Re: [PATCH] kbuild: Disable extra debugging info in .s output

Borislav Petkov
2019-02-18T14:31:49Z

On Mon, Feb 18, 2019 at 06:10:57PM +0900, Masahiro Yamada wrote:
> Could you send v2 as I suggested?
> Your commit log describes your motivation perfectly.
>
> (I lost the applicable patch because
> I just modified the code locally and pasted it
> into the previous email.)

Sure, here it is.

Thx.

---
From: Masahiro Yamada <yamada.masahiro@socionext.com>
Date: Sun, 25 Nov 2018 23:58:00 +0100
Subject: [PATCH] kbuild: Disable extra debugging info in .s output

Modern gcc adds view assignments, reset assertion checking in .loc
directives and a couple more additional debug markers, which clutters
the asm output unnecessarily:

For example:

bsp_resume:
.LFB3466:
.loc 1 1868 1 is_stmt 1 view -0
.cfi_startproc
.loc 1 1869 2 view .LVU73
# arch/x86/kernel/cpu/common.c:1869: if (this_cpu->c_bsp_resume)
.loc 1 1869 14 is_stmt 0 view .LVU74
movq this_cpu(%rip), %rax # this_cpu, this_cpu
movq 64(%rax), %rax # this_cpu.94_1->c_bsp_resume, _2
# arch/x86/kernel/cpu/common.c:1869: if (this_cpu->c_bsp_resume)
.loc 1 1869 5 view .LVU75
testq %rax, %rax # _2
je .L8 #,
.loc 1 1870 3 is_stmt 1 view .LVU76
movq $boot_cpu_data, %rdi #,
jmp __x86_indirect_thunk_rax

or
.loc 2 57 9 view .LVU478
.loc 2 57 9 view .LVU479
.loc 2 57 9 view .LVU480
.loc 2 57 9 view .LVU481
.LBB1385:
.LBB1383:
.LBB1379:
.LBB1377:
.LBB1375:
.loc 2 57 9 view .LVU482
.loc 2 57 9 view .LVU483
movl %edi, %edx # cpu, cpu
.LVL87:
.loc 2 57 9 is_stmt 0 view .LVU484

That MOV in there is drowned in debugging information and latter makes
it hard to follow the asm. And that DWARF info is not really needed for
asm output staring.

Disable the debug information generation which clutters the asm output
unnecessarily:

bsp_resume:
# arch/x86/kernel/cpu/common.c:1869: if (this_cpu->c_bsp_resume)
movq this_cpu(%rip), %rax # this_cpu, this_cpu
movq 64(%rax), %rax # this_cpu.94_1->c_bsp_resume, _2
# arch/x86/kernel/cpu/common.c:1869: if (this_cpu->c_bsp_resume)
testq %rax, %rax # _2
je .L8 #,
# arch/x86/kernel/cpu/common.c:1870: this_cpu->c_bsp_resume(&boot_cpu_data);
movq $boot_cpu_data, %rdi #,
jmp __x86_indirect_thunk_rax
.L8:
# arch/x86/kernel/cpu/common.c:1871: }
rep ret
.size bsp_resume, .-bsp_resume

[ bp: write commit message. ]

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: linux-kbuild@vger.kernel.org
---
Makefile | 13 ++++++++-----
scripts/Makefile.build | 2 +-
2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 96c5335e7ee4..c5e7d3ac8bd2 100644
--- a/Makefile
+++ b/Makefile
@@ -729,25 +729,28 @@ KBUILD_CFLAGS += -fomit-frame-pointer
endif
endif

-KBUILD_CFLAGS += $(call cc-option, -fno-var-tracking-assignments)
+DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)

ifdef CONFIG_DEBUG_INFO
ifdef CONFIG_DEBUG_INFO_SPLIT
-KBUILD_CFLAGS += $(call cc-option, -gsplit-dwarf, -g)
+DEBUG_CFLAGS += $(call cc-option, -gsplit-dwarf, -g)
else
-KBUILD_CFLAGS += -g
+DEBUG_CFLAGS += -g
endif
KBUILD_AFLAGS += -Wa,-gdwarf-2
endif
ifdef CONFIG_DEBUG_INFO_DWARF4
-KBUILD_CFLAGS += $(call cc-option, -gdwarf-4,)
+DEBUG_CFLAGS += $(call cc-option, -gdwarf-4,)
endif

ifdef CONFIG_DEBUG_INFO_REDUCED
-KBUILD_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
+DEBUG_CFLAGS += $(call cc-option, -femit-struct-debug-baseonly) \
$(call cc-option,-fno-var-tracking)
endif

+KBUILD_CFLAGS += $(DEBUG_CFLAGS)
+export DEBUG_CFLAGS
+
ifdef CONFIG_FUNCTION_TRACER
ifdef CONFIG_FTRACE_MCOUNT_RECORD
# gcc 5 supports generating the mcount tables directly
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index fd03d60f6c5a..8f5c86da77b1 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -104,7 +104,7 @@ modkern_cflags = \
quiet_modtag = $(if $(part-of-module),[M], )

quiet_cmd_cc_s_c = CC $(quiet_modtag) $@
-cmd_cc_s_c = $(CC) $(c_flags) $(DISABLE_LTO) -fverbose-asm -S -o $@ $<
+cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) $(DISABLE_LTO) -fverbose-asm -S -o $@ $<

$(obj)/%.s: $(src)/%.c FORCE
$(call if_changed_dep,cc_s_c)
--
2.19.1