If i have a test.c file with the following
#include ...
int global = 0;
int main() {
int local1 = 0;
while(1) {
int local2 = 0;
// Do some operation with one of them
}
return 0;
}
So if I had to use one of this variables in the while loop, which one would be preferred?
Maybe I'm being a little vague here, but I want to know if the difference in time/space allocation is actually relevant.
If you are wondering whether declaring a variable inside a for loop causes it to be created/destroyed at every iteration, there is nothing really to worry about. These variables are not dynamically allocated at runtime, nothing is being malloced here - just some memory is being set aside for use inside the loop. So having the variable inside is just the same as having it outside the loop in terms of performance.
The real difference here is scope not performance. Whether you use a global or local variable only affects where you want this variable to be visible.
In case you're wondering about performance differences: most likely there aren't any. If there are theoretical performance differences, you'll find it hard to actually devise a test to measure them.
A decision like this should not be based on performance but semantics. Unless the semantic behavior of a global variable is required, you should always use automatic (local non-static) variables.
As others have said and surely will say, there are unlikely to be any differences in performance. If there are, the automatic variable will be faster.
The C compiler will have an easier time making optimizations on the variables declared local to the function. The global variable would require an optimizer to perform "Inter-Procedural Data Flow Analysis", which isn't that commonly done.
As an example of the difference, consider that all your declarations initialize the variable to zero. However, in the case of the global variable, the compiler cannot use that information unless it verifies that no flow of control in your program can change the global prior to using it in your example function. In the case of the locally declared ("automatic") variables, there is no way the initial value can be changed by another function (in particular, the compiler verifies that their address is never passed to a sub-function) and the compiler can perform "killed definitions" and "value liveness" analysis to determine whether the zero value can be assumed in some code paths.
Of the two local variables, as a guideline, the optimizer will always have an easier time optimizing access to the variable with the smaller (more limited) scope.
Having stated the above, I would suggest that other answers concerning a bias toward semantics over optimizer-meta-optimization is correct. Use the variable which causes the code to read best, and you will be rewarded with more time returned to you than assisting the def-use optimization calculation.
In general, avoid using a global variable, or any variable which can be accessed more broadly than absolutely necessary. Limited scoping of variables helps prevent bugs from being introduced during later program maintenance.
There are three broad classes of variables: static (global), stack (auto), and register.
Register variables are stored in CPU registers. Registers are very fast word-sized memories, which are integrated in the CPU pipeline. They are free to access, but there are a very limited number of them (typically between 8 and 32 depending on your processor and what operations you're doing).
Stack variables are stored in an area of RAM called the stack. The stack is almost always going to be in the cache, so stack variables typically take 1-4 cycles to access.
Generally, local variables can be either in registers or on the stack. It doesn't matter whether they are allocated at the top of a function or in a loop; they will only be allocated once per function call, and allocation is basically free. The compiler will put variables in registers if at all possible, but if you have more active variables than registers, they won't all fit. Also, if you take the address of a variable, it must be stored on the stack since registers don't have addresses.
Global and static variables are a different beast. Since they are not usually accessed frequently, they may not be in cache, so it could take hundreds of cycles to access them. Also, since the compiler may not know the address of a global variable ahead of time, it may need to be looked up, which is also expensive.
As others have said, don't worry too much about this stuff. It's definitely good to know, but it shouldn't affect the way you write your programs. Write code that makes sense, and let the compiler worry about optimization. If you get into compiler development, then you can start worrying about it. :)
Edit: more details on allocation:
Register variables are allocated by the compiler, so there is no runtime cost. The code will just put a value in a register as soon as the value is produced.
Stack variables are allocated by your program at runtime. Typically, when a function is called, the first thing it will do is reserve enough stack space for all of its local variables. So there is no per-variable cost.
Related
Like we do with macros:
#undef SOMEMACRO
Can we also undeclare or delete the variables in C, so that we can save a lot of memory?
I know about malloc() and free(), but I want to delete the variables completely so that if I use printf("%d", a); I should get error
test.c:4:14: error: ‘a’ undeclared (first use in this function)
No, but you can create small minimum scopes to achieve this since all scope local variables are destroyed when the scope is exit. Something like this:
void foo() {
// some codes
// ...
