linux-kernel - Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Message-ID: <20090112001255.GR26290@one.firstfloor.org>
Date:	Mon, 12 Jan 2009 01:12:55 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>, jh@...e.cz
Subject: Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Sun, Jan 11, 2009 at 03:05:53PM -0800, Linus Torvalds wrote:
> 
> 
> On Sun, 11 Jan 2009, Linus Torvalds wrote:
> > On Sun, 11 Jan 2009, Andi Kleen wrote:
> > > 
> > > Was -- i think that got fixed in gcc. But again only in newer versions.
> > 
> > I doubt it. People have said that about a million times, it has never 
> > gotten fixed, and I've never seen any actual proof.
> 
> In fact, I just double-checked.
> Try this:

Hmm, I actually had tested it some time ago with my own program
(supposed to emulate an ioctl)

extern void f5(char *);

static void f3(void)
{
	char y[100];
	f5(y);
}

static void f2(void)
{
	char x[100];
	f5(x);
}

int f(int cmd)
{
	switch (cmd) { 
	case 1: f3(); break;
	case 2: f2(); break;
	}
	return 1;
}

and with gcc 4.3.1 I get:

.globl f
	.type	f, @function
f:
.LFB4:
	subq	$120, %rsp			<---- not 200 bytes, stack gets reused; dunno where the 20 comes from
.LCFI0:
	cmpl	$1, %edi
	je	.L4
	cmpl	$2, %edi
	je	.L4
	movl	$1, %eax
	addq	$120, %rsp
	ret
	.p2align 4,,10
	.p2align 3
.L4:
	movq	%rsp, %rdi
	call	f5
	movl	$1, %eax
	addq	$120, %rsp
	ret
.LFE4:

so at least least for this case it works. Your case also doesn't work for me.
So it looks like gcc didn't like something you did in your test program.
Could be a pointer aliasing problem of some sort.

But yes it doesn't work as well as we hoped.

-Andi
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