diff mbox series

[v4,2/3] Kbuild: make DWARF version a choice

Message ID 20210113003235.716547-3-ndesaulniers@google.com (mailing list archive)
State New, archived
Headers show
Series Kbuild: DWARF v5 support | expand

Commit Message

Nick Desaulniers Jan. 13, 2021, 12:32 a.m. UTC 
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 | 21 ++++++++++++++++-----
 2 files changed, 25 insertions(+), 10 deletions(-)

Comments

Nathan Chancellor Jan. 13, 2021, 5:51 p.m. UTC | #1 
On Tue, Jan 12, 2021 at 04:32:34PM -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>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
>  Makefile          | 14 +++++++++-----
>  lib/Kconfig.debug | 21 ++++++++++++++++-----
>  2 files changed, 25 insertions(+), 10 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index d49c3f39ceb4..656fff17b331 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -826,12 +826,16 @@ else
>  DEBUG_CFLAGS	+= -g
>  endif
>  
> -ifneq ($(LLVM_IAS),1)
> -KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> +dwarf-version-$(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))
> +ifdef CONFIG_CC_IS_CLANG
> +DEBUG_CFLAGS	+= $(dwarf-aflag)
>  endif
> -
> -ifdef CONFIG_DEBUG_INFO_DWARF4
> -DEBUG_CFLAGS	+= -gdwarf-4
> +KBUILD_AFLAGS	+= $(dwarf-aflag)
>  endif
>  
>  ifdef CONFIG_DEBUG_INFO_REDUCED
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index dd7d8d35b2a5..e80770fac4f0 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -256,13 +256,24 @@ config DEBUG_INFO_SPLIT
>  	  to know about the .dwo files and include them.
>  	  Incompatible with older versions of ccache.
>  
> +choice
> +	prompt "DWARF version"
> +	help
> +	  Which version of DWARF debug info to emit.
> +
> +config DEBUG_INFO_DWARF2
> +	bool "Generate DWARF Version 2 debuginfo"
> +	help
> +	  Generate DWARF v2 debug info.
> +
>  config DEBUG_INFO_DWARF4
> -	bool "Generate dwarf4 debuginfo"
> +	bool "Generate DWARF Version 4 debuginfo"
>  	help
> -	  Generate dwarf4 debug info. This requires recent versions
> -	  of gcc and gdb. It makes the debug information larger.
> -	  But it significantly improves the success of resolving
> -	  variables in gdb on optimized code.
> +	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> +	  It makes the debug information larger, but it significantly
> +	  improves the success of resolving variables in gdb on optimized code.
> +
> +endchoice # "DWARF version"
>  
>  config DEBUG_INFO_BTF
>  	bool "Generate BTF typeinfo"
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>
Sedat Dilek Jan. 13, 2021, 9:24 p.m. UTC | #2 
On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
<ndesaulniers@google.com> 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 | 21 ++++++++++++++++-----
>  2 files changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index d49c3f39ceb4..656fff17b331 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -826,12 +826,16 @@ else
>  DEBUG_CFLAGS   += -g
>  endif
>
> -ifneq ($(LLVM_IAS),1)
> -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> +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))
> +ifdef CONFIG_CC_IS_CLANG
> +DEBUG_CFLAGS   += $(dwarf-aflag)
>  endif

Why is that "ifdef CONFIG_CC_IS_CLANG"?
When I use GCC v10.2.1 DEBUG_CFLAGS are not set.

