[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>

1 files changed, 3 insertions, 0 deletions

diff --git a/arch/ppc64/Makefile b/arch/ppc64/Makefile
index 743f0dbdebf3..db1b25cdb5be 100644
--- a/arch/ppc64/Makefile
+++ b/arch/ppc64/Makefile
@@ -75,6 +75,9 @@ else
         CFLAGS += $(call cc-option,-mtune=power4)
 endif
 
+# No AltiVec instruction when building kernel
+CFLAGS  += $(call cc-option, -mno-altivec)
+
 # Enable unit-at-a-time mode when possible. It shrinks the
 # kernel considerably.
 CFLAGS += $(call cc-option,-funit-at-a-time)