On Fri, Jan 15, 2021 at 1:46 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote: > > On Fri, Jan 15, 2021 at 10:06 PM Nick Desaulniers > <ndesaulniers@xxxxxxxxxx> 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@xxxxxxxxx> > > Suggested-by: Arvind Sankar <nivedita@xxxxxxxxxxxx> > > Suggested-by: Caroline Tice <cmtice@xxxxxxxxxx> > > Suggested-by: Fangrui Song <maskray@xxxxxxxxxx> > > Suggested-by: Jakub Jelinek <jakub@xxxxxxxxxx> > > Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx> > > Suggested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > --- > > Makefile | 6 ++++++ > > include/asm-generic/vmlinux.lds.h | 6 +++++- > > lib/Kconfig.debug | 18 ++++++++++++++++++ > > scripts/test_dwarf5_support.sh | 8 ++++++++ > > 4 files changed, 37 insertions(+), 1 deletion(-) > > create mode 100755 scripts/test_dwarf5_support.sh > > > > diff --git a/Makefile b/Makefile > > index 4eb3bf7ee974..1dcea03861ef 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -828,10 +828,16 @@ 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) > > # Binutils 2.35+ required for -gdwarf-4+ support. > > dwarf-aflag := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y)) > > KBUILD_AFLAGS += $(dwarf-aflag) > > +ifdef CONFIG_CC_IS_CLANG > > +ifneq ($(LLVM_IAS),1) > > +DEBUG_CFLAGS += $(dwarf-aflag) > > +endif > > +endif > > > > ifdef CONFIG_DEBUG_INFO_REDUCED > > DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ > > 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..658f32ec0c05 100644 > > --- a/lib/Kconfig.debug > > +++ b/lib/Kconfig.debug > > @@ -273,6 +273,24 @@ 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)) > > Better keep GCC depends in one line: > > + depends on CC_IS_GCC && GCC_VERSION >= 50000 || CC_IS_CLANG > + depends on $(success,$(srctree)/scripts/test_dwarf5_support.sh > $(CC) $(CLANG_FLAGS)) It's intentional, if a bit obtuse: We don't want to check the assembler support for -Wa,-gdwarf-5 via compiler driver when CC=gcc; instead, we'll rely on how GCC was configured as per Arvind. > > As said in the other patch: > > Use consistently: s/DWARF Version/DWARF version/g Ah right, and I forget your point about kbuild/Kbuild. Will wait for more feedback then send a v6 next week. Thanks as always for the feedback. > > - Sedat - > > > + depends on !DEBUG_INFO_BTF > > + 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..1a00484d0b2e > > --- /dev/null > > +++ b/scripts/test_dwarf5_support.sh > > @@ -0,0 +1,8 @@ > > +#!/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 > > +echo '.file 0 "filename"' | $* -gdwarf-5 -Wa,-gdwarf-5 -c -x assembler -o /dev/null - > > -- > > 2.30.0.284.gd98b1dd5eaa7-goog > > -- Thanks, ~Nick Desaulniers