- Sedat -

> -
> -ifdef CONFIG_DEBUG_INFO_DWARF4
> -DEBUG_CFLAGS   += -gdwarf-4
> +KBUILD_AFLAGS  += $(dwarf-aflag)
>  endif
>
>  ifdef CONFIG_DEBUG_INFO_REDUCED
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index dd7d8d35b2a5..e80770fac4f0 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -256,13 +256,24 @@ config DEBUG_INFO_SPLIT
>           to know about the .dwo files and include them.
>           Incompatible with older versions of ccache.
>
> +choice
> +       prompt "DWARF version"
> +       help
> +         Which version of DWARF debug info to emit.
> +
> +config DEBUG_INFO_DWARF2
> +       bool "Generate DWARF Version 2 debuginfo"
> +       help
> +         Generate DWARF v2 debug info.
> +
>  config DEBUG_INFO_DWARF4
> -       bool "Generate dwarf4 debuginfo"
> +       bool "Generate DWARF Version 4 debuginfo"
>         help
> -         Generate dwarf4 debug info. This requires recent versions
> -         of gcc and gdb. It makes the debug information larger.
> -         But it significantly improves the success of resolving
> -         variables in gdb on optimized code.
> +         Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> +         It makes the debug information larger, but it significantly
> +         improves the success of resolving variables in gdb on optimized code.
> +
> +endchoice # "DWARF version"
>
>  config DEBUG_INFO_BTF
>         bool "Generate BTF typeinfo"
> --
> 2.30.0.284.gd98b1dd5eaa7-goog
>
Nick Desaulniers Jan. 13, 2021, 11:27 p.m. UTC | #3 
Sedat,
Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
I always appreciate you taking the time to help test my work, and
other Clang+Linux kernel patches!

On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -826,12 +826,16 @@ else
> >  DEBUG_CFLAGS   += -g
> >  endif
> >
> > -ifneq ($(LLVM_IAS),1)
> > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)

^ DEBUG_CFLAGS are set for everyone (all toolchains) if
CONFIG_DEBUG_INFO is defined.

> > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)

^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
&& CONFIG_CC_IS_GCC

> > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > +ifdef CONFIG_CC_IS_CLANG

^ "if clang"

> > +DEBUG_CFLAGS   += $(dwarf-aflag)
> >  endif
>
> Why is that "ifdef CONFIG_CC_IS_CLANG"?

That's what Arvind requested on v2, IIUC:
https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/

> When I use GCC v10.2.1 DEBUG_CFLAGS are not set.

You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
please confirm? (Perhaps you may have accidentally disabled
CONFIG_DEBUG_INFO by rerunning `make defconfig`?)
Sedat Dilek Jan. 14, 2021, 7:20 a.m. UTC | #4 
On Thu, Jan 14, 2021 at 12:27 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Sedat,
> Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
> I always appreciate you taking the time to help test my work, and
> other Clang+Linux kernel patches!
>

Hi Nick,

cool, again in the top 15 :-).

I should ask Mr. Corbet for a LWN subscription.

> On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -826,12 +826,16 @@ else
> > >  DEBUG_CFLAGS   += -g
> > >  endif
> > >
> > > -ifneq ($(LLVM_IAS),1)
> > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
>
> ^ DEBUG_CFLAGS are set for everyone (all toolchains) if
> CONFIG_DEBUG_INFO is defined.
>
> > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
>
> ^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
> && CONFIG_CC_IS_GCC
>

OK, I know DWARF v2 and LLVM_IAS=1 is broken.

Looks like DWARF v5 with GCC v10.2.1 and binutils v2.35.1 is currently
(here) no good choice.

> > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > +ifdef CONFIG_CC_IS_CLANG
>
> ^ "if clang"
>
> > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> > >  endif
> >
> > Why is that "ifdef CONFIG_CC_IS_CLANG"?
>
> That's what Arvind requested on v2, IIUC:
> https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/
>
> > When I use GCC v10.2.1 DEBUG_CFLAGS are not set.
>
> You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
> when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
> please confirm? (Perhaps you may have accidentally disabled
> CONFIG_DEBUG_INFO by rerunning `make defconfig`?)
>

$ egrep 'CC_IS_|LD_IS|BTF|DWARF'
config-5.11.0-rc3-5-amd64-gcc10-llvm11 | grep ^CONFIG
CONFIG_CC_IS_GCC=y
CONFIG_LD_IS_LLD=y
CONFIG_DEBUG_INFO_DWARF4=y
CONFIG_DEBUG_INFO_BTF=y
CONFIG_DEBUG_INFO_BTF_MODULES=y

