Miosix code size optimization - Revision history

Andreabont: Adding Categories

2015-01-22T20:04:57Z

Adding Categories

← Older revision		Revision as of 20:04, 22 January 2015
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	By applying also this last tip code size can be reduced to around 6KB.		By applying also this last tip code size can be reduced to around 6KB.

			[[Category:Installation and Configuration]]
			[[Category:Optimization]]

Fede.tft at 11:31, 4 May 2014

2014-05-04T11:31:32Z

← Older revision		Revision as of 11:31, 4 May 2014
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	</source>		</source>

	~~TODO: how much size is saved?~~		This saves aroud 5KB.

	== Disable the filesystem entirely ==		== Disable the filesystem entirely ==

Fede.tft at 11:01, 4 May 2014

2014-05-04T11:01:02Z

← Older revision		Revision as of 11:01, 4 May 2014
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	</source>		</source>

	This saves nearly 20KB but comes with quite an important side effect, though: if the heap is full, the kernel will reboot the machine. To explain why it is so, consider that the normal behaviour in Miosix to handle a full heap is that the C ''malloc()'' returns ''NULL'', while the C++ ''operator new'' throws an exception of type ''bad_alloc''. Now, when exceptions are disabled, 'new'' can no longer throw, but C++ code doesn't check for ''NULL'', as it expects an exception, and that would result in undefined behaviour. The only way to prevent undefined behaviour, is to reboot the machine, and that is what happens.		This saves nearly 20KB but comes with quite an important side effect, though: if the heap is full, the kernel will reboot the machine. To explain why it is so, consider that the normal behaviour in Miosix to handle a full heap is that the C ''malloc()'' returns ''NULL'', while the C++ ''operator new'' throws an exception of type ''bad_alloc''. Now, when exceptions are disabled, ''new'' can no longer throw, but C++ code doesn't check for ''NULL'', as it expects an exception, and that would result in undefined behaviour. The only way to prevent undefined behaviour, is to reboot the machine, and that is what happens.

	== Putting it all together ==		== Putting it all together ==

Fede.tft at 11:00, 4 May 2014

2014-05-04T11:00:18Z

← Older revision		Revision as of 11:00, 4 May 2014
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	== Disable the C++ exception runtime ==		== Disable the C++ exception runtime ==

	The Miosix kernel is by default compiled with support for C++ exceptions, and unless you're also enabling [[Miosix processes\|processes]] that depend on C++ exceptions, it is possible to disable their support. To di so, open [Makefile.inc\|miosix/config/Makefile.inc]] and uncomment the following line:		The Miosix kernel is by default compiled with support for C++ exceptions, and unless you're also enabling [[Miosix processes\|processes]] that depend on C++ exceptions, it is possible to disable their support. To di so, open [[Makefile.inc\|miosix/config/Makefile.inc]] and uncomment the following line:

	<source lang="bash">		<source lang="bash">

Fede.tft at 10:59, 4 May 2014

2014-05-04T10:59:15Z

← Older revision		Revision as of 10:59, 4 May 2014
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	Maybe you're not using ''printf()'' at all in your application, however in this case you're still paying for the code size of ''iprintf()'' as the kernel uses it to print boot logs and error logs. To disable them, you can open [[miosix_settings.h\|miosix/config/miosix_settings.h]] and uncomment ''WITH_BOOTLOG'' and ''WITH_ERRLOG''. This allows to save an additional 17KB of code size, but this is only true if there's no ''printf()'' call in your code, otherewise the code size saving will be minimal.		Maybe you're not using ''printf()'' at all in your application, however in this case you're still paying for the code size of ''iprintf()'' as the kernel uses it to print boot logs and error logs. To disable them, you can open [[miosix_settings.h\|miosix/config/miosix_settings.h]] and uncomment ''WITH_BOOTLOG'' and ''WITH_ERRLOG''. This allows to save an additional 17KB of code size, but this is only true if there's no ''printf()'' call in your code, otherewise the code size saving will be minimal.

