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

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Message-ID: <f058a9c30901200943p4f453806sc2448893584e0ce3@mail.gmail.com>
Date:	Tue, 20 Jan 2009 17:43:02 +0000
From:	"Miguel F Mascarenhas Sousa Filipe" <miguel.filipe@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Nick Piggin" <npiggin@...e.de>, "Ingo Molnar" <mingo@...e.hu>,
	"Bernd Schmidt" <bernds_cb1@...nline.de>,
	"Andi Kleen" <andi@...stfloor.org>,
	"David Woodhouse" <dwmw2@...radead.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"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>,
	"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 Mon, Jan 19, 2009 at 9:01 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
> On Mon, 19 Jan 2009, Nick Piggin wrote:
>>
>> I want to know what is the problem with the restrict keyword?
>> I'm sure I've read Linus ranting about how bad it is in the
>> past...
>
> No, I don't think I've ranted about 'restrict'. I think it's a reasonable
> solution for performance-critical code, and unlike the whole type-aliasing
> model, it actually works for the _sane_ cases (ie doing some operation
> over two arrays of the same type, and letting the compiler know that it
> can access the arrays without fearing that writing to one would affect
> reading from the other).
>
> The problem with 'restrict' is that almost nobody uses it, and it does
> obviously require programmer input rather than the compiler doing it
> automatically. But it should work well as a way to get Fortran-like
> performance from HPC workloads written in C - which is where most of the
> people are who really want the alias analysis.

while working in my college thesis, a fortran HPC workload (10k lines
of fortran), converted to C resulted in performance speedups. this was
with gcc 3.4.
A simple f2c conversion + adaptations, resulted in a considerable
speedup. (20% IIRC).
The conversion was not done by performance reasons, the extra
performance was simply a unexpected, but quite nice outcome.
Just to let you guys know, that even with gcc3.4, gcc of C code ran
faster than gfortran of the equiv. fortran code.


... pushing the optimization engine further (-march tunning and -O3)
resulted in bad data.
But I can't swear by the correctness of some of the computations with
REAL's made in the original fortran code.



>
>> it seems like a nice opt-in thing that can be used where the aliases are
>> verified and the code is particularly performance critical...
>
> Yes. I think we could use it in the kernel, although I'm not sure how many
> cases we would ever find where we really care.
>
>                        Linus
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-- 
Miguel Sousa Filipe
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