$ grep '\-Wa,-gdwarf-4' build-log_5.11.0-rc3-5-amd64-gcc10-llvm11.txt
| wc -l
156
Sedat Dilek Jan. 14, 2021, 7:30 a.m. UTC | #5 
On Thu, Jan 14, 2021 at 8:20 AM Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
> On Thu, Jan 14, 2021 at 12:27 AM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > Sedat,
> > Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
> > I always appreciate you taking the time to help test my work, and
> > other Clang+Linux kernel patches!
> >
>
> Hi Nick,
>
> cool, again in the top 15 :-).
>
> I should ask Mr. Corbet for a LWN subscription.
>
> > On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> > >
> > > On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
> > > <ndesaulniers@google.com> wrote:
> > > >
> > > > --- a/Makefile
> > > > +++ b/Makefile
> > > > @@ -826,12 +826,16 @@ else
> > > >  DEBUG_CFLAGS   += -g
> > > >  endif
> > > >
> > > > -ifneq ($(LLVM_IAS),1)
> > > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> >
> > ^ DEBUG_CFLAGS are set for everyone (all toolchains) if
> > CONFIG_DEBUG_INFO is defined.
> >
> > > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
> >
> > ^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
> > && CONFIG_CC_IS_GCC
> >
>
> OK, I know DWARF v2 and LLVM_IAS=1 is broken.
>
> Looks like DWARF v5 with GCC v10.2.1 and binutils v2.35.1 is currently
> (here) no good choice.
>
> > > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > > +ifdef CONFIG_CC_IS_CLANG
> >
> > ^ "if clang"
> >
> > > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> > > >  endif
> > >
> > > Why is that "ifdef CONFIG_CC_IS_CLANG"?
> >
> > That's what Arvind requested on v2, IIUC:
> > https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/
> >
> > > When I use GCC v10.2.1 DEBUG_CFLAGS are not set.
> >
> > You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
> > when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
> > please confirm? (Perhaps you may have accidentally disabled
> > CONFIG_DEBUG_INFO by rerunning `make defconfig`?)
> >
>
> $ egrep 'CC_IS_|LD_IS|BTF|DWARF'
> config-5.11.0-rc3-5-amd64-gcc10-llvm11 | grep ^CONFIG
> CONFIG_CC_IS_GCC=y
> CONFIG_LD_IS_LLD=y
> CONFIG_DEBUG_INFO_DWARF4=y
> CONFIG_DEBUG_INFO_BTF=y
> CONFIG_DEBUG_INFO_BTF_MODULES=y
>
> $ grep '\-Wa,-gdwarf-4' build-log_5.11.0-rc3-5-amd64-gcc10-llvm11.txt
> | wc -l
> 156

I wonder why I see GNU/as here (see above CONFIG_LD_IS_LLD=y)?

$ llvm-dwarfdump-11 vmlinux | head -20 | egrep
'debug_info|format|version|DW_AT_producer'
vmlinux:        file format elf64-x86-64
.debug_info contents:
0x00000000: Compile Unit: length = 0x0000001e, format = DWARF32,
version = 0x0004, abbr_offset = 0x0000, addr_size = 0x08 (next unit at
0x00000022)
             DW_AT_producer    ("GNU AS 2.35.1")
0x00000022: Compile Unit: length = 0x0000c1d2, format = DWARF32,
version = 0x0004, abbr_offset = 0x0012, addr_size = 0x08 (next unit at
0x0000c1f8)
             DW_AT_producer    ("GNU C89 10.2.1 20210110 -mno-sse
-mno-mmx -mno-sse2 -mno-3dnow -mno-avx -m64 -mno-80387
-mno-fp-ret-in-387 -mpreferred-stack-boundary
=3 -mskip-rax-setup -mtune=generic -mno-red-zone -mcmodel=kernel
-mindirect-branch=thunk-extern -mindirect-branch-register
-mrecord-mcount -mfentry -march=x86-64 -g -g
dwarf-4 -O2 -std=gnu90 -fno-strict-aliasing -fno-common -fshort-wchar
-fno-PIE -falign-jumps=1 -falign-loops=1
-fno-asynchronous-unwind-tables -fno-jump-tables -fno-de
lete-null-pointer-checks -fno-allow-store-data-races
-fstack-protector-strong -fno-strict-overflow -fstack-check=no
-fconserve-stack -fcf-protection=none -fno-stack-pr
otector")

Maybe, I should set all LLVM utils and linker manually, not using LLVM=1.

