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

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Message-ID: <55829A0B.8070804@suse.cz>
Date:	Thu, 18 Jun 2015 12:14:35 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	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

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.

Michal
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