	Also, keep in mind that error logs are a powerful tool to debug crashes of your application or the kernel during code development, so you may want to disable them only in release builds, and keep them enabled during ~~developemnt~~.		Also, keep in mind that error logs are a powerful tool to debug crashes of your application or the kernel during code development, so you may want to disable them only in release builds, and keep them enabled during development.

	Finally, note that you can still access the serial port without using ''printf()'', by means of the POSIX calls ''write()'' and ''read()'', using ''STDOUT_FILENO'' and ''STDIN_FILENO'' as the ''fd'' argument.		Finally, note that you can still access the serial port without using ''printf()'', by means of the POSIX calls ''write()'' and ''read()'', using ''STDOUT_FILENO'' and ''STDIN_FILENO'' as the ''fd'' argument.

Fede.tft at 10:58, 4 May 2014

2014-05-04T10:58:18Z

← Older revision		Revision as of 10:58, 4 May 2014
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	== Disable boot logs ==		== Disable boot logs ==

	Maybe you're no using ''printf()'' at all in your application, however in this case you're still paying for the code size of ''iprintf()'' as the kernel uses it to print boot logs and error logs. To disable them, you can open [[miosix_settings.h\|miosix/config/miosix_settings.h]] and uncomment ''WITH_BOOTLOG'' and ''WITH_ERRLOG''. This allows to save an additional 17KB of code size, but this is only true if there's no ''printf()'' call in your code, otherewise the code size saving will be minimal.		Maybe you're not using ''printf()'' at all in your application, however in this case you're still paying for the code size of ''iprintf()'' as the kernel uses it to print boot logs and error logs. To disable them, you can open [[miosix_settings.h\|miosix/config/miosix_settings.h]] and uncomment ''WITH_BOOTLOG'' and ''WITH_ERRLOG''. This allows to save an additional 17KB of code size, but this is only true if there's no ''printf()'' call in your code, otherewise the code size saving will be minimal.

	Also, keep in mind that error logs are a powerful tool to debug crashes of your application or the kernel during code development, so you may want to disable them only in release builds, and keep them enabled during developemnt.		Also, keep in mind that error logs are a powerful tool to debug crashes of your application or the kernel during code development, so you may want to disable them only in release builds, and keep them enabled during developemnt.

Fede.tft at 10:55, 4 May 2014

2014-05-04T10:55:37Z

← Older revision		Revision as of 10:55, 4 May 2014
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	OPT_OPTIMIZATION := -Os		OPT_OPTIMIZATION := -Os
	</source>		</source>
	The code size will usually decrease as a percentage of the total code size, as this option affects how the compiler generates code. 10% decrease is a common figure. Although I haven't found benchmarks showing how much this affects execution speed, usually the impact isn't that high, and code runs definitely much faster with respect to disabling optimizations altogether (with ''-O0'').ì, so if you are tight on code size this is an easy option that does not require to sacrifice any feature of the kernel.		The code size will usually decrease as a percentage of the total code size, as this option affects how the compiler generates code. 10% decrease is a common figure. Although I haven't found benchmarks showing how much this affects execution speed, usually the impact isn't that high, and code runs definitely much faster with respect to disabling optimizations altogether (with ''-O0''), so if you are tight on code size this is an easy option that does not require to sacrifice any feature of the kernel.

	== Disable DevFs ==		== Disable DevFs ==

Fede.tft at 10:54, 4 May 2014

2014-05-04T10:54:46Z

← Older revision		Revision as of 10:54, 4 May 2014
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	== Compile with Os ==		== Compile with Os ==

	The default [http://fedetft.wordpress.com/2009/10/01/gcc-optimization-flags GCC optimization flag] used in Miosix is ''-O2'', which generates code optimized for speed. You can however choose ''-Os'' which optimizes for size by opening the file [Makefile.inc\|miosix/config/Makefile.inc]], commenting out the ''-O2'' line, and uncommenting the ''-Os'' line, like this:		The default [http://fedetft.wordpress.com/2009/10/01/gcc-optimization-flags GCC optimization flag] used in Miosix is ''-O2'', which generates code optimized for speed. You can however choose ''-Os'' which optimizes for size by opening the file [[Makefile.inc\|miosix/config/Makefile.inc]], commenting out the ''-O2'' line, and uncommenting the ''-Os'' line, like this:
	<source lang="bash">		<source lang="bash">
	#OPT_OPTIMIZATION := -O0		#OPT_OPTIMIZATION := -O0

Fede.tft: Created page with "Miosix is designed with the end goal to become an "unix on a chip" written in C++, so it was built with the idea of packing as much standard compliance as possible, without cu..."

