How much is 32 kB of compiled code

Question

I am planning to use an Arduino programmable board. Those have quite limited flash memories ranging between 16 and 128 kB to store compiled C or C++ code.

Are there ways to estimate how much (standard) code it will represent ?

I suppose this is very vague, but I'm only looking for an order of magnitude.

Answer 1

The output of the size command is a good starting place, but does not give you all of the information you need.

$ avr-size program.elf
text            data    bss     dec     hex filename

The size of your image is usually a little bit more than the sum of the text and the data sections. The bss section is essentially compressed because it is all 0s. There may be other sections which are relevant which aren't listed by size.

If your build system is set up like ones that I've used before for AVR microcontrollers then you will end up with an *.elf file as well as a *.bin file, and possibly a *.hex file. The *.bin file is the actual image that would be stored in the program flash of the processor, so you can examine its size to determine how your program is growing as you make edits to it. The *.bin file is extracted from the *.elf file with the objdump command and some flags which I can't remember right now.

If you are wanting to know how to guess-timate how your much your C or C++ code will produce when compiled, this is a lot more difficult. I have observed a 10x blowup in a function when I tried to use a uint64_t rather than a uint32_t when all I was doing was incrementing it (this was about 5 times more code than I thought it would be). This was mostly to do with gcc's avr optimizations not being the best, but smaller changes in code size can creep in from seemingly innocent code.

This will likely be amplified with the use of C++, which tends to hide more things that turn into code than C does. Chief among the things C++ hides are destructor calls and lots of pointer dereferencing which has to do with the this pointer in objects as well as a secret pointer many objects have to their virtual function table and class static variables.

On AVR all of this pointer stuff is likely to really add up because pointers are twice as big as registers and take multiple instructions to load. Also AVR has only a few register pairs that can be used as pointers, which results in lots of moving things into and out of those registers.

Some tips for small programs on AVR:

• Use uint8_t and int8_t instead of int whenever you can. You could also use uint_fast8_t and int_fast8_t if you want your code to be portable. This can lead to many operations taking up only half as much code, because int is two bytes.

• Be very aware of things like string and struct constants and literals and how/where they are stored.

• If you're not scared of it, read the AVR assembly manual. You can get an idea of the types of instructions, and from that the type of C code that easily maps to those instructions. Use that kind of C code.

Answer 2

You can't really say there. The length of the uncompiled code has little to do with the length of the compiled code. For example:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

int main()
{
  std::vector<std::string> strings;
  strings.push_back("Hello");
  strings.push_back("World");
  std::sort(strings.begin(), strings.end());
  std::copy(strings.begin(), strings.end(), std::ostream_iterator<std::string>(std::cout, ""));
}

vs

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

int main()
{
  std::vector<std::string> strings;
  strings.push_back("Hello");
  strings.push_back("World");
  for ( int idx = 0; idx < strings.size(); idx++ )
    std::cout << strings[idx];
}

Both are the exact same number of lines, and produce the same output, but the first example involves an instantiation of std::sort, which is probably an order of magnitude more code than the rest of the code here.

If you absolutely need to count number of bytes used in the program, use assembler.

Answer 3

Download the arduino IDE and 'verify' some of your existing code, or look at the sample sketches. It will tell you how many bytes that code is, which will give you an idea of how much more you can fit into a given device. Picking a couple of the examples at random, the web server example is 5816 bytes, and the LCD hello world is 2616. Both use external libraries.