Re: [2.6 patch] add a config option for -Os compilation

From: Adrian Bunk (bunk_at_fs.tum.de)
Date: 10/18/03

  • Next message: Nico Schottelius: "Re: 3c59x problem with 2.4.6-test[34]" 
    Date:	Sat, 18 Oct 2003 12:21:27 +0200
To: Andrew Morton <akpm@osdl.org>

    On Wed, Oct 15, 2003 at 04:52:05PM -0700, Andrew Morton wrote:
    >...
    > I really doubt it. Kernel CPU footprint is dominated by dcache misses. If
    > -Os reduces icache footprint it may even be a net win; people tend to
    > benchmark things in tight loops, which favours fast code over small code.

    The main effect of -Os compared to -O2 (besides disabling some
    reordering of the code and prefetching) is the disabling of various
    alignments. I doubt that's a win on all CPUs.

    > > - I've already seen a report for an ICE in gcc 2.95 of a user compiling
    > > kernel 2.4 with -Os [1]
    >
    > Well there's only one way to find out if we'll hit that. How's about you
    > cook me a patch which switches to -Os unconditionally and we'll see how it
    > goes?

    I still dislike it, but the patch is below.

    cu
    Adrian

    --- linux-2.6.0-test5-mm4/arch/arm/Makefile.old 2003-09-25 14:38:18.000000000 +0200
    +++ linux-2.6.0-test5-mm4/arch/arm/Makefile 2003-09-25 14:40:47.000000000 +0200
    @@ -14,8 +14,6 @@
     GZFLAGS :=-9
     #CFLAGS +=-pipe
     
    -CFLAGS :=$(CFLAGS:-O2=-Os)
    -
     ifeq ($(CONFIG_FRAME_POINTER),y)
     CFLAGS +=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
     endif
    --- linux-2.6.0-test5-mm4/arch/h8300/Makefile.old 2003-09-25 14:38:18.000000000 +0200
    +++ linux-2.6.0-test5-mm4/arch/h8300/Makefile 2003-09-25 14:38:24.000000000 +0200
    @@ -34,7 +34,7 @@
     ldflags-$(CONFIG_CPU_H8S) := -mh8300self
     
     CFLAGS += $(cflags-y)
    -CFLAGS += -mint32 -fno-builtin -Os
    +CFLAGS += -mint32 -fno-builtin
     CFLAGS += -g
     CFLAGS += -D__linux__
     CFLAGS += -DUTS_SYSNAME=\"uClinux\"
    --- linux-2.6.0-test8/Makefile.old 2003-10-18 12:15:51.000000000 +0200
    +++ linux-2.6.0-test8/Makefile 2003-10-18 12:16:26.000000000 +0200
    @@ -275,7 +275,7 @@
     CPPFLAGS := -D__KERNEL__ -Iinclude \
                        $(if $(KBUILD_SRC),-Iinclude2 -I$(srctree)/include)
     
    -CFLAGS := -Wall -Wstrict-prototypes -Wno-trigraphs -O2 \
    +CFLAGS := -Wall -Wstrict-prototypes -Wno-trigraphs -Os \
                          -fno-strict-aliasing -fno-common
     AFLAGS := -D__ASSEMBLY__
     
    -
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