Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

[Arvind Sankar <nivedita@alum.mit.edu>]

On Tue, Nov 03, 2020 at 04:53:42PM -0800, Nick Desaulniers wrote:
> Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
> explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
> way that's forward compatible with existing configs, and makes adding
> future versions more straightforward.
> 
> Suggested-by: Fangrui Song <maskray@google.com>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  Makefile          | 14 ++++++++------
>  lib/Kconfig.debug | 19 +++++++++++++++----
>  2 files changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 75b1a3dcbf30..e23786a4c1c7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -826,12 +826,14 @@ else
>  DEBUG_CFLAGS	+= -g
>  endif
>  
> -ifndef LLVM_IAS
> -KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> -endif
> -
> -ifdef CONFIG_DEBUG_INFO_DWARF4
> -DEBUG_CFLAGS	+= -gdwarf-4
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> +DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
> +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
> +# Binutils 2.35+ required for -gdwarf-4+ support.
> +dwarf-aflag	:= $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> +DEBUG_CFLAGS	+= $(dwarf-aflag)
> +KBUILD_AFLAGS	+= $(dwarf-aflag)
>  endif
>  

For LLVM_IAS=1, adding dwarf-aflag to DEBUG_CFLAGS should not be
necessary, no? This seems to add it for dwarf-4.

The as-option check will only work on binutils 2.35.1 onwards: earlier
versions will silently accept any -gdwarf-N option. Do we care? I think
it'll just get dwarf-2 for assembly files even though dwarf-4 might have
been configured. The earlier versions only error if the double-hyphen
form --gdwarf-N is used.

More generally, do we want to force this option via -Wa or should we
leave it up to the compiler driver when we can? For both Clang/IAS and
gcc/binutils, passing -gdwarf-N in KBUILD_AFLAGS will allow the compiler
to pass on the appropriate option to the assembler (with gcc you only
get --gdwarf-2 for the assembler except on trunk though). The only case
that would absolutely require -Wa is Clang without IAS, might be worth
adding the ability to pass on the flag to the external assembler?

Btw, is -gsplit-dwarf at all useful for assembler files?