linux-kernel - Re: [PATCH 2/3] Kconfig.debug: add toolchain checks for DEBUG_INFO_DWARF_TOOLCHAIN

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Date:   Tue, 4 Oct 2022 05:18:49 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/3] Kconfig.debug: add toolchain checks for DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT

On Tue, Oct 4, 2022 at 2:10 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Mon, Oct 03, 2022 at 03:11:06AM +0900, Masahiro Yamada wrote:
> > CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT passes the -g option to the
> > command line. The actual DWARF version is up to the toolchain.
> >
> > The combination of GCC and GAS works fine, and Clang with the integrated
> > assembler is good too.
> >
> > The combination of Clang and GAS is a bit tricky, but at least, the
> > default -g flag worked until LLVM 14 was released because Clang <=13
> > defaults to DWARF v4.
> >
> > Clang 14 switched to DWARF v5 by default.
> >
> > Now, CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT has the same issue as
> > addressed by commit 98cd6f521f10 ("Kconfig: allow explicit opt in to
> > DWARF v5").
> >
> > CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y for Clang >= 14 and
> > GAS < 2.35 produces a ton of errors like follows:
> >
> >   /tmp/main-c2741c.s: Assembler messages:
> >   /tmp/main-c2741c.s:109: Error: junk at end of line, first unrecognized character is `"'
> >   /tmp/main-c2741c.s:109: Error: file number less than one
> >
> > Add 'depends on' to check toolchains.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > ---
> >
> >  lib/Kconfig.debug | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index f4b2165f24db..cc90414d492e 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -253,6 +253,7 @@ config DEBUG_INFO_NONE
> >  config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> >       bool "Rely on the toolchain's implicit default DWARF version"
> >       select DEBUG_INFO
> > +     depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502)
> >       help
> >         The implicit default version of DWARF debug info produced by a
> >         toolchain changes over time.
> > --
> > 2.34.1
> >
> >
>
> Could we do something like this instead? Bikeshed the configuration
> names as much as you want but we are duplicating most of this
> conditional in a few different places so it would be good to consolidate
> those in my opinion. Even though the DWARF4 and DWARF5 conditions are
> the same, they might diverge so I think it makes sense to keep them
> separate.
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index cc90414d492e..8e34a639cd40 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -231,6 +231,17 @@ config DEBUG_INFO
>           in the "Debug information" choice below, indicating that debug
>           information will be generated for build targets.
>
> +config CC_AND_AS_SUPPORT_DWARF4
> +       def_bool !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)
> +
> +config CC_AND_AS_SUPPORT_DWARF5
> +       def_bool !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)
> +
> +config CC_AND_AS_SUPPORT_DWARF_DEFAULT
> +       bool
> +       default y if CC_AND_AS_SUPPORT_DWARF5 && (CLANG_VERSION >= 140000 || GCC_VERSION >= 110000)
> +       default y if CC_AND_AS_SUPPORT_DWARF4 && (CLANG_VERSION < 140000 || GCC_VERSION < 110000)


No, this is an overkill.




In my patch, you can enable DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
for the combination of:
    CC_IS_CLANG=y
    CLANG_VERSION < 140000
    AS_IS_GNU=y
    AS_VERSION < 23502

But, your code disallows it.

You are requiring binutils 2.35.2 for Clang+GAS, but
it is not necessary.







>  choice
>         prompt "Debug information"
>         depends on DEBUG_KERNEL
> @@ -253,7 +264,7 @@ config DEBUG_INFO_NONE
>  config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>         bool "Rely on the toolchain's implicit default DWARF version"
>         select DEBUG_INFO
> -       depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502)
> +       depends on CC_AND_AS_SUPPORT_DWARF_DEFAULT
>         help
>           The implicit default version of DWARF debug info produced by a
>           toolchain changes over time.
> @@ -265,7 +276,7 @@ config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>  config DEBUG_INFO_DWARF4
>         bool "Generate DWARF Version 4 debuginfo"
>         select DEBUG_INFO
> -       depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)
> +       depends on CC_AND_AS_SUPPORT_DWARF4
>         help
>           Generate DWARF v4 debug info. This requires gcc 4.5+, binutils 2.35.2
>           if using clang without clang's integrated assembler, and gdb 7.0+.
> @@ -277,7 +288,7 @@ config DEBUG_INFO_DWARF4
>  config DEBUG_INFO_DWARF5
>         bool "Generate DWARF Version 5 debuginfo"
>         select DEBUG_INFO
> -       depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)
> +       depends on CC_AND_AS_SUPPORT_DWARF5
>         help
>           Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
>           5.0+ accepts the -gdwarf-5 flag but only had partial support for some



-- 
Best Regards
Masahiro Yamada