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

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Date:	Thu, 18 Jun 2015 14:15:27 +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 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

 # small data is default for elf32 tool-chain. If not usable, disable it
-- 
1.9.1

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