From: H.J. Lu Date: Tue, 29 May 2012 21:31:23 +0000 (-0700) Subject: x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported X-Git-Tag: accepted/tizen/common/20141203.182822~4178^2 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27;p=platform%2Fkernel%2Flinux-arm64.git 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 --- 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)