2014-05-04T10:48:58Z

Created page with "Miosix is designed with the end goal to become an "unix on a chip" written in C++, so it was built with the idea of packing as much standard compliance as possible, without cu..."

New page

Miosix is designed with the end goal to become an "unix on a chip" written in C++, so it was built with the idea of packing as much standard compliance as possible, without cutting corners. Things like full support to the C and C++ standard libraries, including full support for the C++ STL, thread-safe exception handling, and seldom used features of the C standard libraries, including stdout redirection via
<source lang="CPP">
fclose(stdout);
stdout=fopen("/sd/log.txt","w");
puts("writing to a file\n");
fflush(stdout);
</source>
are supported. This obviously comes with a code size penalty, especially considering that most features are enabled by default.
However, the kernel is very modular, so you don't have to pay the code size cost of unused features in your application.

This guide lists a number of tips to adapt the Miosix kernel to the needs of code-size constrained embedded applications. The exact figures presented here may change as the kernel is developed further, but the tips are expected to remain valid. Considering the [[stm32f407vg_stm32f4discovery]] which is a common board used with Miosix, compiling an empty ''main()'' with the default settings leads to a code size of over 90KB. With this guide it is possible to reduce code size down to 6KB by removing various features you may not need.

== Compile with Os ==

The default [http://fedetft.wordpress.com/2009/10/01/gcc-optimization-flags GCC optimization flag] used in Miosix is ''-O2'', which generates code optimized for speed. You can however choose ''-Os'' which optimizes for size by opening the file [Makefile.inc|miosix/config/Makefile.inc]], commenting out the ''-O2'' line, and uncommenting the ''-Os'' line, like this:
<source lang="bash">
#OPT_OPTIMIZATION := -O0
#OPT_OPTIMIZATION := -O2
#OPT_OPTIMIZATION := -O3
OPT_OPTIMIZATION := -Os
</source>
The code size will usually decrease as a percentage of the total code size, as this option affects how the compiler generates code. 10% decrease is a common figure. Although I haven't found benchmarks showing how much this affects execution speed, usually the impact isn't that high, and code runs definitely much faster with respect to disabling optimizations altogether (with ''-O0'').ì, so if you are tight on code size this is an easy option that does not require to sacrifice any feature of the kernel.

== Disable DevFs ==

Miosix 2.0 comes with an in-memory filesystem, mounted at ''/dev'' where device files can be addes, similarly to how a Linux machine works. The main purpose of this is to allow [[Miosix processes|processes]] to access peripherals, since due to memory protection they can't just access the peripheral registers. However, processes are currently disabled by default because they're still an experimental feature, so you can disable DevFs too. This can be done by commenting out the ''WITH_DEVFS'' line in [[miosix_settings.h|miosix/config/miosix_settings.h]], like this:

<source lang="CPP">
//#define WITH_DEVFS
</source>

TODO: how much size is saved?

== Disable the filesystem entirely ==

Miosix 2.0 has a completely rewritten filesystem subsystem, supporting multiple mountpoints, extensible filesystem types (currently Fat32 and DevFs), unicode UTF8 in file names, directory listing via the standard ''opendir()''/''readdir()''/''closedir()'' and more POSIX compilance, including inode emulation for Fat32. This comes at a code size cost, though. If your application does not need to read/write from files, you can disable the filesystem subsystem entirely. To do so, open [[miosix_settings.h|miosix/config/miosix_settings.h]] and comment out both the ''WITH_DEVFS'' and ''WITH_FILESYSTEM'' lines. When compared to the default when both filesystem and DevFs are enabled, about 40KB of code size can be saved.

