[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>
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@ -26,6 +26,10 @@ CPPFLAGS += -Iarch/$(ARCH) -Iarch/$(ARCH)/include
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AFLAGS += -Iarch/$(ARCH)
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CFLAGS += -Iarch/$(ARCH) -msoft-float -pipe \
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-ffixed-r2 -mmultiple
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# No AltiVec instruction when building kernel
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CFLAGS += $(call cc-option, -mno-altivec)
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CPP = $(CC) -E $(CFLAGS)
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# Temporary hack until we have migrated to asm-powerpc
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LINUXINCLUDE += -Iarch/$(ARCH)/include
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@ -75,6 +75,9 @@ else
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CFLAGS += $(call cc-option,-mtune=power4)
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endif
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# No AltiVec instruction when building kernel
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CFLAGS += $(call cc-option, -mno-altivec)
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# Enable unit-at-a-time mode when possible. It shrinks the
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# kernel considerably.
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CFLAGS += $(call cc-option,-funit-at-a-time)
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