RE: [Patch][AVR]: Support tail calls

["Boyapati, Anitha" <Anitha dot Boyapati at atmel dot com>]

Hi Georg,

>This is a patch to test/review/comment on. It adds tail call
>optimization to avr backend.
>
>The implementation uses struct machine_function to pass information
>around, i.e. from avr_function_arg_advance to avr_function_ok_for_sibcall.
>
>Tail call support is more general than avr-ld's replacement of
>call/ret sequences with --relax which are sometimes wrong, see
>http://sourceware.org/PR12494
>
>gcc can, e.g. tail-call bar1 in
>
>void bar0 (void);
>void bar1 (int);
>
>int foo (int x)
>{
>  bar0();
>  return bar1 (x);
>}

To be on same page, can you explain how gcc optimizes above case? As I understand, in a tail-call optimization, bar1 can return to the caller of foo(). There can be different cases of handling this. But how is this handled in gcc after recognizing that foo() is a candidate for tail call? 

Also, I have applied the patch, and used it for a small test case as below:

int bar1(int x) {
        x++;
        return x;
}

int foo (int x)
{
  return bar1 (x);
}

int main() {
        volatile int i;
        return foo(i);

}

avr-gcc -S -foptimize-sibling-calls tail-call.c


I find no difference in the code generated with and without tail call optimization. (I am assuming -foptimize-sibling-calls should turn on this). Let me know if I am doing something wrong.

Anitha