On Thu, Dec 03, 2020 at 05:11:27PM -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 <nived...@alum.mit.edu>
> Suggested-by: Jakub Jelinek <ja...@redhat.com>
> Suggested-by: Masahiro Yamada <masahi...@kernel.org>
> Suggested-by: Fangrui Song <mask...@google.com>
> Suggested-by: Caroline Tice <cmt...@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulni...@google.com>
Reviewed-by: Nathan Chancellor <natechancel...@gmail.com>
> ---
> Makefile | 1 +
> include/asm-generic/vmlinux.lds.h | 6 +++++-
> lib/Kconfig.debug | 14 ++++++++++++++
> scripts/test_dwarf5_support.sh | 9 +++++++++
> 4 files changed, 29 insertions(+), 1 deletion(-)
> create mode 100755 scripts/test_dwarf5_support.sh
>
> diff --git a/Makefile b/Makefile
> index 2430e1ee7c44..45231f6c1935 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 b2b3d81b1535..76ce62c77029 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -829,7 +829,11 @@
> .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 04719294a7a3..987815771ad6 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -274,6 +274,20 @@ 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 >= 70000 || CC_IS_CLANG
> + depends on $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC)
> $(CLANG_FLAGS))
> + help
> + Generate DWARF v5 debug info. Requires binutils 2.35, gcc 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..156ad5ec4274
> --- /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"' | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
> --
> 2.29.2.576.ga3fc446d84-goog
>
