[PATCH] prefer Oz optimization if the compiler supports it

[PATCH] prefer Oz optimization if the compiler supports it

[Dmitry Golovin]

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Hi all!

This is a very small and simple patch, it adds support for Oz level of optimization (clang supports it). Since the kernel switched to Oz, I think it's a good idea to use it as long as the compiler supports it, on the other hand I didn't test if it has any advantages in case of musl.

The patch is attached.

Regards,
Dmitry

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From 4f9a8ae81b7c3810b1ec8e31fc11cf72c0f82dee Mon Sep 17 00:00:00 2001
From: Dmitry Golovin <dima@golovin.in>
Date: Wed, 19 Jul 2017 13:35:25 +0300
Subject: [PATCH] prefer Oz optimization if the compiler supports it

---
 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure b/configure
index 539c9fcb..1212af91 100755
--- a/configure
+++ b/configure
@@ -425,7 +425,7 @@ xno|x) printf "disabled\n" ; optimize=no ;;
 *) printf "custom\n" ;;
 esac
 
-test "$optimize" = no || tryflag CFLAGS_AUTO -Os || tryflag CFLAGS_AUTO -O2
+test "$optimize" = no || tryflag CFLAGS_AUTO -Oz || tryflag CFLAGS_AUTO -Os || tryflag CFLAGS_AUTO -O2
 test "$optimize" = yes && optimize="internal,malloc,string"
 
 if fnmatch 'no|size' "$optimize" ; then :
-- 
2.13.1

Re: [PATCH] prefer Oz optimization if the compiler supports it

[Rich Felker]

On Wed, Jul 19, 2017 at 01:44:17PM +0300, Dmitry Golovin wrote:
> Hi all!
> 
> This is a very small and simple patch, it adds support for Oz level
> of optimization (clang supports it). Since the kernel switched to
> Oz, I think it's a good idea to use it as long as the compiler
> supports it, on the other hand I didn't test if it has any
> advantages in case of musl.
> 
> The patch is attached.

I'm not clear what the goal is. Why would Oz be preferred to Os? A
quick search shows that all it seems to do is disabling some
vectorization, which sounds undesirable.

Rich

Re: [PATCH] prefer Oz optimization if the compiler supports it

[Dmitry Golovin]

In theory Oz should produce slimmer binary than Os, but I want first to perform tests to see how optimization level really affects size and performance.

Regards,
Dmitry

19.07.2017, 15:52, "Rich Felker" <dalias@libc.org>:
> On Wed, Jul 19, 2017 at 01:44:17PM +0300, Dmitry Golovin wrote:
>>  Hi all!
>>
>>  This is a very small and simple patch, it adds support for Oz level
>>  of optimization (clang supports it). Since the kernel switched to
>>  Oz, I think it's a good idea to use it as long as the compiler
>>  supports it, on the other hand I didn't test if it has any
>>  advantages in case of musl.
>>
>>  The patch is attached.
>
> I'm not clear what the goal is. Why would Oz be preferred to Os? A
> quick search shows that all it seems to do is disabling some
> vectorization, which sounds undesirable.
>
> Rich

Re: [PATCH] prefer Oz optimization if the compiler supports it

[Rich Felker]

On Wed, Jul 19, 2017 at 04:48:49PM +0300, Dmitry Golovin wrote:
> In theory Oz should produce slimmer binary than Os, but I want first
> to perform tests to see how optimization level really affects size
> and performance.

My impression is that clang's -O2 and -Os are comparable in goals to
gcc's -Os, and that -Oz just makes some minor size-improvement
tradeoffs that could be bad for performance.

I've been wanting for a while to actually drop use of -Os with
selective -O3 for some dirs, and instead use -O2 for everything by
default. With recent compilers, -Os seems to do gratuitously stupid
things, and doesn't make much of a size difference vs -O2 (and even
less so if you use -fno-align-*). The main thing that's kept me from
actually pursuing such a change is not having had time to test the
effects myself or work with someone else to get it done.

Rich


> 19.07.2017, 15:52, "Rich Felker" <dalias@libc.org>:
> > On Wed, Jul 19, 2017 at 01:44:17PM +0300, Dmitry Golovin wrote:
> >>  Hi all!
> >>
> >>  This is a very small and simple patch, it adds support for Oz level
> >>  of optimization (clang supports it). Since the kernel switched to
> >>  Oz, I think it's a good idea to use it as long as the compiler
> >>  supports it, on the other hand I didn't test if it has any
> >>  advantages in case of musl.
> >>
> >>  The patch is attached.
> >
> > I'm not clear what the goal is. Why would Oz be preferred to Os? A
> > quick search shows that all it seems to do is disabling some
> > vectorization, which sounds undesirable.
> >
> > Rich