{ // create an extra minimum scope where a is needed
int a;
}
// a doesn't exist here
}
It's not a direct answer to the question, but it might bring some order and understanding on why this question has no proper answer and why "deleting" variables is impossible in C.
Point #1 What are variables?
Variables are a way for a programmer to assign a name to a memory space. This is important, because this means that a variable doesn't have to occupy any actual space! As long as the compiler has a way to keep track of the memory in question, a defined variable could be translated in many ways to occupy no space at all.
Consider: const int i = 10; A compiler could easily choose to substitute all instances of i into an immediate value. i would occupy 0 data memory in this case (depending on architecture it could increase code size). Alternatively, the compiler could store the value in a register and again, no stack nor heap space will be used. There's no point in "undefining" a label that exists mostly in the code and not necessarily in runtime.
Point #2 Where are variables stored?
After point #1 you already understand that this is not an easy question to answer as the compiler could do anything it wants without breaking your logic, but generally speaking, variables are stored on the stack. How the stack works is quite important for your question.
When a function is being called the machine takes the current location of the CPU's instruction pointer and the current stack pointer and pushes them into the stack, replacing the stack pointer to the next location on stack. It then jumps into the code of the function being called.
That function knows how many variables it has and how much space they need, so it moves the frame pointer to capture a frame that could occupy all the function's variables and then just uses stack. To simplify things, the function captures enough space for all it's variables right from the start and each variable has a well defined offset from the beginning of the function's stack frame*. The variables are also stored one after the other.
While you could manipulate the frame pointer after this action, it'll be too costly and mostly pointless - The running code only uses the last stack frame and could occupy all remaining stack if needed (stack is allocated at thread start) so "releasing" variables gives little benefit. Releasing a variable from the middle of the stack frame would require a defrag operation which would be very CPU costly and pointless to recover few bytes of memory.
Point #3: Let the compiler do its job
The last issue here is the simple fact that a compiler could do a much better job at optimizing your program than you probably could. Given the need, the compiler could detect variable scopes and overlap memory which can't be accessed simultaneously to reduce the programs memory consumption (-O3 compile flag).
There's no need for you to "release" variables since the compiler could do that without your knowledge anyway.
This is to complement all said before me about the variables being too small to matter and the fact that there's no mechanism to achieve what you asked.
* Languages that support dynamic-sized arrays could alter the stack frame to allocate space for that array only after the size of the array was calculated.
There is no way to do that in C nor in the vast majority of programming languages, certainly in all programming languages that I know.
And you would not save "a lot of memory". The amount of memory you would save if you did such a thing would be minuscule. Tiny. Not worth talking about.
The mechanism that would facilitate the purging of variables in such a way would probably occupy more memory than the variables you would purge.
The invocation of the code that would reclaim the code of individual variables would also occupy more space than the variables themselves.
So if there was a magic method purge() that purges variables, not only the implementation of purge() would be larger than any amount of memory you would ever hope to reclaim by purging variables in your program, but also, in int a; purge(a); the call to purge() would occupy more space than a itself.
That's because the variables that you are talking about are very small. The printf("%d", a); example that you provided shows that you are thinking of somehow reclaiming the memory occupied by individual int variables. Even if there was a way to do that, you would be saving something of the order of 4 bytes. The total amount of memory occupied by such variables is extremely small, because it is a direct function of how many variables you, as a programmer, declare by hand-typing their declarations. It would take years of typing on a keyboard doing nothing but mindlessly declaring variables before you would declare a number of int variables occupying an amount of memory worth speaking of.
Well, you can use blocks ({ }) and defining a variable as late as possible to limit the scope where it exists.
But unless the variable's address is taken, doing so has no influence on the generated code at all, as the compiler's determination of the scope where it has to keep the variable's value is not significantly impacted.
