[PATCH v4 0/3] Kbuild: DWARF v5 support

[PATCH v4 0/3] Kbuild: DWARF v5 support

[Nick Desaulniers]

DWARF v5 is the latest standard of the DWARF debug info format.

DWARF5 wins significantly in terms of size when mixed with compression
(CONFIG_DEBUG_INFO_COMPRESSED).

Link: http://www.dwarfstd.org/doc/DWARF5.pdf

Patch 1 is a cleanup from Masahiro and isn't DWARF v5 specific.
Patch 2 is a cleanup that lays the ground work and isn't DWARF
v5 specific.
Patch 3 implements Kconfig and Kbuild support for DWARFv5.

Changes from v3:

Changes as per Arvind:
* only add -Wa,-gdwarf-5 for (LLVM=1|CC=clang)+LLVM_IAS=0 builds.
* add -gdwarf-5 to Kconfig shell script.
* only run Kconfig shell script for Clang.

Apologies to Sedat and Nathan; I appreciate previous testing/review, but
I did no carry forward your Tested-by and Reviewed-by tags, as the
patches have changed too much IMO.

Changes from v2:
* Drop two of the earlier patches that have been accepted already.
* Add measurements with GCC 10.2 to commit message.
* Update help text as per Arvind with help from Caroline.
* Improve case/wording between DWARF Versions as per Masahiro.

Changes from the RFC:
* split patch in 3 patch series, include Fangrui's patch, too.
* prefer `DWARF vX` format, as per Fangrui.
* use spaces between assignment in Makefile as per Masahiro.
* simplify setting dwarf-version-y as per Masahiro.
* indent `prompt` in Kconfig change as per Masahiro.
* remove explicit default in Kconfig as per Masahiro.
* add comments to test_dwarf5_support.sh.
* change echo in test_dwarf5_support.sh as per Masahiro.
* remove -u from test_dwarf5_support.sh as per Masahiro.
* add a -gdwarf-5 cc-option check to Kconfig as per Jakub.

Masahiro Yamada (1):
  Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

Nick Desaulniers (2):
  Kbuild: make DWARF version a choice
  Kbuild: implement support for DWARF v5

 Makefile                          | 15 +++++++----
 include/asm-generic/vmlinux.lds.h |  6 ++++-
 lib/Kconfig.debug                 | 41 +++++++++++++++++++++++++------
 scripts/test_dwarf5_support.sh    |  9 +++++++
 4 files changed, 58 insertions(+), 13 deletions(-)
 create mode 100755 scripts/test_dwarf5_support.sh

-- 
2.30.0.284.gd98b1dd5eaa7-goog

[PATCH v4 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

[Nick Desaulniers]

From: Masahiro Yamada <masahiroy@kernel.org>

The -gdwarf-4 flag is supported by GCC 4.5+, and also by Clang.

You can see it at https://godbolt.org/z/6ed1oW

  For gcc 4.5.3 pane,    line 37:    .value 0x4
  For clang 10.0.1 pane, line 117:   .short 4

Given Documentation/process/changes.rst stating GCC 4.9 is the minimal
version, this cc-option is unneeded.

Note
----

CONFIG_DEBUG_INFO_DWARF4 controls the DWARF version only for C files.

As you can see in the top Makefile, -gdwarf-4 is only passed to CFLAGS.

  ifdef CONFIG_DEBUG_INFO_DWARF4
  DEBUG_CFLAGS    += -gdwarf-4
  endif

This flag is used when compiling *.c files.

On the other hand, the assembler is always given -gdwarf-2.

  KBUILD_AFLAGS   += -Wa,-gdwarf-2

Hence, the debug info that comes from *.S files is always DWARF v2.
This is simply because GAS supported only -gdwarf-2 for a long time.

Recently, GAS gained the support for --dwarf-[3|4|5] options. [1]
And, also we have Clang integrated assembler. So, the debug info
for *.S files might be improved if we want.

In my understanding, the current code is intentional, not a bug.

[1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31bf18645d98b4d3d7357353be840e320649a67d

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
---
 lib/Kconfig.debug | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 78361f0abe3a..dd7d8d35b2a5 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -258,7 +258,6 @@ config DEBUG_INFO_SPLIT
 
 config DEBUG_INFO_DWARF4
 	bool "Generate dwarf4 debuginfo"
-	depends on $(cc-option,-gdwarf-4)
 	help
 	  Generate dwarf4 debug info. This requires recent versions
 	  of gcc and gdb. It makes the debug information larger.
-- 
2.30.0.284.gd98b1dd5eaa7-goog

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

[Nick Desaulniers]

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
-
-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

[PATCH v4 3/3] Kbuild: implement support for DWARF v5

[Nick Desaulniers]

DWARF v5 is the latest standard of the DWARF debug info format.

