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authorH.J. Lu <hjl.tools@gmail.com>2012-05-29 14:31:23 -0700
committerH. Peter Anvin <hpa@zytor.com>2012-06-23 19:25:22 -0700
commitd9b0cde91c60da0ed5f92cdc3ac878142e6b5f27 (patch)
tree4b90bee8a829aa5ec07118626247c12138c0c434 /arch/x86/Makefile
parent485802a6c524e62b5924849dd727ddbb1497cc71 (diff)
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@gmail.com> Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@mail.gmail.com Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Diffstat (limited to 'arch/x86/Makefile')
-rw-r--r--arch/x86/Makefile3
1 files changed, 3 insertions, 0 deletions
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 1f2521434554..b0c5276861ec 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)