Hi Masahiro, I plan to incorporate Fangrui's recommendation into a v2. Do you have additional thoughts on changes I should make in v2? Have you had the chance to test the patch? Should I split it into a series? What do you think about the Kconfig `choice` changes? On Wed, Oct 21, 2020 at 6:21 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > DWARF5 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. Further -gdwarf-X where X is an unsupported value doesn't > produce an error in $(CC). 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 5 | 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 > > Make CONFIG_DEBUG_INFO_DWARF4 part of a Kconfig choice to preserve > forward compatibility. > > Link: http://www.dwarfstd.org/doc/DWARF5.pdf > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > RFC because this patch is super half baked, but I'm looking for > feedback. > > I would logically split this into a series of patches; > 1. disable -Wa,gdwarf-2 for LLVM_IAS=1, see also > https://github.com/ClangBuiltLinux/linux/issues/716 > https://github.com/ClangBuiltLinux/continuous-integration/blob/master/patches/llvm-all/linux-next/arm64/silence-dwarf2-warnings.patch > that way we can backport for improved LLVM_IAS support. > 2. move CONFIG_DEBUG_INFO_DWARF4 to choice. > 3. implement the rest on top. > > I'm pretty sure GNU `as` only recently gained the ability to specify > -gdwarf-4 without erroring in binutils 2.35, so that part likely needs > to be fixed. > > Makefile | 19 ++++++++++++++++--- > include/asm-generic/vmlinux.lds.h | 6 +++++- > lib/Kconfig.debug | 29 +++++++++++++++++++++++++---- > scripts/test_dwarf5_support.sh | 4 ++++ > 4 files changed, 50 insertions(+), 8 deletions(-) > create mode 100755 scripts/test_dwarf5_support.sh > > diff --git a/Makefile b/Makefile > index e71979882e4f..0862df5b1a24 100644 > --- a/Makefile > +++ b/Makefile > @@ -828,10 +828,23 @@ else > DEBUG_CFLAGS += -g > endif > > -KBUILD_AFLAGS += -Wa,-gdwarf-2 > - > +DWARF_VERSION=2 > ifdef CONFIG_DEBUG_INFO_DWARF4 > -DEBUG_CFLAGS += -gdwarf-4 > +DWARF_VERSION=4 > +endif > +ifdef CONFIG_DEBUG_INFO_DWARF5 > +DWARF_VERSION=5 > +endif > +DEBUG_CFLAGS += -gdwarf-$(DWARF_VERSION) > + > +ifneq ($(DWARF_VERSION)$(LLVM_IAS),21) > +KBUILD_AFLAGS += -Wa,-gdwarf-$(DWARF_VERSION) > +endif > + > +ifdef CONFIG_CC_IS_CLANG > +ifneq ($(LLVM_IAS),1) > +KBUILD_CFLAGS += -Wa,-gdwarf-$(DWARF_VERSION) > +endif > endif > > ifdef CONFIG_DEBUG_INFO_REDUCED > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index cd14444bf600..0382808ef9fe 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -828,7 +828,11 @@ > .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 537cf3c2937d..6b01f0e2dad8 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -256,14 +256,35 @@ config DEBUG_INFO_SPLIT > to know about the .dwo files and include them. > Incompatible with older versions of ccache. > > +choice > +prompt "DWARF version" > + depends on DEBUG_INFO > + default DEBUG_INFO_DWARF2 > + help > + Which version of DWARF debug info to emit. > + > +config DEBUG_INFO_DWARF2 > + bool "Generate dwarf2 debuginfo" > + help > + Generate dwarf2 debug info. > + > 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. > - But it significantly improves the success of resolving > - variables in gdb on optimized code. > + Generate dwarf4 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. > + > +config DEBUG_INFO_DWARF5 > + bool "Generate dwarf5 debuginfo" > + depends on DEBUG_INFO > + depends on $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS)) > + help > + Genereate dwarf5 debug info. Requires binutils 2.35+, gcc 5.1+, and > + gdb 8.0+. > + > +endchoice # "DWARF version" > > config DEBUG_INFO_BTF > bool "Generate BTF typeinfo" > diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh > new file mode 100755 > index 000000000000..82c0eea45845 > --- /dev/null > +++ b/scripts/test_dwarf5_support.sh > @@ -0,0 +1,4 @@ > +#!/bin/sh > +# SPDX-License-Identifier: GPL-2.0 > +set -eu > +echo ".file 0 \"asdf\"" | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null - > -- > 2.29.0.rc1.297.gfa9743e501-goog > -- Thanks, ~Nick Desaulniers