If the variable's address is taken, failure of escape-analysis, mostly due to inlining-barriers like separate compilation or allowing semantic interpositioning, can make the compiler assume it has to keep it alive till later in the block than strictly neccessary. That's rarely significant (don't worry about a handful of ints, and most often a few lines of code longer keeping it alive are insignificant), but best to keep it in mind for the rare case where it might matter.
If you are that concerned about the tiny amount of memory that is on the stack, then you're probably going to be interested in understanding the specifics of your compiler as well. You'll need to find out what it does when it compiles. The actual shape of the stack-frame is not specified by the C language. It is left to the compiler to figure out. To take an example from the currently accepted answer:
void foo() {
// some codes
// ...
{ // create an extra minimum scope where a is needed
int a;
}
// a doesn't exist here
}
This may or may not affect the memory usage of the function. If you were to do this in a mainstream compiler like gcc or Visual Studio, you would find that they optimize for speed rather than stack size, so they pre-allocate all of the stack space they need at the start of the function. They will do analysis to figure out the minimum pre-allocation needed, using your scoping and variable-usage analysis, but those algorithms literally wont' be affected by extra scoping. They're already smarter than that.
Other compilers, especially those for embedded platforms, may allocate the stack frame differently. On these platforms, such scoping may be the trick you needed. How do you tell the difference? The only options are:
Read the documentation
Try it, and see what works
Also, make sure you understand the exact nature of your problem. I worked on a particular embedded project which eschewed the stack for everything except return values and a few ints. When I pressed the senior developers about this silliness, they explained that on this particular application, stack space was at more of a premium than space for globally allocated variables. They had a process they had to go through to prove that the system would operate as intended, and this process was much easier for them if they allocated everything up front and avoided recursion. I guarantee you would never arrive at such a convoluted solution unless you first knew the exact nature of what you were solving.
As another solution you could look at, you could always build your own stack frames. Make a union of structs, where each struct contains the variables for one stack frame. Then keep track of them yourself. You could also look at functions like alloca, which can allow for growing the stack frame during the function call, if your compiler supports it.
Would a union of structs work? Try it. The answer is compiler dependent. If all variables are stored in memory on your particular device, then this approach will likely minimize stack usage. However, it could also substantially confuse register coloring algorithms, and result in an increase in stack usage! Try and see how it goes for you!
All,
I have C functions that are called many times a second as they are part of a control loop on a PIC18 board. These functions have variables that only need method scope, but I was wondering what if any overhead there was to constantly allocating these variables vs. using a global or at least higher scoped variable. (Thought of typedef'ing a struct to pass around from a higher scope to avoid global variable use if performance dictates not using method local varables)
There are some good threads on here that cover this topic, but I have yet to see a definitive answer as most preach best practices which I agree and would follow as long as there are not performance gains to be had as every microsecond counts.
One thread mentioned using file scoped static variables as a substitute for global variables, but I can't help wonder if even that is necessary.
What does everyone think?
Accessing a local variable requires doing something like *(SP + offset) (where SP is the stack-pointer), whereas accessing a static (which includes globals) requires something like *(address).
From what I recall, the PIC instruction set has very limited addressing modes. So it's very likely that accessing the global will be faster, at least for the first time it's accessed. Subsequent accesses may be identical if the compiler holds the computed address in a register.
As #unwind said in the comments, you should take a look at the compiler output, and profile to confirm. I would only sacrifice clarity/maintainability if you've proved that it's worthwhile in terms of the runtime of your program.
While I've not used every single PIC compiler in existence, there are two styles. The style I've used allocates all local variables statically by analyzing the program's call graph. If every possible call were in fact performed, the amount of stack memory consumed by locals would match what would be required by static allocation, with a couple of caveats (describing the behavior of HiTech's PICC-18 "standard" compiler--others may vary)
Variadic functions are handled by defining local-variable storage in the scope of the caller, and passing a two-byte pointer to that storage to the function being called.
For every different signature of indirect function pointer, the compiler generates a "pseudo-function" in the call graph; everything that calls a function of that signature calls the pseudo-function, and that pseudo-function calls every function with that signature that has its address taken.