Feature detection of DWARF5 is onerous, especially given that we've
removed $(AS), so we must query $(CC) for DWARF5 assembler directive
support.  GNU `as` only recently gained support for specifying
-gdwarf-5.

The DWARF version of a binary can be validated with:
$ llvm-dwarfdump vmlinux | head -n 4 | grep version
or
$ readelf --debug-dump=info vmlinux 2>/dev/null | grep Version

DWARF5 wins significantly in terms of size when mixed with compression
(CONFIG_DEBUG_INFO_COMPRESSED).

363M    vmlinux.clang12.dwarf5.compressed
434M    vmlinux.clang12.dwarf4.compressed
439M    vmlinux.clang12.dwarf2.compressed
457M    vmlinux.clang12.dwarf5
536M    vmlinux.clang12.dwarf4
548M    vmlinux.clang12.dwarf2

515M    vmlinux.gcc10.2.dwarf5.compressed
599M    vmlinux.gcc10.2.dwarf4.compressed
624M    vmlinux.gcc10.2.dwarf2.compressed
630M    vmlinux.gcc10.2.dwarf5
765M    vmlinux.gcc10.2.dwarf4
809M    vmlinux.gcc10.2.dwarf2

Though the quality of debug info is harder to quantify; size is not a
proxy for quality.

Jakub notes:
  All [GCC] 5.1 - 6.x did was start accepting -gdwarf-5 as experimental
  option that enabled some small DWARF subset (initially only a few
  DW_LANG_* codes newly added to DWARF5 drafts).  Only GCC 7 (released
  after DWARF 5 has been finalized) started emitting DWARF5 section
  headers and got most of the DWARF5 changes in...

Version check GCC so that we don't need to worry about the difference in
command line args between GNU readelf and llvm-readelf/llvm-dwarfdump to
validate the DWARF Version in the assembler feature detection script.

Link: http://www.dwarfstd.org/doc/DWARF5.pdf
Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
Suggested-by: Jakub Jelinek <jakub@redhat.com>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Suggested-by: Fangrui Song <maskray@google.com>
Suggested-by: Caroline Tice <cmtice@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 Makefile                          |  1 +
 include/asm-generic/vmlinux.lds.h |  6 +++++-
 lib/Kconfig.debug                 | 17 +++++++++++++++++
 scripts/test_dwarf5_support.sh    |  9 +++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)
 create mode 100755 scripts/test_dwarf5_support.sh

diff --git a/Makefile b/Makefile
index 656fff17b331..1067cfd98249 100644
--- a/Makefile
+++ b/Makefile
@@ -828,6 +828,7 @@ endif
 
 dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
 dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
 DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
 ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
 # Binutils 2.35+ required for -gdwarf-4+ support.
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 49944f00d2b3..37dc4110875e 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -843,7 +843,11 @@
 		.debug_types	0 : { *(.debug_types) }			\
 		/* DWARF 5 */						\
 		.debug_macro	0 : { *(.debug_macro) }			\
-		.debug_addr	0 : { *(.debug_addr) }
+		.debug_addr	0 : { *(.debug_addr) }			\
+		.debug_line_str	0 : { *(.debug_line_str) }		\
+		.debug_loclists	0 : { *(.debug_loclists) }		\
+		.debug_rnglists	0 : { *(.debug_rnglists) }		\
+		.debug_str_offsets	0 : { *(.debug_str_offsets) }
 
 /* Stabs debugging sections. */
 #define STABS_DEBUG							\
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index e80770fac4f0..60a4f5e27ada 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -273,6 +273,23 @@ config DEBUG_INFO_DWARF4
 	  It makes the debug information larger, but it significantly
 	  improves the success of resolving variables in gdb on optimized code.
 
