How to determine maximum stack usage in embedded system with gcc?

Question

I'm writing the startup code for an embedded system -- the code that loads the initial stack pointer before jumping to the main() function -- and I need to tell it how many bytes of stack my application will use (or some larger, conservative estimate).

I've been told the gcc compiler now has a -fstack-usage option and -fcallgraph-info option that can somehow be used to statically calculates the exact "Maximum Stack Usage" for me. ( "Compile-time stack requirements analysis with GCC" by Botcazou, Comar, and Hainque ).

Nigel Jones says that recursion is a really bad idea in embedded systems ("Computing your stack size" 2009), so I've been careful not to make any mutually recursive functions in this code.

Also, I make sure that none of my interrupt handlers ever re-enable interrupts until their final return-from-interrupt instruction, so I don't need to worry about re-entrant interrupt handlers.

Without recursion or re-entrant interrupt handlers, it should possible to statically determine the maximum stack usage. (And so most of the answers to How to determine maximum stack usage? do not apply). My understanding is I (or preferably, some bit of code on my PC that is automatically run every time I rebuild the executable) first find the maximum stack depth for each interrupt handler when it's not interrupted by a higher-priority interrupt, and the maximum stack depth of the main() function when it is not interrupted. Then I add them all up to find the total (worst-case) maximum stack depth. That occurs (in my embedded system) when the main() background task is at its maximum depth when it is interrupted by the lowest-priority interrupt, and that interrupt is at its maximum depth when it is interrupted by the next-lowest-priority interrupt, and so on.

I'm using YAGARTO with gcc 4.6.0 to compile code for the LM3S1968 ARM Cortex-M3.

So how do I use the -fstack-usage option and -fcallgraph-info option with gcc to calculate the maximum stack depth? Or is there some better approach to determine maximum stack usage?

(See How to determine maximum stack usage in embedded system? for almost the same question targeted to the Keil compiler .)

Answer 1

GCC docs:

-fstack-usage

Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:

• The name of the function.
• A number of bytes.
• One or more qualifiers: static, dynamic, bounded.

The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile-time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile-time and the second field only represents the bounded part.

I can't find any references to -fcallgraph-info

You could potentially create the information you need from -fstack-usage and -fdump-tree-optimized

For each leaf in -fdump-tree-optimized, get its parents and sum their stack size number (keeping in mind that this number lies for any function with "dynamic" but not "bounded") from -fstack-usage, find the max of these values and this should be your maximum stack usage.

Answer 2

Just in case no one comes up with a better answer, I'll post what I had in the comment to your other question, even though I have no experience using these options and tools:

GCC 4.6 adds the -fstack-usage option which gives the stack usage statistics on a function-by-function basis.

If you combine this information with a call graph produced by cflow or a similar tool you can get the kind of stack depth analysis you're looking for (a script could probably be written pretty easily to do this). Have the script read the stack-usage info and load up a map of function names with the stack used by the function. Then have the script walk the cflow graph (which can be an easy-to-parse text tree), adding up the stack usage associated with each line for each branch in the call graph.

So, it looks like this can be done with GCC, but you might have to cobble together the right set of tools.

Answer 3

I ended up writing a python script to implement τεκ's answer. It's too much code to post here, but can be found on github

Answer 4

Quite late, but for anyone looking at this, the answers given involving combining the outputs from fstack-usage and call graph tools like cflow can end up being wildly incorrect for any dynamic allocation, even bounded, because there's no information about when that dynamic stack allocation occurs. It's therefore not possible to know to what functions you should apply the value towards. As a contrived example, if (simplified) fstack-usage output is:

main        1024     dynamic,bounded
functionA    512     static
functionB     16     static

and a very simple call tree is:

main
    functionA
    functionB

The naive approach to combine these may result in main -> functionA being chosen as the path of maximum stack usage, at 1536 bytes. But, if the largest dynamic stack allocation in main() is to push a large argument like a record to functionB() directly on the stack in a conditional block that calls functionB (I already said this was contrived), then really main -> functionB is the path of maximum stack usage, at 1040 bytes. Depending on existing software design, and also for other more restricted targets that pass everything on the stack, cumulative errors may quickly lead you toward looking at entirely wrong paths claiming significantly overstated maximum stack sizes.

Also, depending on your classification of "reentrant" when talking about interrupts, it's possible to miss some stack allocations entirely. For instance, many Coldfire processors' level 7 interrupt is edge-sensitive and therefore ignores the interrupt disable mask, so if a semaphore is used to leave the instruction early, you may not consider it reentrant, but the initial stack allocation will still happen before the semaphore is checked.

In short, you have to be extremely careful about using this approach.

Answer 5

I am not familiar with the -fstack-usage and -fcallgraph-info options. However, it is always possible to figure out actual stack usage by:

1. Allocate adequate stack space (for this experiment), and initialize it to something easily identifiable. I like 0xee.
2. Run the application and test all its internal paths (by all combinations of input and parameters). Let it run for more than "long enough".
3. Examine the stack area and see how much of the stack was used.
4. Make that the stack size, plus 10% or 20% to tolerate software updates and rare conditions.