In this style of compiler, consecutive accesses to local variables will be just as fast as consecutive accesses to globals. Other than global and static variables explicitly-declared as "near", however, which must total no more than 64-128 bytes (varies with different models of PIC), the global and static variables for each module are located separately from local variables, and bank-switching instructions are needed to access things in different banks.
Some compilers which I have not used employ the "enhanced instruction set" option. This option gobbles up 96 bytes of the "near" bank (or all of it, on PICs with less than 96 bytes) and uses it to access 96 bytes relative to the FSR2 register. This would be a wonderful concept if it used the first 16, or maybe 32, bytes as a stack frame. Using 96 bytes means giving up all of the "near" storage, which is a pretty severe limitation. Nonetheless, compilers which use this instruction set can access local variables on a stack just as fast, if not faster, than global variables (no bank-switch required). I really wish Microchip had an option to only set aside 16 bytes or so for the stack frame, leaving a useful amount of 'common bank' RAM, but nonetheless some people have good luck with that mode.
I would imagine that this depends a lot on which compiler you are using. I don't know PIC but I'm guessing some (all?) PIC compilers will optimize the code so that local variables are stored in CPU registers whenever possible. If so, then local variables will likely be equally fast as globals.
Otherwise if the local variable is allocated on the stack the global may be a bit faster to access (see Oli's answer).
My function will be called thousands of times. If i want to make it faster, will changing the local function variables to static be of any use? My logic behind this is that, because static variables are persistent between function calls, they are allocated only the first time, and thus, every subsequent call will not allocate memory for them and will become faster, because the memory allocation step is not done.
Also, if the above is true, then would using global variables instead of parameters be faster to pass information to the function every time it is called? i think space for parameters is also allocated on every function call, to allow for recursion (that's why recursion uses up more memory), but since my function is not recursive, and if my reasoning is correct, then taking off parameters will in theory make it faster.
I know these things I want to do are horrible programming habits, but please, tell me if it is wise. I am going to try it anyway but please give me your opinion.
The overhead of local variables is zero. Each time you call a function, you are already setting up the stack for the parameters, return values, etc. Adding local variables means that you're adding a slightly bigger number to the stack pointer (a number which is computed at compile time).
Also, local variables are probably faster due to cache locality.
If you are only calling your function "thousands" of times (not millions or billions), then you should be looking at your algorithm for optimization opportunities after you have run a profiler.
Re: cache locality (read more here):
Frequently accessed global variables probably have temporal locality. They also may be copied to a register during function execution, but will be written back into memory (cache) after a function returns (otherwise they wouldn't be accessible to anything else; registers don't have addresses).
Local variables will generally have both temporal and spatial locality (they get that by virtue of being created on the stack). Additionally, they may be "allocated" directly to registers and never be written to memory.
The best way to find out is to actually run a profiler. This can be as simple as executing several timed tests using both methods and then averaging out the results and comparing, or you may consider a full-blown profiling tool which attaches itself to a process and graphs out memory use over time and execution speed.
Do not perform random micro code-tuning because you have a gut feeling it will be faster. Compilers all have slightly different implementations of things and what is true on one compiler on one environment may be false on another configuration.
To tackle that comment about fewer parameters: the process of "inlining" functions essentially removes the overhead related to calling a function. Chances are a small function will be automatically in-lined by the compiler, but you can suggest a function be inlined as well.
In a different language, C++, the new standard coming out supports perfect forwarding, and perfect move semantics with rvalue references which removes the need for temporaries in certain cases which can reduce the cost of calling a function.
I suspect you're prematurely optimizing, however, you should not be this concerned with performance until you've discovered your real bottlenecks.
Absolutly not! The only "performance" difference is when variables are initialised
int anint = 42;
vs
static int anint = 42;
In the first case the integer will be set to 42 every time the function is called in the second case ot will be set to 42 when the program is loaded.
However the difference is so trivial as to be barely noticable. Its a common misconception that storage has to be allocated for "automatic" variables on every call. This is not so C uses the already allocated space in the stack for these variables.
Static variables may actually slow you down as its some aggresive optimisations are not possible on static variables. Also as locals are in a contiguous area of the stack they are easier to cache efficiently.
