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authorNick Desaulniers <ndesaulniers@google.com>2021-02-05 12:22:20 -0800
committerMasahiro Yamada <masahiroy@kernel.org>2021-02-16 12:01:45 +0900
commit98cd6f521f1016171e9e263effc7d6edfbf61da1 (patch)
tree582b97c4f47f43065652f59645619225975faccb /Makefile
parenta66049e2cf0ef166dba5bafdbb3062287fc965ad (diff)
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Kconfig: allow explicit opt in to DWARF v5
DWARF v5 is the latest standard of the DWARF debug info format. GCC 11 will change the implicit default DWARF version, if left unspecified, to DWARF v5. Allow users of Clang and older versions of GCC that have not changed the implicit default DWARF version to DWARF v5 to opt in. This can help testing consumers of DWARF debug info in preparation of v5 becoming more widespread, as well as result in significant binary size savings of the pre-stripped vmlinux image. 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: One thing is GCC DWARF-5 support, that is whether the compiler will support -gdwarf-5 flag, and that support should be there from GCC 7 onwards. 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... Another separate thing is whether the assembler does support the -gdwarf-5 option (i.e. if you can compile assembler files with -Wa,-gdwarf-5) ... That option is about whether the assembler will emit DWARF5 or DWARF2 .debug_line. It is fine to compile C sources with -gdwarf-5 and use DWARF2 .debug_line for assembler files if as doesn't support it. 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. Most issues with clang produced assembler were fixed in binutils 2.35.1, but 2.35.2 fixed issues related to requiring the flag -Wa,-gdwarf-5 explicitly. The added shell script test checks for the latter, and is only required when using clang without its integrated assembler, though we use for clang regardless as we do not yet have a way to query the assembler from Kconfig. Disabled for now if CONFIG_DEBUG_INFO_BTF is set; pahole doesn't yet recognize the new additions to the DWARF debug info. This only modifies the DWARF version emitted by the compiler, not the assembler. The DWARF version of a binary can be validated with: $ llvm-dwarfdump <object file> | head -n 4 | grep version or $ readelf --debug-dump=info <object file> 2>/dev/null | grep Version Parts of the tree don't reuse DEBUG_CFLAGS as they should; such cleanup is left as a follow up. Link: http://www.dwarfstd.org/doc/DWARF5.pdf Link: https://bugzilla.redhat.com/show_bug.cgi?id=1922707 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> Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v12.0.0-rc1 x86-64 Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Diffstat (limited to 'Makefile')
-rw-r--r--Makefile1
1 files changed, 1 insertions, 0 deletions
diff --git a/Makefile b/Makefile
index c567c4343880..681bdb5d2f41 100644
--- a/Makefile
+++ b/Makefile
@@ -832,6 +832,7 @@ endif
ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
+dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
DEBUG_CFLAGS += -gdwarf-$(dwarf-version-y)
endif