Messages in this thread | | | Date | Fri, 29 Jan 2021 14:47:50 -0800 | From | Fangrui Song <> | Subject | Re: [PATCH v6 2/2] Kbuild: implement support for DWARF v5 |
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On 2021-01-29, 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 $(CC) for DWARF5 assembler directive >support. > >The DWARF version of a binary can be validated with: >$ 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 | 12 ++++++++++++ > include/asm-generic/vmlinux.lds.h | 6 +++++- > lib/Kconfig.debug | 18 ++++++++++++++++++ > scripts/test_dwarf5_support.sh | 8 ++++++++ > 4 files changed, 43 insertions(+), 1 deletion(-) > create mode 100755 scripts/test_dwarf5_support.sh > >diff --git a/Makefile b/Makefile >index 20141cd9319e..bed8b3b180b8 100644 >--- a/Makefile >+++ b/Makefile >@@ -832,8 +832,20 @@ 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) > >+# If using clang without the integrated assembler, we need to explicitly tell >+# GAS that we will be feeding it DWARF v5 assembler directives. Kconfig should >+# detect whether the version of GAS supports DWARF v5. >+ifdef CONFIG_CC_IS_CLANG >+ifneq ($(LLVM_IAS),1) >+ifeq ($(dwarf-version-y),5) >+DEBUG_CFLAGS += -Wa,-gdwarf-5 >+endif >+endif >+endif >+ > ifdef CONFIG_DEBUG_INFO_REDUCED > DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \ > $(call cc-option,-fno-var-tracking) >diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h >index 34b7e0d2346c..f8d5455cd87f 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 1850728b23e6..09146b1af20d 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)) >+ 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 - >-- >2.30.0.365.g02bc693789-goog >
Since the other thread suggested that you keep just DWARF v4 and v5 and drop DWARF v2, you'll need v7...
I suggest:
Change the -Wa,-gdwarf-2 logic to add either -Wa,-gdwarf-4 or -Wa,-gdwarf-5 and drop the CONFIG_CC_IS_CLANG ifdef. (CONFIG_CC_IS_CLANG is there because clang -Wa,-gdwarf-2 does not allow DW_AT_ranges which is not in DWARF v2).
GNU as before 2020 did not support -gdwarf-[345] (https://sourceware.org/bugzilla/show_bug.cgi?id=25612), so you can make -Wa,-gdwarf-2 a DWARF v4 workaround for older GNU as.
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