Re: [tip:x86/urgent] x86/build: Mostly disable '-maccumulate-outgoing-args'
From: Josh Poimboeuf
Date: Tue Mar 28 2017 - 12:18:37 EST
Dear tip-bot,
If it's not too late, here's a version with Ingo's suggested changes
(improving the makefile comment readability and improving the ftrace
build error message).
----
From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Subject: [PATCH v3] x86: 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: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
v3:
- improve makefile comment readability
- improve precision of ftrace build error message
arch/x86/Makefile | 35 +++++++++++++++++++++++++++++++----
arch/x86/Makefile_32.cpu | 18 ------------------
arch/x86/kernel/ftrace.c | 6 ++++++
scripts/Kbuild.include | 4 ++++
4 files changed, 41 insertions(+), 22 deletions(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2d44933..a94a4d1 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -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
diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
index 6647ed4..a45eb15 100644
--- a/arch/x86/Makefile_32.cpu
+++ b/arch/x86/Makefile_32.cpu
@@ -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)
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 8f3d9cf..cbd73eb 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -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)
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index d6ca649..afe3fd3 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -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,\
--
2.7.4