- Sedat -
Masahiro Yamada Jan. 15, 2021, 3:54 a.m. UTC | #6 
On Thu, Jan 14, 2021 at 8:27 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Sedat,
> Thanks for testing, and congrats on https://lwn.net/Articles/839772/.
> I always appreciate you taking the time to help test my work, and
> other Clang+Linux kernel patches!
>
> On Wed, Jan 13, 2021 at 1:24 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Wed, Jan 13, 2021 at 1:32 AM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -826,12 +826,16 @@ else
> > >  DEBUG_CFLAGS   += -g
> > >  endif
> > >
> > > -ifneq ($(LLVM_IAS),1)
> > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
>
> ^ DEBUG_CFLAGS are set for everyone (all toolchains) if
> CONFIG_DEBUG_INFO is defined.
>
> > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
>
> ^ "If not using dwarf 2 and LLVM_IAS=1", ie. CONFIG_DEBUG_INFO_DWARF5
> && CONFIG_CC_IS_GCC
>
> > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > +ifdef CONFIG_CC_IS_CLANG
>
> ^ "if clang"
>
> > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> > >  endif
> >
> > Why is that "ifdef CONFIG_CC_IS_CLANG"?
>
> That's what Arvind requested on v2, IIUC:
> https://lore.kernel.org/lkml/X8psgMuL4jMjP%2FOy@rani.riverdale.lan/



If CONFIG_CC_IS_CLANG is set,
both -gdwarf and -Wa,-gdwarf-4 are passed to DEBUG_CFLAGS.

Is it necessary?



IIUC, -Wa,-gdwarf is meaningless
when you build *.c files.


I passed -v option to see
how gas is invoked behind the scene.


See the following results
for [1] GCC + GAS and [2] Clang + GAS cases




[1] GCC + GAS


masahiro@grover:~$ cat test.c
int main(void) { return 0; }
masahiro@grover:~$ gcc -v -gdwarf-4 -c -o test.o test.c
Using built-in specs.
COLLECT_GCC=gcc
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10.2.0-13ubuntu1'
--with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2
--prefix=/usr --with-gcc-major-version-only --program-suffix=-10
--program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --libdir=/usr/lib
--enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (Ubuntu 10.2.0-13ubuntu1)
COLLECT_GCC_OPTIONS='-v' '-gdwarf-4' '-c' '-o' 'test.o'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/10/cc1 -quiet -v -imultiarch
x86_64-linux-gnu test.c -quiet -dumpbase test.c -mtune=generic
-march=x86-64 -auxbase-strip test.o -gdwarf-4 -version
-fasynchronous-unwind-tables -fstack-protector-strong -Wformat
-Wformat-security -fstack-clash-protection -fcf-protection -o
/tmp/cc4hKJeo.s
GNU C17 (Ubuntu 10.2.0-13ubuntu1) version 10.2.0 (x86_64-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.2.0-rc1, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/10/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/10/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/10/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C17 (Ubuntu 10.2.0-13ubuntu1) version 10.2.0 (x86_64-linux-gnu)
compiled by GNU C version 10.2.0, GMP version 6.2.0, MPFR version
4.1.0, MPC version 1.2.0-rc1, isl version isl-0.22.1-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 4831429547eb0be4fec215fca56ed5cf
COLLECT_GCC_OPTIONS='-v' '-gdwarf-4' '-c' '-o' 'test.o'
'-mtune=generic' '-march=x86-64'
 as -v --64 -o test.o /tmp/cc4hKJeo.s
GNU assembler version 2.35.1 (x86_64-linux-gnu) using BFD version (GNU
Binutils for Ubuntu) 2.35.1
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/10/:/usr/lib/gcc/x86_64-linux-gnu/10/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/10/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/10/:/usr/lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/10/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/10/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-gdwarf-4' '-c' '-o' 'test.o'
'-mtune=generic' '-march=x86-64'
masahiro@grover:~$ readelf  --debug-dump=info test.o
Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x4f (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  8
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <c>   DW_AT_producer    : (indirect string, offset: 0x16): GNU C17
10.2.0 -mtune=generic -march=x86-64 -gdwarf-4
-fasynchronous-unwind-tables -fstack-protector-strong
-fstack-clash-protection -fcf-protection
    <10>   DW_AT_language    : 12 (ANSI C99)
    <11>   DW_AT_name        : (indirect string, offset: 0xf): test.c
    <15>   DW_AT_comp_dir    : (indirect string, offset: 0x0): /home/masahiro
    <19>   DW_AT_low_pc      : 0x0
    <21>   DW_AT_high_pc     : 0xf
    <29>   DW_AT_stmt_list   : 0x0
 <1><2d>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2e>   DW_AT_external    : 1
    <2e>   DW_AT_name        : (indirect string, offset: 0xab): main
    <32>   DW_AT_decl_file   : 1
    <33>   DW_AT_decl_line   : 1
    <34>   DW_AT_decl_column : 5
    <35>   DW_AT_prototyped  : 1
    <35>   DW_AT_type        : <0x4b>
    <39>   DW_AT_low_pc      : 0x0
    <41>   DW_AT_high_pc     : 0xf
    <49>   DW_AT_frame_base  : 1 byte block: 9c (DW_OP_call_frame_cfa)
    <4b>   DW_AT_GNU_all_call_sites: 1
 <1><4b>: Abbrev Number: 3 (DW_TAG_base_type)
    <4c>   DW_AT_byte_size   : 4
    <4d>   DW_AT_encoding    : 5 (signed)
    <4e>   DW_AT_name        : int
 <1><52>: Abbrev Number: 0





