GDB and -gdwarf-5

GDB and -gdwarf-5

[Eli Zaretskii]

I have GCC 6.3.0 installed, and it seems to support DWARF2 version 5
via the -gdwarf-5 switch.  But I wanted to make sure I really get that
version of debug info, so I tried "info source" in GDB.  But that
command doesn't tell the version of DWARF2.  Is there a GDB command
which does?

I also tried "objdump --dwarf=info" (Binutils 2.29), but that says the
DWARF2 version is 4.  So now I'm confused.

Can someone please tell me how to verify the version of DWARF2 I get
with that GCC switch?

Thanks.

Re: GDB and -gdwarf-5

[Jan Kratochvil]

On Thu, 07 Sep 2017 21:16:44 +0200, Eli Zaretskii wrote:
> I have GCC 6.3.0 installed, and it seems to support DWARF2 version 5
> via the -gdwarf-5 switch.  But I wanted to make sure I really get that
> version of debug info, so I tried "info source" in GDB.  But that
> command doesn't tell the version of DWARF2.  Is there a GDB command
> which does?
> 
> I also tried "objdump --dwarf=info" (Binutils 2.29), but that says the
> DWARF2 version is 4.  So now I'm confused.

Here gcc-6.3.1-1.fc25.x86_64 really accepts -gdwarf-5 but still it does produce
only DWARF 4.  I do not know why.

gcc-7.1.1-3.fc26.x86_64 does produce DWARF 5.

You should really see from objdump/readelf:
  Compilation Unit @ offset 0x0:
   Length:        0x13c (32-bit)
   Version:       5


Jan

Re: GDB and -gdwarf-5

[Joel Brobecker]

> I have GCC 6.3.0 installed, and it seems to support DWARF2 version 5
> via the -gdwarf-5 switch.  But I wanted to make sure I really get that
> version of debug info, so I tried "info source" in GDB.  But that
> command doesn't tell the version of DWARF2.  Is there a GDB command
> which does?
> 
> I also tried "objdump --dwarf=info" (Binutils 2.29), but that says the
> DWARF2 version is 4.  So now I'm confused.
> 
> Can someone please tell me how to verify the version of DWARF2 I get
> with that GCC switch?

I would compile a unit, and then dump the DWARF info using objdump
or readelf. The beginning of that section includes the DWARF version
number. Alternatively, compile with -save-temps -dA, and look at
the assembly file instead. -dA annotates the directives used to
produce the DWARF, and should also make it easy to verify the version
number.

-- 
Joel

Re: GDB and -gdwarf-5

[Eli Zaretskii]

> Date: Thu, 7 Sep 2017 21:24:47 +0200
> From: Jan Kratochvil <jan.kratochvil@redhat.com>
> Cc: gdb@sourceware.org
> 
> > I also tried "objdump --dwarf=info" (Binutils 2.29), but that says the
> > DWARF2 version is 4.  So now I'm confused.
> 
> Here gcc-6.3.1-1.fc25.x86_64 really accepts -gdwarf-5 but still it does produce
> only DWARF 4.  I do not know why.

OK, so I guess it's a bug or a missing feature in GCC 6.3.  Thanks.

> gcc-7.1.1-3.fc26.x86_64 does produce DWARF 5.
> 
> You should really see from objdump/readelf:
>   Compilation Unit @ offset 0x0:
>    Length:        0x13c (32-bit)
>    Version:       5

Yes, I see version 4 instead.  I guess I have to wait for GCC 7, then.

Re: GDB and -gdwarf-5

[Eli Zaretskii]

> Date: Thu, 7 Sep 2017 12:28:00 -0700
> From: Joel Brobecker <brobecker@adacore.com>
> Cc: gdb@sourceware.org
> 
> > I also tried "objdump --dwarf=info" (Binutils 2.29), but that says the
> > DWARF2 version is 4.  So now I'm confused.
> > 
> > Can someone please tell me how to verify the version of DWARF2 I get
> > with that GCC switch?
> 
> I would compile a unit, and then dump the DWARF info using objdump
> or readelf.

That's what I did, and objdump said it was version 4.  As Jan
explained, it seems to be a deficiency in GCC 6.3.

Thanks.