linux-kernel - Re: subtle side effect of commit a1c48bb160f836

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Message-ID: <55829E51.3060007@synopsys.com>
Date:	Thu, 18 Jun 2015 16:02:49 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Michal Marek <mmarek@...e.cz>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: subtle side effect of commit a1c48bb160f836

On Thursday 18 June 2015 03:44 PM, Michal Marek wrote:
> Dne 18.6.2015 v 10:45 Vineet Gupta napsal(a):
>> > On Thursday 18 June 2015 01:43 PM, Michal Marek wrote:
>>>>> >>>> Alternatively, as we already have CONFIG_CC_OPTIMIZE_FOR_SIZE,
>>>>> >>>> a(nother) Kconfig option may make sense.
>>> >> We can also introduce some ARCH_CFLAGS that is appended near the end of
>>> >> the list, and have arc/Makefile add its -O3 there. But I'd like to why
>>> >> the -O3 needs to be there in first place. 
>> > 
>> > This is how historically ARC kernels have been built. We do track performance
>> > results LMBench/hackbench... and going from -O3 to -O2 caused a sudden dip in some
>> > of the numbers when we measured 3.18 (vs. 3.13)
>> > 
>>> >> Obviously, the kernel works
>>> >> with -O2, otherwise the regression would have been identified earlier.
>> > 
>> > Its a performance thing - so yeah -O2 works, but -O3 works even better :-)
>> > 
>>> >> So why can't users specify -O3 in KCFLAGS like on any other
>>> >> architecture.
>> > 
>> > Sweet, I didn't know about this. But I don't see any arch setting this - only tile
>> > using it. So yeah - below does the trick !
>> > 
>> > --------->
>> > From 5e44cd2ed69b1d030b4cb87600d2767b69c35537 Mon Sep 17 00:00:00 2001
>> > From: Vineet Gupta <vgupta@...opsys.com>
>> > Date: Thu, 18 Jun 2015 13:54:01 +0530
>> > Subject: [PATCH] ARC: Override toplevel default -O2 with -O3
>> > 
>> > ARC kernels have historically been built with -O3, despite top level
>> > Makefile defaulting to -O2. This was facilitated by implicitly ordering
>> > of arch makefile include AFTER top level assigned -O2.
>> > 
>> > An upstream fix to top level a1c48bb160f ("Makefile: Fix unrecognized
>> > cross-compiler command line options") changed the ordering, making ARC
>> > -O3 defunt.
>> > 
>> > Fix that by NOT relying on any ordering whatsoever and use the right
>> > mechanism to do the over-rides.
>> > 
>> > Suggested-by: Michal Marek <mmarek@...e.cz>
>> > Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
>> > Cc: <stable@...r.kernel.org>
>> > Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
>> > ---
>> >  arch/arc/Makefile | 1 +
>> >  1 file changed, 1 insertion(+)
>> > 
>> > diff --git a/arch/arc/Makefile b/arch/arc/Makefile
>> > index 86c71b2089d2..c23f3f2b0ff8 100644
>> > --- a/arch/arc/Makefile
>> > +++ b/arch/arc/Makefile
>> > @@ -44,6 +44,7 @@ endif
>> >  ifndef CONFIG_CC_OPTIMIZE_FOR_SIZE
>> >  # Generic build system uses -O2, we want -O3
>> >  cflags-y  += -O3
>> > +KCFLAGS += -O3
>> >  endif
> Uh, this is not what I meant. KCFLAGS is a *user* setting. It's meant to
> be set in the environment or command line.

Well I don't want to rely on external settings whatsoever to enforce this. If this
is not the right way, what do u suggest I do to help fix this.

-Vineet
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