[tip:x86/build] x86-64, gcc: Use -mpreferred-stack-boundary= 3 if supported

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Commit-ID:  d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27
Gitweb:     http://git.kernel.org/tip/d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27
Author:     H.J. Lu <hjl.tools@xxxxxxxxx>
AuthorDate: Tue, 29 May 2012 14:31:23 -0700
Committer:  H. Peter Anvin <hpa@xxxxxxxxx>
CommitDate: Sat, 23 Jun 2012 19:25:22 -0700

x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported

On x86-64, the standard ABI requires alignment to 16 bytes.  However,
this is not actually necessary in the kernel (we don't do SSE except
in very controlled ways); and furthermore, the standard kernel entry
on x86-64 actually leaves the stack on an odd 8-byte boundary, which
means that gcc will generate extra instructions to keep the stack
*mis*aligned!

gcc 4.8 adds an -mpreferred-stack-boundary=3 option to override this
and lets us save some stack space and a handful of instructions.

Note that this causes us to pass -mno-sse twice; this is redundant,
but necessary since the cc-option test will fail unless -mno-sse is
passed on the same command line.

[ hpa: rewrote the patch description ]

Signed-off-by: H.J. Lu <hjl.tools@xxxxxxxxx>
Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@xxxxxxxxxxxxxx
Signed-off-by: H. Peter Anvin <hpa@xxxxxxxxx>
---
 arch/x86/Makefile |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1f25214..b0c5276 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -49,6 +49,9 @@ else
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64
 
+	# Use -mpreferred-stack-boundary=3 if supported.
+	KBUILD_CFLAGS += $(call cc-option,-mno-sse -mpreferred-stack-boundary=3)
+
         # FIXME - should be integrated in Makefile.cpu (Makefile_32.cpu)
         cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
         cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)
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