[Xen-changelog] [xen master] x86/Rules: Use -mskip-rax-setup if the compiler supports it

[patchbot@xxxxxxx]

commit 2d70b54e055635ff60526b6949156504b6194b7c
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Oct 6 13:21:32 2017 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Jan 12 18:53:19 2018 +0000

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/Rules.mk | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
index 568657e..71ed371 100644
--- 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
--
generated by git-patchbot for /home/xen/git/xen.git#master

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