+config DEBUG_INFO_DWARF5
+	bool "Generate DWARF Version 5 debuginfo"
+	depends on GCC_VERSION >= 50000 || CC_IS_CLANG
+	depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
+	help
+	  Generate DWARF v5 debug info. Requires binutils 2.35, gcc 5.0+ (gcc
+	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
+	  draft features until 7.0), and gdb 8.0+.
+
+	  Changes to the structure of debug info in Version 5 allow for around
+	  15-18% savings in resulting image and debug info section sizes as
+	  compared to DWARF Version 4. DWARF Version 5 standardizes previous
+	  extensions such as accelerators for symbol indexing and the format
+	  for fission (.dwo/.dwp) files. Users may not want to select this
+	  config if they rely on tooling that has not yet been updated to
+	  support DWARF Version 5.
+
 endchoice # "DWARF version"
 
 config DEBUG_INFO_BTF
diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
new file mode 100755
index 000000000000..142a1b5c7fa2
--- /dev/null
+++ b/scripts/test_dwarf5_support.sh
@@ -0,0 +1,9 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
+# in binutils < 2.35.
+# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
+# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
+set -e
+echo '.file 0 "filename"' | $* -gdwarf-5 -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
-- 
2.30.0.284.gd98b1dd5eaa7-goog

Re: [PATCH v4 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

[Nathan Chancellor]

On Tue, Jan 12, 2021 at 04:32:33PM -0800, Nick Desaulniers wrote:
> From: Masahiro Yamada <masahiroy@kernel.org>
> 
> The -gdwarf-4 flag is supported by GCC 4.5+, and also by Clang.
> 
> You can see it at https://godbolt.org/z/6ed1oW
> 
>   For gcc 4.5.3 pane,    line 37:    .value 0x4
>   For clang 10.0.1 pane, line 117:   .short 4
> 
> Given Documentation/process/changes.rst stating GCC 4.9 is the minimal
> version, this cc-option is unneeded.
> 
> Note
> ----
> 
> CONFIG_DEBUG_INFO_DWARF4 controls the DWARF version only for C files.
> 
> As you can see in the top Makefile, -gdwarf-4 is only passed to CFLAGS.
> 
>   ifdef CONFIG_DEBUG_INFO_DWARF4
>   DEBUG_CFLAGS    += -gdwarf-4
>   endif
> 
> This flag is used when compiling *.c files.
> 
> On the other hand, the assembler is always given -gdwarf-2.
> 
>   KBUILD_AFLAGS   += -Wa,-gdwarf-2
> 
> Hence, the debug info that comes from *.S files is always DWARF v2.
> This is simply because GAS supported only -gdwarf-2 for a long time.
> 
> Recently, GAS gained the support for --dwarf-[3|4|5] options. [1]
> And, also we have Clang integrated assembler. So, the debug info
> for *.S files might be improved if we want.
> 
> In my understanding, the current code is intentional, not a bug.
> 
> [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31bf18645d98b4d3d7357353be840e320649a67d
> 
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

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

> ---
>  lib/Kconfig.debug | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 78361f0abe3a..dd7d8d35b2a5 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -258,7 +258,6 @@ config DEBUG_INFO_SPLIT
>  
>  config DEBUG_INFO_DWARF4
>  	bool "Generate dwarf4 debuginfo"
> -	depends on $(cc-option,-gdwarf-4)
>  	help
>  	  Generate dwarf4 debug info. This requires recent versions
>  	  of gcc and gdb. It makes the debug information larger.
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
> 

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

[Nathan Chancellor]

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
> 

Re: [PATCH v4 3/3] Kbuild: implement support for DWARF v5

[Nathan Chancellor]

On Tue, Jan 12, 2021 at 04:32:35PM -0800, Nick Desaulniers wrote:
> DWARF v5 is the latest standard of the DWARF debug info format.
> 
> Feature detection of DWARF5 is onerous, especially given that we've
> removed $(AS), so we must query $(CC) for DWARF5 assembler directive
> support.  GNU `as` only recently gained support for specifying
> -gdwarf-5.
> 
> The DWARF version of a binary can be validated with:
> $ llvm-dwarfdump vmlinux | head -n 4 | grep version
> or
> $ readelf --debug-dump=info vmlinux 2>/dev/null | grep Version
> 
> DWARF5 wins significantly in terms of size when mixed with compression
> (CONFIG_DEBUG_INFO_COMPRESSED).
> 
> 363M    vmlinux.clang12.dwarf5.compressed
> 434M    vmlinux.clang12.dwarf4.compressed
> 439M    vmlinux.clang12.dwarf2.compressed
> 457M    vmlinux.clang12.dwarf5
> 536M    vmlinux.clang12.dwarf4
> 548M    vmlinux.clang12.dwarf2
> 
> 515M    vmlinux.gcc10.2.dwarf5.compressed
> 599M    vmlinux.gcc10.2.dwarf4.compressed
> 624M    vmlinux.gcc10.2.dwarf2.compressed
> 630M    vmlinux.gcc10.2.dwarf5
> 765M    vmlinux.gcc10.2.dwarf4
> 809M    vmlinux.gcc10.2.dwarf2
> 
> Though the quality of debug info is harder to quantify; size is not a
> proxy for quality.
> 
> Jakub notes:
>   All [GCC] 5.1 - 6.x did was start accepting -gdwarf-5 as experimental
>   option that enabled some small DWARF subset (initially only a few
>   DW_LANG_* codes newly added to DWARF5 drafts).  Only GCC 7 (released
>   after DWARF 5 has been finalized) started emitting DWARF5 section
>   headers and got most of the DWARF5 changes in...
> 
> Version check GCC so that we don't need to worry about the difference in
> command line args between GNU readelf and llvm-readelf/llvm-dwarfdump to
> validate the DWARF Version in the assembler feature detection script.
> 
> Link: http://www.dwarfstd.org/doc/DWARF5.pdf
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-by: Jakub Jelinek <jakub@redhat.com>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Fangrui Song <maskray@google.com>
> Suggested-by: Caroline Tice <cmtice@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