[2] Clang + GAS

masahiro@grover:~$ clang -v -fno-integrated-as -gdwarf-4 -c -o test.o test.c
Ubuntu clang version 11.0.0-2
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/usr/lib/llvm-11/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -S
-disable-free -disable-llvm-verifier -discard-value-names
-main-file-name test.c -mrelocation-model static -mframe-pointer=all
-fmath-errno -fno-rounding-math -no-integrated-as
-mconstructor-aliases -munwind-tables -target-cpu x86-64
-fno-split-dwarf-inlining -debug-info-kind=limited -dwarf-version=4
-debugger-tuning=gdb -v -resource-dir
/usr/lib/llvm-11/lib/clang/11.0.0 -internal-isystem /usr/local/include
-internal-isystem /usr/lib/llvm-11/lib/clang/11.0.0/include
-internal-externc-isystem /usr/include/x86_64-linux-gnu
-internal-externc-isystem /include -internal-externc-isystem
/usr/include -fno-dwarf-directory-asm -fdebug-compilation-dir
/home/masahiro -ferror-limit 19 -fgnuc-version=4.2.1
-fcolor-diagnostics -o /tmp/test-f43580.s -x c test.c
clang -cc1 version 11.0.0 based upon LLVM 11.0.0 default target
x86_64-pc-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/llvm-11/lib/clang/11.0.0/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/bin/as" --64 -o test.o /tmp/test-f43580.s
masahiro@grover:~$ readelf  --debug-dump=info test.o
Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x47 (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  8
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <c>   DW_AT_producer    : (indirect string, offset: 0x0): Ubuntu
clang version 11.0.0-2
    <10>   DW_AT_language    : 12 (ANSI C99)
    <12>   DW_AT_name        : (indirect string, offset: 0x1e): test.c
    <16>   DW_AT_stmt_list   : 0x0
    <1a>   DW_AT_comp_dir    : (indirect string, offset: 0x25): /home/masahiro
    <1e>   DW_AT_low_pc      : 0x0
    <26>   DW_AT_high_pc     : 0xf
 <1><2a>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2b>   DW_AT_low_pc      : 0x0
    <33>   DW_AT_high_pc     : 0xf
    <37>   DW_AT_frame_base  : 1 byte block: 56 (DW_OP_reg6 (rbp))
    <39>   DW_AT_name        : (indirect string, offset: 0x34): main
    <3d>   DW_AT_decl_file   : 1
    <3e>   DW_AT_decl_line   : 1
    <3f>   DW_AT_prototyped  : 1
    <3f>   DW_AT_type        : <0x43>
    <43>   DW_AT_external    : 1
 <1><43>: Abbrev Number: 3 (DW_TAG_base_type)
    <44>   DW_AT_name        : (indirect string, offset: 0x39): int
    <48>   DW_AT_encoding    : 5 (signed)
    <49>   DW_AT_byte_size   : 4
 <1><4a>: Abbrev Number: 0






In [1],  "as -v --64 -o test.o /tmp/cc4hKJeo.s"
is the command that invoked gas.

