[PATCH] compile with -funit-at-a-time option of gcc

[PATCH] compile with -funit-at-a-time option of gcc

[aq]

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This patch makes xen compiled with gcc option -funit-at-a-time if
supported. This option is available for gcc 3.4 and upward, which
reduces the .text binary size considerably.

Signed-off-by: Nguyen Anh Quynh <aquynh@gmail.com>

$ diffstat configmk.patch 
 Config.mk |    6 ++++++
 1 files changed, 6 insertions(+)

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===== Config.mk 1.6 vs edited =====
--- 1.6/Config.mk	2005-06-18 18:24:47 +09:00
+++ edited/Config.mk	2005-06-30 13:41:06 +09:00
@@ -29,8 +29,14 @@
 EXTRA_LIB += $(EXTRA_PREFIX)/$(LIBDIR)
 endif
 
+cc-option := $(shell if $(CC) $(CFLAGS) $(1) -S -o /dev/null -xc /dev/null \
+             > /dev/null 2>&1; then echo "$(1)"; else echo "$(2)"; fi ;)
+
+check_gcc := $(call cc-option, $(1),$(2))
+
 LDFLAGS += $(foreach i, $(EXTRA_LIB), -L$(i)) 
 CFLAGS += $(foreach i, $(EXTRA_INCLUDES), -I$(i))
+CFLAGS += $(call check_gcc,-funit-at-a-time,)
 
 # Choose the best mirror to download linux kernel
 KERNEL_REPO = http://www.kernel.org

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RE: [PATCH] compile with -funit-at-a-time option of gcc

[Ian Pratt]

> This patch makes xen compiled with gcc option 
> -funit-at-a-time if supported. This option is available for 
> gcc 3.4 and upward, which reduces the .text binary size considerably.

I haven't come across this option before. What does it actually do?  

Thanks,
Ian

Re: [PATCH] compile with -funit-at-a-time option of gcc

[Keir Fraser]

On 30 Jun 2005, at 05:53, aq wrote:

> This patch makes xen compiled with gcc option -funit-at-a-time if
> supported. This option is available for gcc 3.4 and upward, which
> reduces the .text binary size considerably.

This option is the default for -O2 and above in gcc4 anyway. It might 
explain why I end up with a buggy Xen when I build with gcc4 (Linux has 
had a number of problems with -funit-at-a-time, because odf extra stack 
usage and code reordering).

  -- Keir

Re: [PATCH] compile with -funit-at-a-time option of gcc

[Keir Fraser]

On 30 Jun 2005, at 08:21, Ian Pratt wrote:

>> This patch makes xen compiled with gcc option
>> -funit-at-a-time if supported. This option is available for
>> gcc 3.4 and upward, which reduces the .text binary size considerably.
>
> I haven't come across this option before. What does it actually do?

Considers whole file at a time when doing optimisation. It's the 
default on gcc4 with -O2 and above. We probably don;t want to enable it 
before a general move to gcc4 as it does break stuff (albeit stuff that 
was buggy/broken already ;-) ). Adding another skanky CFLAG that will 
be defaulted anyway in future, to get the benefit of maybe subtly 
breaking Xen, doesn;t sound like a big win to me.

  -- Keir

Re: [PATCH] compile with -funit-at-a-time option of gcc

[Andi Kleen]

Keir Fraser <Keir.Fraser@cl.cam.ac.uk> writes:

> On 30 Jun 2005, at 08:21, Ian Pratt wrote:
> 
> >> This patch makes xen compiled with gcc option
> >> -funit-at-a-time if supported. This option is available for
> >> gcc 3.4 and upward, which reduces the .text binary size considerably.
> >
> > I haven't come across this option before. What does it actually do?
> 
> Considers whole file at a time when doing optimisation. It's the

Basically it allows inlining even when a function is defined after
the caller and makes the inliner more aggressive. Everything static with
only one caller will be always inlined.

> default on gcc4 with -O2 and above. We probably don;t want to enable
> it before a general move to gcc4 as it does break stuff (albeit stuff
> that was buggy/broken already ;-) ). Adding another skanky CFLAG that
> will be defaulted anyway in future, to get the benefit of maybe subtly
> breaking Xen, doesn;t sound like a big win to me.

The main breakage that can usually happen with unit-at-a-time
(short of broken inline asms) is that your stack frames grow
too big because gcc before 4 is not very good at reusing the slots, 
but they add up with aggressive inlining.

You can check for that with a simple 
objdump -S ... | grep sub.*[re]sp | sort ...
and check for anything big enough that might overflow your kernel stack.

The main kernel has a special check script for this too.

-Andi