Re: [PATCH][PR/backtrace 16721] Fix erroneous backtrace on AVR

[Tristan Gingold <gingold at adacore dot com>]

On 18 Mar 2014, at 18:53, Pierre Langlois <pierre.langlois@embecosm.com> wrote:

> Hi all,
> 
> Looking at stack unwinding on AVR, I noticed the current frame was not always correctly
> detected from the function prologue.
> 
> This bug occurs only with the following prologue, referred to as "Method 2: Adjust stack pointer"
> in GCC: gcc/config/avr/avr.c (avr_prologue_setup_frame).
> 
> --> push the old frame pointer
>  push r28
>  push r29
> 
> --> allocate new space
>  rcall .+0
>  push r1
> 
> --> move fp <- sp
>  in r28, 0x3d
>  in r29, 0x3e
> 
> GCC uses "rcall .+0" and "push r1" to adjust the stack pointer, "rcall" pushing
> automatically 2 or 3 bytes on the stack, depending on the target.
> 
> GDB should scan this prologue and find out the size of the frame but it is incorrect by one
> because it expects "push r0" and not "push r1".
> 
> I believe this register was changed in GCC withcommit 915f904be.

Looks good to me.

Tristan (as former AVR maintainer)

> 
> Best,
> 
> Pierre
> 
> 2014-03-18  Pierre Langlois  <pierre.langlois@embecosm.com>
> 
>      * avr-tdep.c (avr_scan_prologue): Accept push r1 instruction for small
>        stack allocation.
> 
> -----------------------------------------------------------------------------------------
> 
> GNU gdb (AVR 8-bit toolchain (built 20140310)) 7.7.50.20140318-cvs
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> (gdb) file atmega-test.elf
> Reading symbols from atmega-test.elf...done.
> (gdb) target remote :51000
> Remote debugging using :51000
> 0x00000116 in multiply (a=25, b=8) at main.c:4
> 4		return a * b;
> (gdb) monitor reset
> (gdb) load
> Loading section .text, size 0x1a4 lma 0x0
> Start address 0x0, load size 420
> Transfer rate: 3360 bits in <1 sec, 210 bytes/write.
> (gdb) b multiply
> Breakpoint 1 at 0x114: file main.c, line 4.
> (gdb) c
> Continuing.
> 
> Program received signal SIGTRAP, Trace/breakpoint trap.
> 0x00000116 in multiply (a=25, b=8) at main.c:4
> 4		return a * b;
> (gdb) bt
> #0  0x00000116 in multiply (a=25, b=8) at main.c:4
> #1  0x01e00000 in ?? ()
> (gdb) q
> A debugging session is active.
> 
> 	Inferior 1 [Remote target] will be killed.
> 
> Quit anyway? (y or n)
> 
> <pr-16721.patch>