How to get GNU AS to emit per-line debugging info or GDB to single step lines in (AVR) Assembly?

Question

I cannot figure out how to get GAS to emit line number debugging information for gdb in assembly. Note that I'm using the avr-gcc cross compiler, but doubt that is relevant.

I'm using the following command line options to assemble: avr-gcc -g -ggdb -Os -mmcu=atmega644 -ffunction-sections -fsigned-char -Winvalid-pch -Wall -Wno-long-long -x assembler-with-cpp -Wa,-ggdb -c -o z813.o z813.s

When I use a nearly identical set of arguments to compile C code, I can see the line number and source file augmentation using 'objdump -l -d'.

However the assembly objects have none. ie

000000d4 <run_timetable>:
      d4:       0e 94 57 00     call    0xae    ; 0xae <run_timetable_row>
      d8:       0e 94 b4 00     call    0x168   ; 0x168 <delay>

vs

00000f9c :
main():
/home/braddock/work/tviki/tviki/scratch/z813-vid.c:68
     f9c:       0e 94 ae 07     call    0xf5c   ; 0xf5c <init>
/home/braddock/work/tviki/tviki/scratch/z813-vid.c:70
     fa0:       0e 94 6a 00     call    0xd4    ; 0xd4 <run_timetable>

When in avr-gdb (via simulavr remote gdb connection), I cannot get the debugger to single step through my assemble code, although it does recognize the symbol names and information. I assume this is related.

I put in good effort trying to find this in info/man/google and permuting the likely flags. Any help appreciated!

Answer 1

When you "compile" assembler source code, invoking the C compiler is somewhat overkill. The compiler will recognize that the input is a C file, and ignore any options that you pass it that affect C compilation, such as -g and -Os. Pass "-v" to the compiler line to see the options that get actually passed to the assembler.

When I assemble with

avr-as --gstabs -mmcu=atmega644 a.s -o a.o

then I get nice-looking source line information in the object file. My version of avr-as (2.18.0) doesn't support a -ggdb option at all, it only has -g, --gstabs, --gstabs+, --gdwarf-2. With stabs, objdump is able to display the lines. With dwarf-2, it's not - I'm not sure whether that's a bug in objdump or in as.

If you absolutely insist on invoking the assembler as "gcc", you should pass -Wa,--gstabs.