Xen devel - Jun 2005 - [PATCH] compile with -funit-at-a-time option of gcc

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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> 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

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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


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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


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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

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