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    Subject[patch 2/7] enable unit-at-a-time optimisations for gcc4
    Subject: allow gcc4 to optimize unit-at-a-time

    From: Ingo Molnar <>

    allow gcc4 compilers to optimize unit-at-a-time.

    This flag enables gcc to "see" the entire C file before making optimisation
    decisions such as inline, which results in gcc making better decisions. One
    of the immediate effects of this is that static functions that are used only
    once now get inlined.

    gcc 3.4 has this flag as well, however gcc 3.x have a problem with inlining
    and stacks and as a result, enabling this flag there would cause excessive
    and unacceptable stack use. This problem is fixed in the gcc 4.x series.
    The x86-64 architecture already enables this feature so it's well tested

    Signed-off-by: Ingo Molnar <>
    Signed-off-by: Arjan van de Ven <>


    arch/i386/Makefile | 6 +++---
    1 files changed, 3 insertions(+), 3 deletions(-)

    Index: linux-2.6.15/arch/i386/Makefile
    --- linux-2.6.15.orig/arch/i386/Makefile
    +++ linux-2.6.15/arch/i386/Makefile
    @@ -42,9 +42,9 @@ include $(srctree)/arch/i386/Makefile.cp
    GCC_VERSION := $(call cc-version)
    cflags-$(CONFIG_REGPARM) += $(shell if [ $(GCC_VERSION) -ge 0300 ] ; then echo "-mregparm=3"; fi ;)

    -# Disable unit-at-a-time mode, it makes gcc use a lot more stack
    -# due to the lack of sharing of stacklots.
    -CFLAGS += $(call cc-option,-fno-unit-at-a-time)
    +# Disable unit-at-a-time mode on pre-gcc-4.0 compilers, it makes gcc use
    +# a lot more stack due to the lack of sharing of stacklots:
    +CFLAGS += $(shell if [ $(GCC_VERSION) -lt 0400 ] ; then echo $(call cc-option,-fno-unit-at-a-time); fi ;)

    CFLAGS += $(cflags-y)

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      Last update: 2006-01-06 11:48    [W:0.020 / U:2.928 seconds]
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