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[on:]

hi,
i discovered a bug with mega2560 and ide 1.0.3 (ubuntu 12.10)
compiling a program with a size of ~33kb and adding Serial.x stuff makes the linker throw the following error:

/home/--/arduino-1.0/hardware/tools/avr/bin/../lib/gcc/avr/4.3.2/../../../avr/lib/avr6/crtm2560.o: In function `__bad_interrupt':
../../../../crt1/gcrt1.S:193: relocation truncated to fit: R_AVR_13_PCREL against symbol `__vector_46' defined in .text section in /home/--/arduino-1.0/hardware/tools/avr/bin/../lib/gcc/avr/4.3.2/../../../avr/lib/avr6/crtm2560.o

removing the --relax linker option fixes that (beginning at line 193 in Compiler.java).

furthermore i dont know why exactly relaxation is enabled on mega2560?
from the avr-gcc description:

Code:

Enables linker relaxations. By default, the linker links functions will a full CALL statement,
which is wasteful if two functions are near each other.
Relaxations will do more in the future, but currently (AFAIK) just replace CALL
statements with RCALL where possible to save a few bytes.

but afaik rcall can only jump to regions +-4kib away! the mega2560 has 256kib flash,
im in doubt about the compiler can optimize every call to rcall to within +-4kib limit.
am i overlooking something?

side hint: hard coding such exception seems not the right solution to me (i searched and grepped a long bit).

[Reply #1 on:]

Is this the same issue?

https://github.com/arduino/Arduino/issues/1071

[Reply #2 on:]

seems so.
i attached a patched version (current git version) of the ide without the --relax parameter at linker stage.
(just copy to /lib path inside the arduino directory)

[Reply #3 on:]

Here's a link to the issue for which we included the relax compiler argument: https://code.google.com/p/arduino/issues/detail?id=729

[Reply #4 on:]

If the interrupt functions can only be a certain distance from 0, then a way to solve the problem and still use the linker relax, is to create a new section that the linker puts into memory before the normal text section and put the interrupt code into that section, so the linker has a better chance of using the appropriate jump.  Looking at the binutils source (probably a few months old), the avr port has several special sections:

• .progmem.gcc* and .progmem for code that must be in the lower 64k of progmem
• .lowtext* for code that must be in the lower 128k of progmem

A quick glance in the 4.7 sources doesn't seem to have a function attribute to put a function into the .progmem or .lowtext sections, but you could use the section attribute to hardwire the function.