linux-kernel - [PATCH] kbuild: Disable extra debugging info in .s output

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Date:   Mon, 21 Jan 2019 10:53:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     linux-kbuild@...r.kernel.org
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: [PATCH] kbuild: Disable extra debugging info in .s output

From: Borislav Petkov <bp@...e.de>

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:
  .LFB3466:
          .loc 1 1868 1
  # arch/x86/kernel/cpu/common.c:1869:    if (this_cpu->c_bsp_resume)
          .loc 1 1869 14
          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
          testq   %rax, %rax      # _2
          je      .L8     #,
  # arch/x86/kernel/cpu/common.c:1870:            this_cpu->c_bsp_resume(&boot_cpu_data);
          .loc 1 1870 3
          movq    $boot_cpu_data, %rdi    #,
          jmp     __x86_indirect_thunk_rax

Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc: Michal Marek <michal.lkml@...kovi.net>
Cc: linux-kbuild@...r.kernel.org
---
 scripts/Makefile.build | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index fd03d60f6c5a..4f33fdab89d2 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -103,8 +103,12 @@ modkern_cflags =                                          \
 		$(KBUILD_CFLAGS_KERNEL) $(CFLAGS_KERNEL))
 quiet_modtag = $(if $(part-of-module),[M],   )
 
+disable_extra_cc_dbg := $(call cc-option,-gno-as-locview-support)
+disable_extra_cc_dbg += $(call cc-option,-fno-dwarf2-cfi-asm)
+disable_extra_cc_dbg += $(call cc-option,-feliminate-unused-debug-symbols)
+disable_extra_cc_dbg += $(call cc-option,-gno-statement-frontiers)
 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) $(c_flags) $(DISABLE_LTO) $(disable_extra_cc_dbg) -fverbose-asm -S -o $@ $<
 
 $(obj)/%.s: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_s_c)
-- 
2.19.1