Messages in this thread Patch in this message | | | From | Rasmus Villemoes <> | Subject | [PATCH] x86_64: use -mskip-rax-setup when available | Date | Mon, 20 Apr 2015 14:28:16 +0200 |
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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@rasmusvillemoes.dk> ---
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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