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[PATCH] ppc: prevent GCC 4 from generating AltiVec instructions in kernel

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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>
This commit is contained in:
Lee Nicks 2005-10-28 17:46:09 -07:00 committed by Paul Mackerras
parent f78541dcec
commit 9e3699ea7b
2 changed files with 7 additions and 0 deletions
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4
arch/ppc/Makefile
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@ -26,6 +26,10 @@ CPPFLAGS += -Iarch/$(ARCH) -Iarch/$(ARCH)/include
AFLAGS += -Iarch/$(ARCH) AFLAGS += -Iarch/$(ARCH)
CFLAGS += -Iarch/$(ARCH) -msoft-float -pipe \ CFLAGS += -Iarch/$(ARCH) -msoft-float -pipe \
-ffixed-r2 -mmultiple -ffixed-r2 -mmultiple
# No AltiVec instruction when building kernel
CFLAGS += $(call cc-option, -mno-altivec)
CPP = $(CC) -E $(CFLAGS) CPP = $(CC) -E $(CFLAGS)
# Temporary hack until we have migrated to asm-powerpc # Temporary hack until we have migrated to asm-powerpc
LINUXINCLUDE += -Iarch/$(ARCH)/include LINUXINCLUDE += -Iarch/$(ARCH)/include

3
arch/ppc64/Makefile
View File

@ -75,6 +75,9 @@ else
CFLAGS += $(call cc-option,-mtune=power4) CFLAGS += $(call cc-option,-mtune=power4)
endif endif
# No AltiVec instruction when building kernel
CFLAGS += $(call cc-option, -mno-altivec)
# Enable unit-at-a-time mode when possible. It shrinks the # Enable unit-at-a-time mode when possible. It shrinks the
# kernel considerably. # kernel considerably.
CFLAGS += $(call cc-option,-funit-at-a-time) CFLAGS += $(call cc-option,-funit-at-a-time)