There is no -gdwarf-4 option passed to gas here,
but the produced object has the correct dwarf4 info.




In [2],   "/usr/bin/as" --64 -o test.o /tmp/test-f43580.s
is the command that invoked gas.

Again, no -gdwarf-4 option here,
but the produced object has the correct dwarf4 info.




So, when you build *.c -> *.o,
passing -gdwarf-* is enough.

The debug info is generated in the compile stage (i.e. by cc1)
and included in the intermediate /tmp/*.s file.

All gas needs to do is to transform the debug sections
in the intermediate /tmp/*.s file
into the binary stream in the .o file.
GAS does it without being instructed by the
explicit -Wa,-gdwarf-* option.


In my understanding, passing -Wa,-gdwarf
makes sense only when you build *.S -> *.o


This is why I think
  DEBUG_CFLAGS += -gdwarf-4   (for source debug of .c files)
and
  KBUILD_AFLAGS += -Wa,gdwarf-4  (for source debug of .S files)

are basically orthogonal (and they can be even controlled by
separate CONFIG options).


As stated above,  DEBUG_CFLAGS += -Wa,gdward-4
does not make sense.


I am not a compiler expert, but
that is what I understood from some experiments.

Please correct me if I am wrong.






> > When I use GCC v10.2.1 DEBUG_CFLAGS are not set.
>
> You should have -gdwarf-4 (and not -Wa,-gwarf-4) set for DEBUG_CFLAGS
> when compiling with GCC and enabling CONFIG_DEBUG_INFO_DWARF4. Can you
> please confirm? (Perhaps you may have accidentally disabled
> CONFIG_DEBUG_INFO by rerunning `make defconfig`?)
> --
> Thanks,
> ~Nick Desaulniers



--
Best Regards


Masahiro Yamada
diff mbox series

Patch

diff --git a/Makefile b/Makefile
index d49c3f39ceb4..656fff17b331 100644
--- a/Makefile
+++ b/Makefile
@@ -826,12 +826,16 @@  else
 DEBUG_CFLAGS	+= -g
 endif
 
-ifneq ($(LLVM_IAS),1)
-KBUILD_AFLAGS	+= -Wa,-gdwarf-2
+dwarf-version-$(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))
+ifdef CONFIG_CC_IS_CLANG
+DEBUG_CFLAGS	+= $(dwarf-aflag)
 endif
-
-ifdef CONFIG_DEBUG_INFO_DWARF4
-DEBUG_CFLAGS	+= -gdwarf-4
+KBUILD_AFLAGS	+= $(dwarf-aflag)
 endif
 
 ifdef CONFIG_DEBUG_INFO_REDUCED
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index dd7d8d35b2a5..e80770fac4f0 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -256,13 +256,24 @@  config DEBUG_INFO_SPLIT
 	  to know about the .dwo files and include them.
 	  Incompatible with older versions of ccache.
 
+choice
+	prompt "DWARF version"
+	help
+	  Which version of DWARF debug info to emit.
+
+config DEBUG_INFO_DWARF2
+	bool "Generate DWARF Version 2 debuginfo"
+	help
+	  Generate DWARF v2 debug info.
+
 config DEBUG_INFO_DWARF4
-	bool "Generate dwarf4 debuginfo"
+	bool "Generate DWARF Version 4 debuginfo"
 	help
-	  Generate dwarf4 debug info. This requires recent versions
-	  of gcc and gdb. It makes the debug information larger.
-	  But it significantly improves the success of resolving
-	  variables in gdb on optimized code.
+	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
+	  It makes the debug information larger, but it significantly
+	  improves the success of resolving variables in gdb on optimized code.
+
+endchoice # "DWARF version"
 
 config DEBUG_INFO_BTF
 	bool "Generate BTF typeinfo"