[Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3

[Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3

[Jin Guojie]

* in configure, the default optimization level is changed to O3.
* Tested successfully on following machines:

  | Host machine | Host OS  | qemu-system | Debian ISO  |
  |-----------------------------------------------------|
  |   x86 64     | Fedora25 |    i386     |    i386     |
  | mips 64 le   | Fedora25 |    i386     |    i386     |

Signed-off-by: Jin Guojie <jinguojie@loongson.cn>

Jin Guojie (1):
  Change default CFLAGS to -O3

 configure | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

-- 
2.9.3

[Qemu-devel] [PATCH 1/1] Change default CFLAGS to -O3

[Jin Guojie]

For better performance.

Signed-off-by: Jin Guojie <jinguojie@loongson.cn>
---
 configure | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/configure b/configure
index 218df87..8ebc286 100755
--- a/configure
+++ b/configure
@@ -4753,9 +4753,9 @@ if test "$gcov" = "yes" ; then
   CFLAGS="-fprofile-arcs -ftest-coverage -g $CFLAGS"
   LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
 elif test "$fortify_source" = "yes" ; then
-  CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
+  CFLAGS="-O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
 elif test "$debug" = "no"; then
-  CFLAGS="-O2 $CFLAGS"
+  CFLAGS="-O3 $CFLAGS"
 fi
 
 ##########################################
-- 
2.9.3

Re: [Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3

[Peter Maydell]

On 9 January 2017 at 10:01, Jin Guojie <jinguojie@loongson.cn> wrote:
> * in configure, the default optimization level is changed to O3.
> * Tested successfully on following machines:
>
>   | Host machine | Host OS  | qemu-system | Debian ISO  |
>   |-----------------------------------------------------|
>   |   x86 64     | Fedora25 |    i386     |    i386     |
>   | mips 64 le   | Fedora25 |    i386     |    i386     |
>
> Signed-off-by: Jin Guojie <jinguojie@loongson.cn>
>
> Jin Guojie (1):
>   Change default CFLAGS to -O3
>
>  configure | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

...so what are the measured performance improvements from
this change? As I understand it -O3 is where the "trade off
code size for speed" optimisations kick in, so we need
to benchmark to be sure that the increased icache usage
doesn't outweigh any speed gains. -O3 is not guaranteed
to be strictly better than -O2 (which is why for instance
Debian and Gentoo recommend -O2 as their default).

thanks
-- PMM

Re: [Qemu-devel] [PATCH 0/1] Change default CFLAGS to -O3

[Christian Borntraeger]

On 01/09/2017 11:01 AM, Jin Guojie wrote:
> * in configure, the default optimization level is changed to O3.
> * Tested successfully on following machines:
> 
>   | Host machine | Host OS  | qemu-system | Debian ISO  |
>   |-----------------------------------------------------|
>   |   x86 64     | Fedora25 |    i386     |    i386     |
>   | mips 64 le   | Fedora25 |    i386     |    i386     |
> 
> Signed-off-by: Jin Guojie <jinguojie@loongson.cn>
> 
> Jin Guojie (1):
>   Change default CFLAGS to -O3
> 
>  configure | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 

There are some compilers that trigger some maybe Warnings with -O3
e.g.

      CC      block/vmdk.o
    /home/cborntra/REPOS/qemu/block/vmdk.c: In function ‘vmdk_open_vmdk4’:
    /home/cborntra/REPOS/qemu/block/vmdk.c:718:24: error: ‘extent’ may be
    used uninitialized in this function [-Werror=maybe-uninitialized]
         extent->has_marker = le32_to_cpu(header.flags) & VMDK4_FLAG_MARKER;
                            ^
    /home/cborntra/REPOS/qemu/block/vmdk.c: In function ‘vmdk_open_sparse’:
    /home/cborntra/REPOS/qemu/block/vmdk.c:530:9: error: ‘extent’ may be
    used uninitialized in this function [-Werror=maybe-uninitialized]
         ret = vmdk_init_tables(bs, extent, errp);
             ^
    /home/cborntra/REPOS/qemu/block/vmdk.c:509:17: note: ‘extent’ was
    declared here
         VmdkExtent *extent;
                     ^
    /home/cborntra/REPOS/qemu/block/vmdk.c: In function
    ‘vmdk_open_desc_file.isra.12’:
    /home/cborntra/REPOS/qemu/block/vmdk.c:873:39: error: ‘extent’ may be
    used uninitialized in this function [-Werror=maybe-uninitialized]
                 extent->flat_start_offset = flat_offset << 9;
                                           ^
    /home/cborntra/REPOS/qemu/block/vmdk.c:803:17: note: ‘extent’ was
    declared here
         VmdkExtent *extent;
                     ^
    cc1: all warnings being treated as errors
    /home/cborntra/REPOS/qemu/rules.mak:60: recipe for target 'block/vmdk.o'
    failed


I have started fixing those some time ago but had no time yet finishing that.
Totally messy patches (in terms of patch description) at
https://github.com/borntraeger/qemu/commits/o3

Christian

Re: [Qemu-devel] [PATCH 1/1] Change default CFLAGS to -O3

[Daniel P. Berrange]

On Mon, Jan 09, 2017 at 06:01:23PM +0800, Jin Guojie wrote:
> For better performance.

The commit message needs to have actual data on what the
measured performance improvements are really.

> 
> Signed-off-by: Jin Guojie <jinguojie@loongson.cn>
> ---
>  configure | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/configure b/configure
> index 218df87..8ebc286 100755
> --- a/configure
> +++ b/configure
> @@ -4753,9 +4753,9 @@ if test "$gcov" = "yes" ; then
>    CFLAGS="-fprofile-arcs -ftest-coverage -g $CFLAGS"
>    LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
>  elif test "$fortify_source" = "yes" ; then
> -  CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
> +  CFLAGS="-O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
>  elif test "$debug" = "no"; then
> -  CFLAGS="-O2 $CFLAGS"
> +  CFLAGS="-O3 $CFLAGS"
>  fi

Regards,
Daniel
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