allow gcc4 to optimize unit-at-a-time

allow gcc4 compilers to optimize unit-at-a-time - which results in gcc having a wider scope when optimizing. This also results in smaller code when optimizing for size. (gcc4 does not have the stack footprint problem of gcc3 compilers.)

Signed-off-by Ingo Molnar <[email protected]>

Signed-off-by Arjan van de Ven <[email protected]>

----

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

Index: linux/arch/i386/Makefile
===================================================================

arch/i386/Makefile include $(srctree)/arch/i386/Makefile.cp · line 47

Quoted 3 lines

arch/i386/Makefile +3 -0

+# 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 "-fno-unit-at-a-time"; fi ;)

-fno-unit-at-a-time option has been introduced in GCC 3.4 (and 3.3-hammer branch). So unless the minimum supported GCC version to compile kernel is 3.4+, you need to replace
echo "-fno-unit-at-a-time"
with
$(call cc-option,-fno-unit-at-a-time)

Quoted 10 lines · Jakub Jelinek

arch/i386/Makefile include $(srctree)/arch/i386/Makefile.cp · line 47

Quoted 3 lines

arch/i386/Makefile +3 -0

+# 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 "-fno-unit-at-a-time"; fi ;)

-fno-unit-at-a-time option has been introduced in GCC 3.4 (and 3.3-hammer branch). So unless the minimum supported GCC version to compile kernel is 3.4+, you need to replace
echo "-fno-unit-at-a-time"
with
$(call cc-option,-fno-unit-at-a-time)

The test "$(GCC_VERSION) -lt 0400" takes care of this.

No.
-fno-unit-at-a-time should be used with GCCs that
a) support it
b) are older than GCC 4.0

The "$(GCC_VERSION) -lt 0400" test cares of b), $(call cc-option,-fno-unit-at-a-time) cares of a).

Quoted 7 lines · Jakub Jelinek

No.
-fno-unit-at-a-time should be used with GCCs that
a) support it
b) are older than GCC 4.0

The "$(GCC_VERSION) -lt 0400" test cares of b), $(call cc-option,-fno-unit-at-a-time) cares of a).

There was a reason for disabling it unconditionally in first place. That was due to unexpected huge stack usage if I understand correct. Ingo's patch enebles unit-at-a-time only for gcc > 4.00 which should have this issue fixed.

If the argument is that we suddenly shall enable unit-at-a-time for gcc before 4.00 then we should visit the reasons why it originally was disabled.

Quoted 1 line

+CFLAGS += $(shell if [ $(GCC_VERSION) -lt 0400 ] ; then echo "-fno-unit-at-a-time"; fi ;)

-fno-unit-at-a-time option has been introduced in GCC 3.4 (and 3.3-hammer branch). So unless the minimum supported GCC version to compile kernel is 3.4+, you need to replace
echo "-fno-unit-at-a-time"
with
$(call cc-option,-fno-unit-at-a-time)
.

allow gcc4 compilers to optimize unit-at-a-time - which results in gcc having a wider scope when optimizing. This also results in smaller code when optimizing for size. (gcc4 does not have the stack footprint problem of gcc3 compilers.)

Quoted 4 lines · Ingo Molnar

allow gcc4 compilers to optimize unit-at-a-time - which results in gcc having a wider scope when optimizing. This also results in smaller code when optimizing for size. (gcc4 does not have the stack footprint problem of gcc3 compilers.)

I never had any trouble with stack footprint even with gcc 3.3 on x86-64 and unit-at-a-time and it was always enabled.

But one caveat: turning on unit-at-a-time makes objdump -S / make foo/bar.lst with CONFIG_DEBUG_INFO essentially useless because objdump cannot deal with functions being out of order in the object file. This can be a big problem while analyzing oopses - essentially you have to analyze the functions without source level information. And with unit-at-a-time they become bigger so it's more difficult.

But I still think it's a good idea.

But one caveat: turning on unit-at-a-time makes objdump -S / make foo/bar.lst with CONFIG_DEBUG_INFO essentially useless because objdump cannot deal with functions being out of order in the object file. This can be a big problem while analyzing oopses - essentially you have to analyze the functions without source level information. And with unit-at-a-time they become bigger so it's more difficult.

But I still think it's a good idea.

Quoted 11 lines

But one caveat: turning on unit-at-a-time makes objdump -S / make foo/bar.lst with CONFIG_DEBUG_INFO essentially useless because objdump
cannot deal with functions being out of order in the object file. This
can be a big problem while analyzing oopses - essentially you have to
analyze the functions without source level information. And with unit-at-a-time they become bigger so it's more difficult.

Quoted 1 line

But I still think it's a good idea.

hm, i dont seem to have problems with DEBUG_INFO. I picked a random address within the kernel:

c035766f T schedule_timeout

(gdb) list *0xc035768f
0xc035768f is in schedule_timeout (kernel/timer.c:1075). 1070 * should never happens anyway). You just have the printk() 1071 * that will tell you if something is gone wrong and where. 1072 */
1073 if (timeout < 0)
1074 {
1075 printk(KERN_ERR "schedule_timeout: wrong timeout " 1076 "value %lx from %p

1077 __builtin_return_address(0));
1078 current->state = TASK_RUNNING;
1079 goto out;
(gdb)

or is it something else that breaks?