[PATCH] ppc: prevent GCC 4 from generating AltiVec instructions in kernel
Depending on how GCC is built, GCC 4 may generate altivec instructions without
user explicitly requesting vector operations in the code. Although this is a
performance booster for user applications, it is a problem for kernel.
This patch explicitly instruct GCC to NOT generate altivec instructions while
building the kernel.
Here are some test cases I ran.
(1) build gcc 4.0.1 with '--with-cpu=7450 --enable-altivec
--enable-cxx-flags=-mcpu=7450', and use this gcc to build kernel WITHOUT
this kernel patch. Kernel fail to boot up on a 7450 board because of
altivec instructions in kernel.
(2) build gcc 4.0.1 with "--with-cpu=7450 --enable-altivec
--enable-cxx-flags=-mcpu=7450", and use this gcc to build kernel WITH this
kernel patch. Kernel boot up on a 7450 board without any problem.
(3) build gcc 4.0.1 with "--with-cpu=750 --enable-cxx-flags=-mcpu=750",
and use this gcc to build kernel with or without this kernel patch.
Kernel boot up on a 7450 board without any problem.
This patch should also work with GCC 3 or even earlier GCC 2.95.3.
Signed-off-by: Lee Nicks <allinux@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/ppc/Makefile b/arch/ppc/Makefile
index 121d234..94d5716 100644
--- a/arch/ppc/Makefile
+++ b/arch/ppc/Makefile
@@ -26,6 +26,10 @@
AFLAGS += -Iarch/$(ARCH)
CFLAGS += -Iarch/$(ARCH) -msoft-float -pipe \
-ffixed-r2 -mmultiple
+
+# No AltiVec instruction when building kernel
+CFLAGS += $(call cc-option, -mno-altivec)
+
CPP = $(CC) -E $(CFLAGS)
# Temporary hack until we have migrated to asm-powerpc
LINUXINCLUDE += -Iarch/$(ARCH)/include