[Bug backtrace/16771] New: AVR: Backtraces fail for functions with no locals and -fno-omit-frame-pointer

[Bug backtrace/16771] New: AVR: Backtraces fail for functions with no locals and -fno-omit-frame-pointer

[justin at jgottula dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16771

            Bug ID: 16771
           Summary: AVR: Backtraces fail for functions with no locals and
                    -fno-omit-frame-pointer
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: backtrace
          Assignee: unassigned at sourceware dot org
          Reporter: justin at jgottula dot com

Stack unwinding consistently fails to work properly when an AVR program (for
atxmega128a3) is compiled with -fno-omit-frame-pointer:

(gdb) bt
#0  0x00000bae in can_spi_xfer (byte=128 '\200') at src/io/stdfile.c:55
#1  0x0000000c in __vectors ()

Frame #0 is correct, #1 is garbage, and nothing else is shown even though this
is several calls deep.

When compiled with -fno-omit-frame-pointer, typical function prologues with NO
local variables look like this:

000019b0 <main>:
    19b0:    cf 93           push    r28
    19b2:    df 93           push    r29
    19b4:    cd b7           in    r28, 0x3d    ; 61
    19b6:    de b7           in    r29, 0x3e    ; 62

or this:

00001436 <fifo_init.1460>:
    1436:    0f 93           push    r16
    1438:    1f 93           push    r17
    143a:    cf 93           push    r28
    143c:    df 93           push    r29
    143e:    cd b7           in    r28, 0x3d    ; 61
    1440:    de b7           in    r29, 0x3e    ; 62

The documentation for the avr_scan_prologue function in gdb/avr-tdep.c says:

   A typical AVR function prologue with a frame pointer might look like this:
        push    rXX        ; saved regs
        ...
        push    r28
        push    r29
        in      r28,__SP_L__
        in      r29,__SP_H__
        sbiw    r28,<LOCALS_SIZE>
        in      __tmp_reg__,__SREG__
        cli
        out     __SP_H__,r29
        out     __SREG__,__tmp_reg__
        out     __SP_L__,r28

And, correspondingly, scan_stage 2 of avr_scan_prologue does indeed require
that the frame pointer subtraction and stack pointer transfer are present,
meaning that the two examples above fail to be processed correctly and aren't 

To fix this problem, scan_stage 2 of avr_scan_prologue needs to be less
restrictive, such that function prologues that use the frame pointer, yet don't
have local variables, will be successfully scanned as AVR_PROLOGUE_NORMAL.

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[Bug backtrace/16771] AVR: Backtraces fail for functions with no locals and -fno-omit-frame-pointer

[justin at jgottula dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16771

Justin Gottula <justin at jgottula dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |justin at jgottula dot com

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