Re: [avr-libc-dev] Possible hole in the assembler interrupt documentation

[Dale]

Hi Joerg                                         >@2012.11.01_17:37:49_+0200

> As Brendan Hanna wrote:
> 
> > It took me some time to realise that the vector would not be
> > considered for linking unless its object file contained a dependency
> > required by the main object file.
> 
> *Every* object (.o) file specified on the linker command-line is being
> linked.  Only .a files, regardless of whether their names are
> implicitly derived from a -l option, or explicitly specified as
> libfoobar.a, are being searched in order to resolve undefined global
> references (and those things not referenced there are not linked at
> all).
> 
> What you guys are probably stumbling about is that you are using the
> --gc-sections linker option.  Then, the linker does exactly what you
> are asking it for: it drops every module that hasn't at least one
> symbol being referenced from outside.
> 

You're correct, it was a while ago and thinking about it further, where
I had a problem was library-related, although I thought I'd had the same
issue elsewhere.

So, I'd say there is still a hole, albeit a lot smaller than first
thought.

The issue then is the case of a library that contains an ISR.

Its not unreasonable to do this and currently you need to ensure that
the ISR is in an object module that will be linked in as a result of
external references.

> Here's a small example demonstrating the linker doesn't throw anything
> away normally:
> 
> $ cat foo.c
> int
> main(void)
> {
>   return 42;
> }
> $ cat bar.s
> .global __vector_8
> __vector_8:
>    nop
>    reti
> $ avr-gcc -mmcu=atmega8 -Os -o foo.elf foo.c bar.s
> $ avr-objdump -d foo.elf
> 
> foo.elf:     file format elf32-avr
> 
> 
> Disassembly of section .text:
> 
> 00000000 <__vectors>:
>    0:   12 c0           rjmp    .+36            ; 0x26 <__ctors_end>
>    2:   19 c0           rjmp    .+50            ; 0x36 <__bad_interrupt>
>    4:   18 c0           rjmp    .+48            ; 0x36 <__bad_interrupt>
>    6:   17 c0           rjmp    .+46            ; 0x36 <__bad_interrupt>
>    8:   16 c0           rjmp    .+44            ; 0x36 <__bad_interrupt>
>    a:   15 c0           rjmp    .+42            ; 0x36 <__bad_interrupt>
>    c:   14 c0           rjmp    .+40            ; 0x36 <__bad_interrupt>
>    e:   13 c0           rjmp    .+38            ; 0x36 <__bad_interrupt>
>   10:   16 c0           rjmp    .+44            ; 0x3e <__vector_8>
>   12:   11 c0           rjmp    .+34            ; 0x36 <__bad_interrupt>
>   14:   10 c0           rjmp    .+32            ; 0x36 <__bad_interrupt>
>   16:   0f c0           rjmp    .+30            ; 0x36 <__bad_interrupt>
>   18:   0e c0           rjmp    .+28            ; 0x36 <__bad_interrupt>
>   1a:   0d c0           rjmp    .+26            ; 0x36 <__bad_interrupt>
>   1c:   0c c0           rjmp    .+24            ; 0x36 <__bad_interrupt>
>   1e:   0b c0           rjmp    .+22            ; 0x36 <__bad_interrupt>
>   20:   0a c0           rjmp    .+20            ; 0x36 <__bad_interrupt>
>   22:   09 c0           rjmp    .+18            ; 0x36 <__bad_interrupt>
>   24:   08 c0           rjmp    .+16            ; 0x36 <__bad_interrupt>
> 
> 00000026 <__ctors_end>:
>   26:   11 24           eor     r1, r1
>   28:   1f be           out     0x3f, r1        ; 63
>   2a:   cf e5           ldi     r28, 0x5F       ; 95
>   2c:   d4 e0           ldi     r29, 0x04       ; 4
>   2e:   de bf           out     0x3e, r29       ; 62
>   30:   cd bf           out     0x3d, r28       ; 61
>   32:   02 d0           rcall   .+4             ; 0x38 <main>
>   34:   06 c0           rjmp    .+12            ; 0x42 <_exit>
> 
> 00000036 <__bad_interrupt>:
>   36:   e4 cf           rjmp    .-56            ; 0x0 <__vectors>
> 
> 00000038 <main>:
>   38:   8a e2           ldi     r24, 0x2A       ; 42
>   3a:   90 e0           ldi     r25, 0x00       ; 0
>   3c:   08 95           ret
> 
> 0000003e <__vector_8>:
>   3e:   00 00           nop
>   40:   18 95           reti
> 
> 00000042 <_exit>:
>   42:   f8 94           cli
> 
> 00000044 <__stop_program>:
>   44:   ff cf           rjmp    .-2             ; 0x44 <__stop_program>
> 
> The "nop" in the ISR is only there to demonstrate it's really the ISR
> from bar.s that is being linked into the final object file.
> 
> (Besides, even with --gc-sections, the ISR remains there, because it
> doesn't allocate an input section of its own.)
> -- 
> cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL
> 
> http://www.sax.de/~joerg/                        NIC: JW11-RIPE
> Never trust an operating system you don't have sources for. ;-)
> 
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