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

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Date:	Thu, 23 Apr 2009 08:36:25 +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: fresh data was Re: [PATCH] X86-32: Let gcc decide whether to
	inline memcpy was Re: New x86 warning


* Andi Kleen <andi@...stfloor.org> wrote:

> Andi Kleen <andi@...stfloor.org> writes:
> 
> >> > 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.
> >
> > But from a quick check (find -name '*.o' | xargs nm | grep memcpy) there are
> > very little files which call it with the patch, so there's some
> > evidence that there isn't a dramatic increase.
> 
> I let the objdumps finish over night. [...]

objdump -d never took me more than a minute - let alone a full 
night. You must be doing something really wrong there. Looking at 
objdump -d is an essential, unavoidable component of my workflow 
with x86 architecture patches, you need to find a way to do it 
efficiently if you want to send patches for this area of the kernel.

> [...] On my setup (defconfig + some additions) there are actually 
> less calls to out of line memcpy/__memcpy with the patch. I see 
> only one for my defconfig, while there are ~10 without the patch. 
> So it makes very little difference. The code size savings must 
> come from more efficient code generation for the inline case. I 
> haven't investigated that in detail though.
> 
> So the patch seems like a overall win.

It's a clear loss here with GCC 3.4, and it took me less than 5 
minutes to figure that out.

With what precise compiler version did you test (please paste the 
gcc -v output), and could you send me the precise .config you used, 
and describe the method you used to determine the number of 
out-of-line memcpy calls? I'd like to double-check your numbers.

	Ingo
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