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x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported
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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@gmail.com>
Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
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hjl-tools authored and H. Peter Anvin committed Jun 24, 2012
1 parent 485802a commit d9b0cde
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3 changes: 3 additions & 0 deletions arch/x86/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -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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