linux-kernel - Re: [PATCH] X86-32: Let gcc decide whether to inline memcpy was Re: New x86 warning

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Message-ID: <20090423074320.GI13896@one.firstfloor.org>
Date:	Thu, 23 Apr 2009 09:43:20 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeff Garzik <jeff@...zik.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH] X86-32: Let gcc decide whether to inline memcpy was Re: New x86 warning

On Thu, Apr 23, 2009 at 08:30:53AM +0200, Ingo Molnar wrote:
> 
> * Andi Kleen <andi@...stfloor.org> wrote:
> 
> > > > Quick test here:
> > > 
> > > How about you just compile the kernel with gcc-3.2 and compare 
> > > the number of calls to memcpy before-and-after instead? That's 
> > > the real test.
> > 
> > I waited over 10 minutes for the full vmlinux objdumps to finish. 
> > sorry lost patience. If someone has a fast disassembler we can try 
> > it. I'll leave them running over night, maybe there are exact 
> > numbers tomorrow.
> 
> Uhm, the test Linus requested is very simple, it doesnt need 'full' 
> objdumps, just a plain defconfig [*] - an objdump takes less than 10 
> seconds here even on an old box i tried it on.
> 
> I just did this - it all took less than 5 minutes to do the whole 
> test with gcc34:
> 
>   vmlinux.gcc34.vanilla:       679 calls to memcpy
>   vmlinux.gcc34.gcc-memcpy:   1393 calls to memcpy
> 
> So your patch more than doubles the number of calls to out-of-line 
> memcpy on older GCC. That's not really acceptable 

How do you determine it's not acceptable?

It seems not nice to me, but not a fatal problem. I think
Linus was more interested in dramatic growth, but factor 2 over
a very large code base doesn't seem to be dramatic to me, especially
since it's very likely most of the are slow path code.

> Next time you send such patches please test with older GCCs straight 
> away - it's a basic act of testing when doing a patch that 'lets GCC 

I tested with 3.2.3, but not with 3.4.

3.2.3 doesn't do that. AFAIK 3.2 was a pretty common compiler; at least
several SUSE releases shipped with it.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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