handler-function-table: Link-time read-only function pointer table in GCC

This answers the question in https://stackoverflow.com/questions/19845635/implementing-a-compile-time-read-only-function-pointer-table-in-gcc and implements a compile time (or rather: link time) read only function pointer table in gcc using weak symbols.

This makes it possible to place that table in what avr-libc calls PROGMEM.

Usage

To clean up all generated files:

make clean

To build all test cases for AVR:

make MCU=atmega644p

To build all test cases natively and run them:

make check

To compare the disassembly of some generated executables:

diff -u build/native/testcase-{1a,2a}.dump
diff -u build/atmega644p/testcase-{1a,2a}.dump

A typo in the function name of an overriding function will result in the override not happening WITHOUT compiler warnings or errors.

avr-libc solves that issue by defining macros for all vectors which map identifiers onto vector numbers, and uses those numbers in the table instead of the user visible identifiers. This can cause a compilation error when a typo in the name of an ISR handler happens.

Even though the assembly instructions emitted by gcc appear to use the same pattern of .weak and .set assembly directives as avr-libc uses in crt1/gcrt1.S to define __vectors, __bad_interrupt, and __vector_default, we cannot override the default handler in our implementation.

Solution: We were missing the non-weak __function_handler_default function which then calls the weakly defined function_handler_default function. In crt1/gcrt1.S can do the indirection step by just jumping to the weakly defined __vector_default function, but calling the weakly defined function works just as well.