series of variable declarations. C module char p. If you didnt know anything about data alignment, you might assume. That is, on a 3. 2 bit machine 4 bytes of pointer would be. And a 6. 4 bit machine would be different only. In fact, the hidden assumption that the allocated order of static. C. standards dont mandate it. Im going to ignore this detail because. Heres what actually happens on an x. ARM or anything else with. The storage for p starts on a self aligned 4. This is. pointer alignment the strictest possible. The storage for c follows immediately. But the 4 byte alignment. The pad3 character array represents the fact that there are three. The old school term for this. The value of the padding bits is undefined in particular. Compare what happens if x is a 2 byte short char. In that case, the actual layout will be this char 4 or 8 bytes. On the other hand, if x is a long on a 6. If you have been following carefully, you are probably now wondering about. If the actual memory layout were written like thischar c. M and N First, in this case N will be zero. The address of x, coming. The value of M is less predictable. If the compiler. happened to map c to the last byte of a machine word, the next. M would be zero. It is more likely that c will be mapped to the first byte of a. In that case M will be whatever padding is needed to. Intermediate cases are possible. M can be anything from 0 to 7. If you wanted to make those variables take up less space. Usually, for the small number of scalar variables in your C programs. The technique. becomes more interesting when applied to nonscalar variables. Before we get to those, lets dispose of arrays of scalars. On a. platform with self aligned types, arrays of. In the next section we will see that the same is not necessarily true of. Structure alignment and padding. In general, a struct instance will have the alignment of its widest scalar. Compilers do this as the easiest way to ensure that all the. Also, in C the address of a struct is the same as the address of its. Beware in C, classes. Whether they do or not. When youre in doubt about this sort of thing, ANSI C provides an. Consider this struct struct foo. Assuming a 6. 4 bit machine, any instance of struct foo. The memory layout of one of these looks. Its laid out exactly as though variables of these types has. But if we put c first, thats. If the members were separate variables, c could start at any byte. Because struct foo. Now c has to be pointer aligned, and following padding of 7 bytes is. Now lets talk about trailing padding on structures. To explain this. I need to introduce a basic concept which Ill call the stride. It is the first address following the. The general rule of trailing structure padding is this the compiler. This rule controls what sizeof will return. Consider this example on a 6. ARM machine struct foo. You might think that sizeofstruct foo. The stride address is that of p2. Thus, in the quad. The memory layout is as though the structure had been. For contrast, consider the following example struct foo. Because s only needs to be 2 byte aligned, the stride address is. It will be laid out like this struct foo. Heres a last important detail If your structure has structure. Suppose you write this struct foo. The char member in the inner struct forces the outer struct to be. Actual layout will be like. This structure gives us a hint of the savings that might be possible. Of 2. 4 bytes, 1. Thats more than 5. Now lets consider bitfields. What they give you the ability to do is. The thing to know about bitfields is that they are implemented with. C9. 9 guarentees that. This restriction is relaxed in C1. C1. 4. class. Its up to the implementation to. GCC leaves it up to the ABI, which for x. Assuming were on a 3. C9. 9 rules imply that the layout may. But this isnt the only possibility, because the C standard does not. So the layout could look. That is, the padding could precede rather than following the. Note also that, as with normal structure padding, the padding bits are. C9. 9 mentions this. Note that the base type of a bit field is interpreted for signedness. It is up to implementors whether. Proceed with caution and check with Wpadded if you have it available. Compilers on exotic hardware, might interpret the. C9. 9 rules in surprising ways older compilers might not quite follow. The restriction that bitfields cannot cross machine word boundaries. C9. 9, in the last of which only one bit is used. Again, C1. 1 and C1. On the other hand, struct foo.