From d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27 Mon Sep 17 00:00:00 2001 From: "H.J. Lu" Date: Tue, 29 May 2012 14:31:23 -0700 Subject: [PATCH] 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 Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@mail.gmail.com Signed-off-by: H. Peter Anvin --- arch/x86/Makefile | 3 +++ 1 file changed, 3 insertions(+) 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) -- 2.7.4