Commit 3f135e57 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Ingo Molnar
Browse files

x86/build: Mostly disable '-maccumulate-outgoing-args' 



The GCC '-maccumulate-outgoing-args' flag is enabled for most configs,
mostly because of issues which are no longer relevant.  For most
configs, and with most recent versions of GCC, it's no longer needed.

Clarify which cases need it, and only enable it for those cases.  Also
produce a compile-time error for the ftrace graph + mcount + '-Os' case,
which will otherwise cause runtime failures.

The main benefit of '-maccumulate-outgoing-args' is that it prevents an
ugly prologue for functions which have aligned stacks.  But removing the
option also has some benefits: more readable argument saves, smaller
text size, and (presumably) slightly improved performance.

Here are the object size savings for 32-bit and 64-bit defconfig
kernels:

      text	   data	    bss	     dec	    hex	filename
  10006710	3543328	1773568	15323606	 e9d1d6	vmlinux.x86-32.before
   9706358	3547424	1773568	15027350	 e54c96	vmlinux.x86-32.after

      text	   data	    bss	     dec	    hex	filename
  10652105	4537576	 843776	16033457	 f4a6b1	vmlinux.x86-64.before
  10639629	4537576	 843776	16020981	 f475f5	vmlinux.x86-64.after

That comes out to a 3% text size improvement on x86-32 and a 0.1% text
size improvement on x86-64.

Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrew Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170316193133.zrj6gug53766m6nn@treble


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent a46f60d7

arch/x86/Makefile

+31 −4
Original line number Diff line number Diff line
@@ -120,10 +120,6 @@ else 
        # -funit-at-a-time shrinks the kernel .text considerably

        # unfortunately it makes reading oopses harder.

        KBUILD_CFLAGS += $(call cc-option,-funit-at-a-time)



        # this works around some issues with generating unwind tables in older gccs

        # newer gccs do it by default

        KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args)

endif



ifdef CONFIG_X86_X32
@@ -147,6 +143,37 @@ ifeq ($(CONFIG_KMEMCHECK),y) 
	KBUILD_CFLAGS += $(call cc-option,-fno-builtin-memcpy)

endif



#

# If the function graph tracer is used with mcount instead of fentry,

# '-maccumulate-outgoing-args' is needed to prevent a GCC bug

# (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42109)

#

ifdef CONFIG_FUNCTION_GRAPH_TRACER

  ifndef CONFIG_HAVE_FENTRY

	ACCUMULATE_OUTGOING_ARGS := 1

  else

    ifeq ($(call cc-option-yn, -mfentry), n)

	ACCUMULATE_OUTGOING_ARGS := 1

    endif

  endif

endif



#

# Jump labels need '-maccumulate-outgoing-args' for gcc < 4.5.2 to prevent a

# GCC bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46226).  There's no way

# to test for this bug at compile-time because the test case needs to execute,

# which is a no-go for cross compilers.  So check the GCC version instead.

#

ifdef CONFIG_JUMP_LABEL

  ifneq ($(ACCUMULATE_OUTGOING_ARGS), 1)

	ACCUMULATE_OUTGOING_ARGS = $(call cc-if-fullversion, -lt, 040502, 1)

  endif

endif



ifeq ($(ACCUMULATE_OUTGOING_ARGS), 1)

	KBUILD_CFLAGS += -maccumulate-outgoing-args

endif



# Stackpointer is addressed different for 32 bit and 64 bit x86

sp-$(CONFIG_X86_32) := esp

sp-$(CONFIG_X86_64) := rsp

arch/x86/Makefile_32.cpu

+0 −18
Original line number Diff line number Diff line
@@ -45,24 +45,6 @@ cflags-$(CONFIG_MGEODE_LX) += $(call cc-option,-march=geode,-march=pentium-mmx) 
# cpu entries

cflags-$(CONFIG_X86_GENERIC) 	+= $(call tune,generic,$(call tune,i686))



# Work around the pentium-mmx code generator madness of gcc4.4.x which

# does stack alignment by generating horrible code _before_ the mcount

# prologue (push %ebp, mov %esp, %ebp) which breaks the function graph

# tracer assumptions. For i686, generic, core2 this is set by the

# compiler anyway

ifeq ($(CONFIG_FUNCTION_GRAPH_TRACER), y)

ADD_ACCUMULATE_OUTGOING_ARGS := y

endif



# Work around to a bug with asm goto with first implementations of it

# in gcc causing gcc to mess up the push and pop of the stack in some

# uses of asm goto.

ifeq ($(CONFIG_JUMP_LABEL), y)

ADD_ACCUMULATE_OUTGOING_ARGS := y

endif



cflags-$(ADD_ACCUMULATE_OUTGOING_ARGS) += $(call cc-option,-maccumulate-outgoing-args)



# Bug fix for binutils: this option is required in order to keep

# binutils from generating NOPL instructions against our will.

ifneq ($(CONFIG_X86_P6_NOP),y)

arch/x86/kernel/ftrace.c

+6 −0
Original line number Diff line number Diff line
@@ -29,6 +29,12 @@ 
#include <asm/ftrace.h>

#include <asm/nops.h>



#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && \

	!defined(CC_USING_FENTRY) && \

	!defined(CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE)

# error The following combination is not supported: ((compiler missing -mfentry) || (CONFIG_X86_32 and !CONFIG_DYNAMIC_FTRACE)) && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE

#endif



#ifdef CONFIG_DYNAMIC_FTRACE



int ftrace_arch_code_modify_prepare(void)

scripts/Kbuild.include

+4 −0
Original line number Diff line number Diff line
@@ -148,6 +148,10 @@ cc-fullversion = $(shell $(CONFIG_SHELL) \ 
# Usage:  EXTRA_CFLAGS += $(call cc-ifversion, -lt, 0402, -O1)

cc-ifversion = $(shell [ $(cc-version) $(1) $(2) ] && echo $(3) || echo $(4))



# cc-if-fullversion

# Usage:  EXTRA_CFLAGS += $(call cc-if-fullversion, -lt, 040502, -O1)

cc-if-fullversion = $(shell [ $(cc-fullversion) $(1) $(2) ] && echo $(3) || echo $(4))



# cc-ldoption

# Usage: ldflags += $(call cc-ldoption, -Wl$(comma)--hash-style=both)

cc-ldoption = $(call try-run,\

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