There is no one answer to this. It will vary with the CPU, the compiler, the compiler flags, the number of local variables you have, what the CPU's been doing before you call the function, and quite possibly the phase of the moon.
Consider two extremes; if you have only one or a few local variables, it/they might easily be stored in registers rather than be allocated memory locations at all. If register "pressure" is sufficiently low that this may happen without executing any instructions at all.
At the opposite extreme there are a few machines (e.g., IBM mainframes) that don't have stacks at all. In this case, what we'd normally think of as stack frames are actually allocated as a linked list on the heap. As you'd probably guess, this can be quite slow.
When it comes to accessing the variables, the situation's somewhat similar -- access to a machine register is pretty well guaranteed to be faster than anything allocated in memory can possible hope for. OTOH, it's possible for access to variables on the stack to be pretty slow -- it normally requires something like an indexed indirect access, which (especially with older CPUs) tends to be fairly slow. OTOH, access to a global (which a static is, even though its name isn't globally visible) typically requires forming an absolute address, which some CPUs penalize to some degree as well.
Bottom line: even the advice to profile your code may be misplaced -- the difference may easily be so tiny that even a profiler won't detect it dependably, and the only way to be sure is to examine the assembly language that's produced (and spend a few years learning assembly language well enough to know say anything when you do look at it). The other side of this is that when you're dealing with a difference you can't even measure dependably, the chances that it'll have a material effect on the speed of real code is so remote that it's probably not worth the trouble.
It looks like the static vs non-static has been completely covered but on the topic of global variables. Often these will slow down a programs execution rather than speed it up.
The reason is that tightly scoped variables make it easy for the compiler to heavily optimise, if the compiler has to look all over your application for instances of where a global might be used then its optimising won't be as good.
This is compounded when you introduce pointers, say you have the following code:
int myFunction()
{
SomeStruct *A, *B;
FillOutSomeStruct(B);
memcpy(A, B, sizeof(A);
return A.result;
}
the compiler knows that the pointer A and B can never overlap and so it can optimise the copy. If A and B are global then they could possibly point to overlapping or identical memory, this means the compiler must 'play it safe' which is slower. The problem is generally called 'pointer aliasing' and can occur in lots of situations not just memory copies.
http://en.wikipedia.org/wiki/Pointer_alias
Using static variables may make a function a tiny bit faster. However, this will cause problems if you ever want to make your program multi-threaded. Since static variables are shared between function invocations, invoking the function simultaneously in different threads will result in undefined behaviour. Multi-threading is the type of thing you may want to do in the future to really speed up your code.
Most of the things you mentioned are referred to as micro-optimizations. Generally, worrying about these kind of things is a bad idea. It makes your code harder to read, and harder to maintain. It's also highly likely to introduce bugs. You'll likely get more bang for your buck doing optimizations at a higher level.
As M2tM suggests, running a profiler is also a good idea. Check out gprof for one which is quite easy to use.
You can always time your application to truly determine what is fastest. Here is what I understand: (all of this depends on the architecture of your processor, btw)
C functions create a stack frame, which is where passed parameters are put, and local variables are put, as well as the return pointer back to where the caller called the function. There is no memory management allocation here. It usually a simple pointer movement and thats it. Accessing data off the stack is also pretty quick. Penalties usually come into play when you're dealing with pointers.
As for global or static variables, they're the same...from the standpoint that they're going to be allocated in the same region of memory. Accessing these may use a different method of access than local variables, depends on the compiler.
The major difference between your scenarios is memory footprint, not so much speed.
Using static variables can actually make your code significantly slower. Static variables must exist in a 'data' region of memory. In order to use that variable, the function must execute a load instruction to read from main memory, or a store instruction to write to it. If that region is not in the cache, you lose many cycles. A local variable that lives on the stack will most surely have an address that is in the cache, and might even be in a cpu register, never appearing in memory at all.
I agree with the others comments about profiling to find out stuff like that, but generally speaking, function static variables should be slower. If you want them, what you are really after is a global. Function statics insert code/data to check if the thing has been initialized already that gets run every time your function is called.
