[PATCH] x86_64: use -mskip-rax-setup when available

[Rasmus Villemoes]

gcc 5 supports the option -mskip-rax-setup to avoid emitting the
two-byte instruction xor %eax,%eax before a vararg function
call. Clearing %eax is redundant in the kernel, since no callee
expects any floating point arguments, and hence never use the incoming
value of %eax for anything.

For a defconfig kernel, the .text saving is around 26 kB:

$ size /tmp/vmlinux.{old,new}
   text    data     bss     dec     hex filename
12221850        1735352 1077248 15034450         e56852 /tmp/vmlinux.old
12195955        1735352 1077248 15008555         e5032b /tmp/vmlinux.new

Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---

Nothing seems to have happened on the kernel side since H.J. Lu
implemented this in December.  Since gcc 5 is officially released
soon, I think it would be nice if this could soak in -next until the
next merge window.

 arch/x86/Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2fda005bb334..00bc42c4cbd1 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -81,6 +81,8 @@ else
         KBUILD_CFLAGS += $(call cc-option,-mno-80387)
         KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
 
+        KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
+
 	# Use -mpreferred-stack-boundary=3 if supported.
 	KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
 
-- 
2.1.3

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