linux-kernel - [PATCH] Make alignment cflags configurable.

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Date:	Wed, 12 Aug 2015 14:32:55 +0200
From:	dl9pf@....de
To:	dl9pf@....de
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	llvmlinux@...ts.linuxfoundation.org, dwmw2@...radead.org,
	behanw@...verseincode.com
Subject: [PATCH] Make alignment cflags configurable.

From: Jan-Simon Möller <dl9pf@....de>

This patch adds switches for
-falign-jumps=1
and
-falign-loops=1

Default is off by intention to allow seamless operation.

Signed-off-by: Jan-Simon Möller <dl9pf@....de>
CC: tglx@...utronix.de
CC: mingo@...hat.com
CC: hpa@...or.com
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
CC: llvmlinux@...ts.linuxfoundation.org
CC: dwmw2@...radead.org
CC: behanw@...verseincode.com
---
 arch/x86/Kconfig  | 22 ++++++++++++++++++++++
 arch/x86/Makefile |  4 ++++
 2 files changed, 26 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b3a1a5d..ebd4b03 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -644,6 +644,28 @@ config SCHED_OMIT_FRAME_POINTER
 
 	  If in doubt, say "Y".
 
+config ALIGN_JUMP_TARGETS_NONDEFAULT
+       bool "Align jump targets to 1 byte"
+       default n
+       ---help---
+         Align jump targets to 1 byte, not the default 16 bytes.
+         This results in a smaller packaging of the Kernel
+         (around 300k in a usual configuration) by removing
+         noops that would otherwise be inserted for
+         alignment reasons.
+         The default is off to make sure all compilers work.
+
+config ALIGN_LOOPS_NONDEFAULT
+       bool "Pack loops tightly"
+       default n
+       ---help---
+         Align loops to 1 byte, not the default 16 bytes.
+         This results in a smaller packaging of the Kernel
+         by removing noops that would otherwise be inserted for
+         alignment reasons.
+         The default is off to make sure all compilers work.
+
+
 menuconfig HYPERVISOR_GUEST
 	bool "Linux guest support"
 	---help---
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 118e6de..38c38f4 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,11 +77,15 @@ else
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64
 
+ifdef ALIGN_JUMP_TARGETS_NONDEFAULT
         # Align jump targets to 1 byte, not the default 16 bytes:
         KBUILD_CFLAGS += -falign-jumps=1
+endif
 
+ifdef ALIGN_LOOPS_NONDEFAULT
         # Pack loops tightly as well:
         KBUILD_CFLAGS += -falign-loops=1
+endif
 
         # Don't autogenerate traditional x87 instructions
         KBUILD_CFLAGS += $(call cc-option,-mno-80387)
-- 
2.5.0

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