x86/asm: Use -mskip-rax-setup if supported
GCC 5 added a compiler option, -mskip-rax-setup, for x86-64. It skips
setting up the RAX register when SSE is disabled and there are no
variable arguments passed in vector registers. (According to the x86_64
ABI, %al is used as a hidden register containing the number of vector
registers used).
Since the kernel doesn't pass vector registers to functions with
variable arguments, this option can be used to optimize the x86-64
kernel.
This GCC feature was suggested by Rasmus Villemoes <linux@rasmusvillemoes.dk>.
This is the corresponding kernel change using it.
For kernel v3.17:
text data bss dec filename
11455921 2204048 5853184 19513153 vmlinux #with -mskip-rax-setup
11480079 2204048 5853184 19537311 vmlinux
For Kernel v4.0+ - custom config:
text data bss dec filename
10231778 3479800 16617472 30329050 vmlinux-gcc5+-mskip-rax-setup
10268797 3547448 16621568 30437813 vmlinux
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5ba2d9c..40af1ba 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -84,6 +84,9 @@
# Use -mpreferred-stack-boundary=3 if supported.
KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
+ # Use -mskip-rax-setup if supported.
+ KBUILD_CFLAGS += $(call cc-option,-mskip-rax-setup)
+
# FIXME - should be integrated in Makefile.cpu (Makefile_32.cpu)
cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)