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SubjectRe: [PATCH v7 2/2] Kbuild: implement support for DWARF v5
On Sat, Jan 30, 2021 at 9:44 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> 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.
>
> 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.
>
> GNU `as` only recently gained support for specifying -gdwarf-5, so when
> compiling with Clang but without Clang's integrated assembler
> (LLVM_IAS=1 is not set), explicitly add -Wa,-gdwarf-5 to DEBUG_CFLAGS.
>
> Disabled for now if CONFIG_DEBUG_INFO_BTF is set; pahole doesn't yet
> recognize the new additions to the DWARF debug info. Thanks to Sedat for
> the report.
>
> Link: http://www.dwarfstd.org/doc/DWARF5.pdf
> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-by: Caroline Tice <cmtice@google.com>
> Suggested-by: Fangrui Song <maskray@google.com>
> Suggested-by: Jakub Jelinek <jakub@redhat.com>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Makefile | 1 +
> include/asm-generic/vmlinux.lds.h | 7 ++++++-
> lib/Kconfig.debug | 18 ++++++++++++++++++
> scripts/test_dwarf5_support.sh | 8 ++++++++
> 4 files changed, 33 insertions(+), 1 deletion(-)
> create mode 100755 scripts/test_dwarf5_support.sh




Nick, the patch set is getting simpler and simpler,
and almost good enough to be merged.


Please let me ask two questions below.

There has been a lot of discussion, and
I might have missed the context.





> diff --git a/Makefile b/Makefile
> index d2b4980807e0..5387a6f2f62d 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,6 +831,7 @@ KBUILD_AFLAGS += -Wa,-gdwarf-2
> endif
>
> dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
> DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
>
> ifdef CONFIG_DEBUG_INFO_REDUCED
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 34b7e0d2346c..1e7cde4bd3f9 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -842,8 +842,13 @@
> /* DWARF 4 */ \
> .debug_types 0 : { *(.debug_types) } \
> /* DWARF 5 */ \
> + .debug_addr 0 : { *(.debug_addr) } \
> + .debug_line_str 0 : { *(.debug_line_str) } \
> + .debug_loclists 0 : { *(.debug_loclists) } \
> .debug_macro 0 : { *(.debug_macro) } \
> - .debug_addr 0 : { *(.debug_addr) }
> + .debug_names 0 : { *(.debug_names) } \
> + .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 94c1a7ed6306..ad6f78989d4f 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -268,6 +268,24 @@ 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))

Q1.

This "CC_IS_GCC ||" was introduced by v4.

GCC never outputs '.file 0', which is why
this test is only needed for Clang, correct?






> + depends on !DEBUG_INFO_BTF
> + 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
> + 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..c46e2456b47a
> --- /dev/null
> +++ b/scripts/test_dwarf5_support.sh
> @@ -0,0 +1,8 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# Test that the assembler doesn't need -Wa,-gdwarf-5 when presented with DWARF
> +# v5 input, such as `.file 0` and `md5 0x00`. Should be fixed in GNU binutils
> +# 2.35.2. https://sourceware.org/bugzilla/show_bug.cgi?id=25611

I saw the following links in v6.

https://sourceware.org/bugzilla/show_bug.cgi?id=25612
https://sourceware.org/bugzilla/show_bug.cgi?id=25614

They were dropped in v7. Why?

I just thought they were good to know...



> +echo '.file 0 "filename" md5 0x7a0b65214090b6693bd1dc24dd248245' | \
> + $* -gdwarf-5 -Wno-unused-command-line-argument -c -x assembler -o /dev/null -
> --
> 2.30.0.365.g02bc693789-goog
>
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--
Best Regards
Masahiro Yamada

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