x86/Rules: Use -mskip-rax-setup if the compiler supports it

This option is available from GCC 5 onwards, and was specifically introduced
as an optimisation for Linux.  When using variadic functions, the caller needs
to know how many floating point arguments were passed.  Xen, like Linux,
doesn't uses floating point arguments, so doesn't need to emit code to inform
variadic functions such as printk() that there are zero arguments.

The net delta for a release build is:

  add/remove: 0/0 grow/shrink: 35/625 up/down: 603/-5489 (-4886)

with the single biggest change being:

  x86_emulate                               101933  101751    -182

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 568657ee52b0cfaa1a4cb9bcf30184cea3e271b6..71ed3711ba25e424f759430e9d6e4efa707408eb 100644 (file)
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -24,8 +24,12 @@ $(call as-insn-check,CFLAGS,CC,".equ \"x\"$$(comma)1", \
                      -U__OBJECT_LABEL__ -DHAVE_GAS_QUOTED_SYM \
                      '-D__OBJECT_LABEL__=$(subst $(BASEDIR)/,,$(CURDIR))/$$@')
 
-CFLAGS += -mno-red-zone -mno-sse -fpic
-CFLAGS += -fno-asynchronous-unwind-tables
+CFLAGS += -mno-red-zone -fpic -fno-asynchronous-unwind-tables
+
+# Xen doesn't use SSE interally.  If the compiler supports it, also skip the
+# SSE setup for variadic function calls.
+CFLAGS += -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
+
 # -fvisibility=hidden reduces -fpic cost, if it's available
 ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
 CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE