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Program size optimization

I am trying to spare a few bytes of program size. I faced a problem which I cannot understand and I hope somebody will help.

Look at this empty sketch, pleas. It takes just 444 bytes:

void setup()
{
}
void loop()
{
}

However, this one – 1332 bytes already!

void f() {
 Serial.println(1);
}
void setup()
{
}
void loop()
{
}

Why? It is supposed that function f() will be cut-off during the linkage stage. But it is not. And what can I do to cut such things off, since they are not used in the code?

Answer*

Using the `Serial` object pulls HardwareSerial0.o into your program
linkage. This file defines, among other things, the ISRs (interrupt
service routines) associated with the serial port. ISRs are defined
using [the `ISR()` macro][isr] from the acr-libc. This macro creates a
function prototype and assigns some attributes to the function,
including [the `used` attribute][used] which tells the linker that it
should consider the function to be used, even if it cannot spot a single
call to it within the compiled program. The ISR cannot then be removed,
and it in turn keeps lots of code from the `Serial` implementation.

**Edit**: As pointed out by the busybee in a comment, although `f()`
cannot be removed at link time, it can be removed at compile time if you
give it the `static` qualifier. This has the added benefit of triggering
a compiler warning:

> **warning**: '_void f()_' defined but not used [**-Wunused-function**]

[isr]: https://www.nongnu.org/avr-libc/user-manual/group__avr__interrupts.html
[used]: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-unused-function-attribute

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You might like to add that the missing static forces the compiler to include f() in the first place. All other stuff is then pulled in by this.

the busybee
– the busybee

2021年02月25日 12:03:56 +00:00
Commented Feb 25, 2021 at 12:03
@thebusybee: Right, I edited my answer.

Edgar Bonet
– Edgar Bonet

2021年02月25日 14:00:24 +00:00
Commented Feb 25, 2021 at 14:00

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