Profiling may not see the difference, disassembling and knowing what to look for might.
I suspect you are only going to get a variation as much as a few clock cycles per loop (on average depending on the compiler, etc). Sometimes the change will be dramatic improvement or dramatically slower, and that wont necessarily be because the variables home has moved to/from the stack. Lets say you save four clock cycles per function call for 10000 calls on a 2ghz processor. Very rough calculation: 20 microseconds saved. Is 20 microseconds a lot or a little compared to your current execution time?
You will likely get more a performance improvement by making all of your char and short variables into ints, among other things. Micro-optimization is a good thing to know but takes lots of time experimenting, disassembling, timing the execution of your code, understanding that fewer instructions does not necessarily mean faster for example.
Take your specific program, disassemble both the function in question and the code that calls it. With and without the static. If you gain only one or two instructions and this is the only optimization you are going to do, it is probably not worth it. You may not be able to see the difference while profiling. Changes in where the cache lines hit could show up in profiling before changes in the code for example.
This question already has answers here:
How is conditional initialization handled and is it a good practice?
(5 answers)
Closed 9 years ago.
How long does it take to declare a variable in C, for example int x or unsigned long long var? I am wondering if it would make my code any faster in something like this.
for (conditions) {
int var = 0;
// code
}
Would it be faster to do this, or is it easier not to?
int var;
for (conditions) {
var = 0;
// code
}
Thanks for the help.
One piece of advice: stop worrying about which language constructs are microscopically faster or slower than which others, and instead focus on which ones let you express yourself best.
Also, to find out where your code is spending time, use a profiler.
And as others have pointed out, declarations are purely compile-time things, they don't affect execution time.
It doesn't make any difference. In a traditional implementation the declaration itself (excluding initialization) generates no machine instructions. Function prologue code typically allocates space in the stack for all local variables at once, regardless of where they are declared.
However, where you declare your local variables can affect the performance of your code indirectly, in theory at least. When you declare the variables as locally as possible (your first variant), in general case it results in smaller size of the stack frame reserved by the function for its local variables (since the same location in the stack can be shared by different local variables at different times). Having smaller stack frame reduces the general stack memory consumption, i.e. as nested function calls are performed stack size doesn't grow as fast (especially noticeable with recursive functions). It generally improves performance since new stack page allocations happen less often, and stack memory locality becomes better.
The latter considerations are platform-dependent, of course. It might have very little or no effect on your platform and/or for your applications.
Whenever you have a question about performance, the best thing to do is wrap a loop around it (millions of iterations) and time it. But, in this case, you will likely find that it makes no difference.
It is more important to properly express the intentions of your code. If you need the variable outside your loop, delare it outside. If you only need the variable inside the loop, declare it inside.
You should always declare and initialize variables in narrowest scope possible.
You shouldn't be worrying about those types of micro-optimizations anyway (except in the rarest, rarest of cases). If you really need to worry about potential nano-second performance improvements, measure the difference. It is very unlikely that your variable declarations will be the largest bottleneck in your application.
It takes no time at all. The memory for global variables is allocated at startup, and "declaring" variables on the stack simply involves how far "up" the stack pointer moves when the function is called.
declarations are purely compile time, they cost nothing at runtime¹. But the first piece of code is still better than the second for two reasons
you should always initialize variables when you declare them, they way they can never have uninitialized values. This goes hand in hand with
always use the narrowest possible scope for variable declarations
So your first example, while no faster than the second, is still better.
And all of the people who chimed in telling him not to prematurely or micro optimize his code are wrong. It is never bad to know how costly various bits of code are. The very best programmers have a solid, almost unconcious, grasp of the cost of various strategies and take that into account automatically when they design. The way you become that programmer is to ask just this sort of question when you are a beginner.
¹ In fact, there is a small cost when each function allocates space for local variables, but that cost is the same regardless of how many local variables there are*.
*ok that's not really true, but the cost depends only on the total amount of space, not the number of variables.
Declaration takes no time at all.
The compiler will interpret that line as a notification that space for it will need to exist on the stack.