== Use iprintf() instead of printf() in your application ==

''iprintf()'' is a non-standard extension of the [https://www.sourceware.org/newlib newlib] C library used by Miosix. It can do all what ''printf()'' can do except printing floating point numbers, and thath's why its code size is significantly lower, printing floating point numbers is quite complicated. Miosix already uses ''iprintf()'' internally, so to see the code size difference you have to write the following simple code in ''main()''

<source lang="CPP">
int main()
{
int i=0;
printf("%d\n",i);
//iprintf("%d\n",i);
}
</source>

Trying with ''iprintf()'' instead of ''printf()'', a 14KB code size reduction is observed. Note that you have to use a format string containing a % argument, such as %d to see the code size difference. If there is no % in the format string, GCC may replace ''printf()''/''iprintf()'' with a call to ''puts()'' as an optimization, and no code size difference can be seen.

== Disable boot logs ==

Maybe you're no using ''printf()'' at all in your application, however in this case you're still paying for the code size of ''iprintf()'' as the kernel uses it to print boot logs and error logs. To disable them, you can open [[miosix_settings.h|miosix/config/miosix_settings.h]] and uncomment ''WITH_BOOTLOG'' and ''WITH_ERRLOG''. This allows to save an additional 17KB of code size, but this is only true if there's no ''printf()'' call in your code, otherewise the code size saving will be minimal.

Also, keep in mind that error logs are a powerful tool to debug crashes of your application or the kernel during code development, so you may want to disable them only in release builds, and keep them enabled during developemnt.

Finally, note that you can still access the serial port without using ''printf()'', by means of the POSIX calls ''write()'' and ''read()'', using ''STDOUT_FILENO'' and ''STDIN_FILENO'' as the ''fd'' argument.

== Disable the C++ exception runtime ==

The Miosix kernel is by default compiled with support for C++ exceptions, and unless you're also enabling [[Miosix processes|processes]] that depend on C++ exceptions, it is possible to disable their support. To di so, open [Makefile.inc|miosix/config/Makefile.inc]] and uncomment the following line:

<source lang="bash">
OPT_EXCEPT := -fno-exceptions -fno-rtti -D__NO_EXCEPTIONS
</source>

This saves nearly 20KB but comes with quite an important side effect, though: if the heap is full, the kernel will reboot the machine. To explain why it is so, consider that the normal behaviour in Miosix to handle a full heap is that the C ''malloc()'' returns ''NULL'', while the C++ ''operator new'' throws an exception of type ''bad_alloc''. Now, when exceptions are disabled, 'new'' can no longer throw, but C++ code doesn't check for ''NULL'', as it expects an exception, and that would result in undefined behaviour. The only way to prevent undefined behaviour, is to reboot the machine, and that is what happens.

== Putting it all together ==

With all those tips presented so far applied simultaneously, the code size of an empty ''main()'' for the [[stm32f407vg_stm32f4discovery]] is reduced to around 10KB.

== One more thing, disable the serial port ==

The kernel, during the boot process, configures a serial port on nearly all the supported boards. If you really have no use for the serial port, you can disable it entirely, saving additional code size. To do so there is no configuration option, but you can edit the board support package file of your board. For the [[stm32f407vg_stm32f4discovery]] it's ''miosix/arch/cortexM4_stm32f4/stm32f407vg_stm32f4discovery/interfaces-impl/bsp.cpp'', and comment out the following lines in ''IRQbspInit()''

<source lang="CPP">
DefaultConsole::instance().IRQset(intrusive_ref_ptr<Device>(
new STM32Serial(defaultSerial,defaultSerialSpeed,
defaultSerialFlowctrl ? STM32Serial::RTSCTS : STM32Serial::NOFLOWCTRL)));
</source>

as well as the following line in ''shutdown()'' and ''reboot()''

<source lang="CPP">
DefaultConsole::instance().get()->ioctl(IOCTL_SYNC,0);
</source>

By applying also this last tip code size can be reduced to around 6KB.