Use link time optimizations to save flash / program storage space (Sketch too big error)

[mfalkvidd]

A friend of mine who is way more savvy in c/c++ programming than me mentioned that enabling link time optimizations (LTO) might save some flash space. I decided to do some research and it turns out that LTO is actually supported by the Arduino IDE toolchain, but it is not enabled by default.

I did some digging and figured out how to activate it. These are the results:
GatewaySerial on Atmega328 without LTO:

Sketch uses 15,170 bytes (49%) of program storage space. Maximum is 30,720 bytes.
Global variables use 711 bytes (34%) of dynamic memory, leaving 1,337 bytes for local variables. Maximum is 2,048 bytes.

GatewaySerial on Atmega328 with LTO:

Sketch uses 14,050 bytes (45%) of program storage space. Maximum is 30,720 bytes.
Global variables use 712 bytes (34%) of dynamic memory, leaving 1,336 bytes for local variables. Maximum is 2,048 bytes.

1,120 bytes flash saved (7.4%)!

SecureActuator on Atmega328 without LTO:

Sketch uses 23,572 bytes (76%) of program storage space. Maximum is 30,720 bytes.
Global variables use 1,258 bytes (61%) of dynamic memory, leaving 790 bytes for local variables. Maximum is 2,048 bytes.

SecureActuator on Atmega328 with LTO:

Sketch uses 21,242 bytes (69%) of program storage space. Maximum is 30,720 bytes.
Global variables use 1,241 bytes (60%) of dynamic memory, leaving 807 bytes for local variables. Maximum is 2,048 bytes.

2,330 bytes flash saved (9.9%)!

Pretty sweet!

Here is how to enable it. Shut down the Arduino IDE. Put the following lines:

compiler.c.extra_flags=-flto -fno-fat-lto-objects
compiler.c.elf.extra_flags=-flto -fuse-linker-plugin
compiler.S.extra_flags=-flto
compiler.cpp.extra_flags=-flto
compiler.ar.cmd=avr-gcc-ar

into a new file at "C:\Program Files (x86)\Arduino\hardware\arduino\avr\platform.local.txt" (or the corresponding place on your OSX/Linux machine)

This might help people trying to fit code into attiny85 or similar mcus with small flash size, or people who try to fit a lot of libraries and/or functionality (for example MY_DEBUG and MY_SIGNING_SOFT at the same time), getting the dreaded "Sketch too big; see http://www.arduino.cc/en/Guide/Troubleshooting#size for tips on reducing it." error.

Caveat: There might be some side-effects of the optimizations. I don't kow what they might be or how common they would be, so please try this and report your results.

[gohan]

I'll try to experiment with it. Thanks

[gohan]

I didn't see any change in compile size... maybe it didn't like the new created file

[mfalkvidd]

@gohan you did have the IDE closed when adding the file?
Which board are you using? The change documented above will only affect Arduino avr boards. Boards using other board definitions will not be affected.

[gohan]

I'm using the usual pro mini 3.3v. Yes it was closed and the file is there.

[mfalkvidd]

@gohan strange. Which version of the Arduino Avr boards are you using? I am using 1.6.20. I think the minimum version is 1.6.10.

[mfalkvidd]

I think I read somewhere that the location of the platform.local.txt file was moved in some version of the Arduino IDE. I am using 1.6.8. Is there a platform.txt file in the folder where you put the local file on your computer?

[gohan]

it is in the same folder as platform.txt

[gohan]

I am still lost with this... I can't get it to work...

[mfalkvidd]

Update: LTO is enabled by default for avr core/board version 1.6.12 and newer.