As others have already said, it shouldn't take any time. Therefore you need to make this decision based on other factors: what would make your code more readable and less prone to bugs. It's generally considered a good practice to declare a variable as close as possible to its usage (so you can see the declaration and usage in one go). If it's only used in the inner scope then just declare it inside that scope - forget about performance on this one.
Declaring variables does take time, as it results in machine language instructions that allocate the space for the variables on the stack. This is simply an increment of the stack pointer, which takes a tiny, but non-zero amount of time.
I believe your question is whether more time will be required (i.e. more stack increment operations) if the variable is declared inside the loop. The answer is no, since the stack is incremented once only for the loop block, not each time the loop is executed. So, there will be no difference in time either way, even if the loop executes zillions of zillions of times.
Disclaimer: Precisely what happens depends on your compiler, architecture, etc. But conceptually here's what's going on:
When you declare a variable within a method, it is allocated on the stack. Allocating something on the stack only involves bumping up the stack pointer by the size of the variable. So, for example, if SP represents the memory address of the top of the stack, declaring char x results in SP += 1 and int x results in SP += 4 (on a 32 bit machine).
When the function exits, the stack pointer is returned to where it was before your method was called. So deallocating everything is fast, too.
So, either way it's just an add, which takes the same amount of time regardless of the amount of data.
A smart compiler will combine several variable declarations into a single add.
When you declare a variable within a loop, in theory it could be changing the stack pointer on each iteration through the loop, but again, a smart compiler probably won't do that.
(A notable exception is C++, which does extra work because it needs to call constructors and destructors when the stack-allocated object is created or destroyed.)
I wouldn't care about a nanosecond here or there. Unless you need to access its value after the for loop ends, leave the variable inside the loop: it will be closer to the code that uses it (your code will be more readable), and its scope will be bounded by the loop itself (your code will be more elegant and less bug-prone).
I bet the compiled binary will be identical for both cases.
Variable declaration is turned into stack space reservation by the compiler. Now how does this work is entirely platform-dependent. On x86 and pretty much every popular architecture this is just a subtraction from the address of the stack frame and\or indexing addressing mode to access from the top of the stack. All these come with the cost of a simple subtraction\addition, which is really irrelevant.
Technically the second example is less efficient, because the declaration happens on every entry into the loop scope, i.e. on every loop iteration. However it is 99.99% chance that the stack space will be reserved only once.Even the assignment operation will be optimized away, although technically it should be done every loop iteration. Now in C++ this can get much worse, if the variable has a constructor which will then be run on every loop iteration.
And as a bottom line, you really should not worry about any of such issues without proper profiling. And even then there are much more valuable questions to ask yourself here, like "what is the most readable way to do this, what is easier to understand and maintain, etc.".
I'm programming in C for RAM limited embedded microcontroller with RTOS.
I regularly break my code to short functions, but every function calling require to more stack memory.
Every task needs his stack, and this is one of the significant memory consumers in the project.
Is there an alternative to keep the code well organized and readable, still preserve the memory?
Try to make the call stack flatter, so instead of a() calling b() which calls c() which calls d(), have a() call b(), c(), and d() itself.
If a function is only referenced once, mark it inline (assuming your compiler supports this).
There are 3 components to your stack usage:
Function Call return addresses
Function Call parameters
automatic(local) variables
The key to minimizing your stack usage is to minimize parameter passing and automatic variables. The space consumption of the actual function call itself is rather minimal.
Parameters
One way to address the parameter issue is to pass a structure (via pointer) instead of a large number of parameters.
foo(int a, int b, int c, int d)
{
...
bar(int a, int b);
}
do this instead:
struct my_params {
int a;
int b;
int c;
int d;
};
foo(struct my_params* p)
{
...
bar(p);
};
This strategy is good if you pass down a lot of parameters. If the parameters are all different, then it might not work well for you. You would end up with a large structure being passed around that contains many different parameters.
Automatic Variables (locals)
This tend to be the biggest consumer of stack space.
Arrays are the killer. Don't define arrays in your local functions!
