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

[Adrian Bunk <bunk@fs.tum.de>]

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__