diff mbox series

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

Message ID 20210113003235.716547-4-ndesaulniers@google.com (mailing list archive)
State New
Headers show
Series Kbuild: DWARF v5 support | expand

Commit Message

Nick Desaulniers Jan. 13, 2021, 12:32 a.m. UTC
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

Comments

Nathan Chancellor Jan. 13, 2021, 5:55 p.m. UTC | #1
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
>
diff mbox series

Patch

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 -