Minimize the number of local variables.
Use the smallest type necessary.
If re-entrancy is not an issue, you can use module static variables.
Keep in mind that if you're simply moving all your local variables from local scope to module scope, you have NOT saved any space. You traded stack space for data segment space.
Some RTOS support thread local storage, which allocates "global" storage on a per-thread basis. This might allow you to have multiple independent global variables on a per task basis, but this will make your code not as straightforward.
In the event you can spare a lot of main memory but have only a small shred of stack, I suggest evaluating static allocations.
In C, all variables declared inside a function are "automatically managed" which means they're allocated on the stack.
Qualifying the declarations as "static" stores them in main memory instead of on the stack. They basically behave like global variables but still allow you to avoid the bad habits that come with overusing globals. You can make a good case for declaring large, long-lived buffers/variables as static to reduce pressure on the stack.
Beware that this doesn't work well/at all if your application is multithreaded or if you use recursion.
Turn on optimization, specifically aggressive inlining. The compiler should be able to inline methods to minimize calls. Depending on the compiler and the optimization switches you use, marking some methods as inline may help (or it may be ignored).
With GCC, try adding the "-finline-functions" (or -O3) flag and possibly the " -finline-limit=n" flag.
One trick that I read somewhere inorder to evaluate the stack requirements of the code in an embedded setup is to fill the stack space at the onset with a known pattern(DEAD in hex being my favorite) and let the system run for a while.
After a normal run, read the stack space and see how much of the stack space has not been replaced during the course of operation. Design so as to leave atleast 150% of that so as to tackle all obsure code paths that might not have been exercised.
Can you replace some of your local variables by globals?
Arrays in particular can eat up stack.
If the situation allows you to share some globals between some those between functions,
there is a chance you can reduce your memory foot print.
The trade off cost is increased complexity, and greater risk of unwanted side effects between functions vs a possibly smaller memory foot print.
What sort of variables do you have in your functions?
What sizes and limits are we talking about?
Depending on your compiler, and how aggressive your optimisation options are, you will have stack usage for every function call you make. So to start with you will probably need to limit the depth of your function calls.
Some compilers do use jumps rather than branches for simple functions which will reduce stack usage. Obviously you can do the same thing by using, say, an assembler macro to jump to your functions rather than a direct function call.
As mentioned in other answers, inlining is one option available although that does come at the cost of greater code size.
The other area that eats stack is the local parameters. This area you do have some control over. Using (file level) statics will avoid stack allocation at the cost of your static ram allocation. Globals likewise.
In (truly) extreme cases you can come up with a convention for functions that uses a fixed number of global variables as temporary storage in lieu of locals on the stack. The tricky bit is making sure that none of the functions that use the same globals ever get called at the same time. (hence the convention)
If you need to start preserving stack space you should either get a better compiler or more memory.
Your software will typically grow (new features,...) , so if you have to start a project by thinking about how to preserve stack space it's doomed from the beginning.
Yes, an RTOS can really eat up RAM for task stack usage. My experience is that as a new user of an RTOS, there's a tendency to use more tasks than necessary.
For an embedded system using an RTOS, RAM can be a precious commodity. To preserve RAM, for simple features it can still be effective to implement several features within one task, running in round-robin fashion, with a cooperative multitasking design. Thus reduce total number of tasks.
I think you may be imagining a problem which doesnt exist here. Most compilers don't actually do anything when they "allocate" automaticic variables on the stack.
The stack is allocated before "main()" is executed. When you call function b() from function a() the address of the storage area immediately after the last variable used by a is passed to b(). This becomes the start of b()'s stack if b() then calls function c() then c's stack starts after the last automatic variable defined by b().
Note that the stack memory is already there and allocated, that no initialisation takes place and the only processing involved is passing a stack pointer.
The only time this becomes a problem would be where all three functions use large amounts of storage the stack then has to accomadate the memory of all three functions. Try to keep functions which allocate large amounts of storage at the bottom of the call stack i.e. dont call another function from them.
Another trick for memory constained systems is to split of the memory hogging parts of a function into separate self contained functions.