One small nit below.

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

> ---
>  Makefile                          |  1 +
>  include/asm-generic/vmlinux.lds.h |  6 +++++-
>  lib/Kconfig.debug                 | 17 +++++++++++++++++
>  scripts/test_dwarf5_support.sh    |  9 +++++++++
>  4 files changed, 32 insertions(+), 1 deletion(-)
>  create mode 100755 scripts/test_dwarf5_support.sh
> 
> diff --git a/Makefile b/Makefile
> index 656fff17b331..1067cfd98249 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -828,6 +828,7 @@ endif
>  
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
>  DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
>  ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
>  # Binutils 2.35+ required for -gdwarf-4+ support.
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 49944f00d2b3..37dc4110875e 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -843,7 +843,11 @@
>  		.debug_types	0 : { *(.debug_types) }			\
>  		/* DWARF 5 */						\
>  		.debug_macro	0 : { *(.debug_macro) }			\
> -		.debug_addr	0 : { *(.debug_addr) }
> +		.debug_addr	0 : { *(.debug_addr) }			\
> +		.debug_line_str	0 : { *(.debug_line_str) }		\
> +		.debug_loclists	0 : { *(.debug_loclists) }		\
> +		.debug_rnglists	0 : { *(.debug_rnglists) }		\
> +		.debug_str_offsets	0 : { *(.debug_str_offsets) }
>  
>  /* Stabs debugging sections. */
>  #define STABS_DEBUG							\
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index e80770fac4f0..60a4f5e27ada 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -273,6 +273,23 @@ config DEBUG_INFO_DWARF4
>  	  It makes the debug information larger, but it significantly
>  	  improves the success of resolving variables in gdb on optimized code.
>  
> +config DEBUG_INFO_DWARF5
> +	bool "Generate DWARF Version 5 debuginfo"
> +	depends on GCC_VERSION >= 50000 || CC_IS_CLANG
> +	depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
> +	help
> +	  Generate DWARF v5 debug info. Requires binutils 2.35, gcc 5.0+ (gcc
> +	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
> +	  draft features until 7.0), and gdb 8.0+.
> +
> +	  Changes to the structure of debug info in Version 5 allow for around
> +	  15-18% savings in resulting image and debug info section sizes as
> +	  compared to DWARF Version 4. DWARF Version 5 standardizes previous
> +	  extensions such as accelerators for symbol indexing and the format
> +	  for fission (.dwo/.dwp) files. Users may not want to select this
> +	  config if they rely on tooling that has not yet been updated to
> +	  support DWARF Version 5.
> +
>  endchoice # "DWARF version"
>  
>  config DEBUG_INFO_BTF
> diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
> new file mode 100755
> index 000000000000..142a1b5c7fa2
> --- /dev/null
> +++ b/scripts/test_dwarf5_support.sh
> @@ -0,0 +1,9 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
> +# in binutils < 2.35.
> +# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
> +# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
> +set -e

This is unnecessary, clang will error without this and a script's exit
code is the exit code of its last command.

> +echo '.file 0 "filename"' | $* -gdwarf-5 -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
> 

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

[Sedat Dilek]

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
>

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

[Nick Desaulniers]

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`?)
-- 
Thanks,
~Nick Desaulniers

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

[Sedat Dilek]

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

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

[Sedat Dilek]

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 -

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

[Masahiro Yamada]

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