2005-09-14 06:12:16

by Lee Nicks

Subject: [PATCH 2.6.14-rc1] 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.
Please review and apply it.

Signed-off-by: Lee Nicks <[email protected]>
======================================

diff -ruN linux-2.6.14-rc1-original/arch/ppc/Makefile
linux-2.6.14-rc1/arch/ppc/Makefile
--- linux-2.6.14-rc1-original/arch/ppc/Makefile 2005-09-13
20:00:34.000000000 -0700
+++ linux-2.6.14-rc1/arch/ppc/Makefile 2005-09-13 20:09:36.000000000 -0700
@@ -26,6 +26,12 @@
AFLAGS += -Iarch/$(ARCH)
CFLAGS += -Iarch/$(ARCH) -msoft-float -pipe \
-ffixed-r2 -mmultiple
+
+# No AltiVec instruction when building kernel
+ifeq ($(call cc-option-yn, -mno-altivec), y)
+CFLAGS += -mno-altivec
+endif
+
CPP = $(CC) -E $(CFLAGS)
# Temporary hack until we have migrated to asm-powerpc
LINUXINCLUDE += -Iarch/$(ARCH)/include
diff -ruN linux-2.6.14-rc1-original/arch/ppc64/Makefile
linux-2.6.14-rc1/arch/ppc64/Makefile
--- linux-2.6.14-rc1-original/arch/ppc64/Makefile 2005-09-13
20:00:36.000000000 -0700
+++ linux-2.6.14-rc1/arch/ppc64/Makefile 2005-09-13
20:08:41.000000000 -0700
@@ -75,6 +75,11 @@
CFLAGS += $(call cc-option,-mtune=power4)
endif

+# No AltiVec instruction when building kernel
+ifeq ($(call cc-option-yn, -mno-altivec), y)
+CFLAGS += -mno-altivec
+endif
+
# Enable unit-at-a-time mode when possible. It shrinks the
# kernel considerably.
CFLAGS += $(call cc-option,-funit-at-a-time)

2005-09-14 06:24:28

by Keith Owens

Subject: Re: [PATCH 2.6.14-rc1] ppc: prevent GCC 4 from generating AltiVec instructions in kernel

On Tue, 13 Sep 2005 23:12:06 -0700,
Lee Nicks <[email protected]> wrote:
>Depending on how GCC is built, GCC 4 may generate altivec instructions
>...
>+# No AltiVec instruction when building kernel
>+ifeq ($(call cc-option-yn, -mno-altivec), y)
>+CFLAGS += -mno-altivec
>+endif

Use the standard format, one line instead of three.

cflags-y += $(call cc-option, -mno-altivec)