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

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Message-ID: <20090423063053.GA9833@elte.hu>
Date:	Thu, 23 Apr 2009 08:30:53 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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

* 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 so i'm NAK-ing 
this patch.

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 
decide' anything. GCC has a very bad track record in that area.

	Ingo
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