linux-kernel - Re: Control Dependencies vs C Compilers

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Message-ID: <20201007212006.GS29330@paulmck-ThinkPad-P72>
Date:   Wed, 7 Oct 2020 14:20:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        linux-toolchains@...r.kernel.org, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, stern@...land.harvard.edu,
        parri.andrea@...il.com, boqun.feng@...il.com, npiggin@...il.com,
        dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        akiyks@...il.com, dlustig@...dia.com, joel@...lfernandes.org,
        torvalds@...ux-foundation.org
Subject: Re: Control Dependencies vs C Compilers

On Wed, Oct 07, 2020 at 11:07:17PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 10:11:07AM -0700, Paul E. McKenney wrote:
> 
> > Challenges include:
> > 
> > o	Unmarked accesses.  Compilers are quite aggressive about
> > 	moving normal code.
> 
> Which is why this thread exists :-) We wants to dis-allow lifting the
> stores over our volatile-if.

Of course.  But you should expect this point to be a continual source
of shock and surprise to compiler folks.  ;-)

> > o	Separately compiled code.  For example, does the compiler have
> > 	unfortunatel optimization opportunities when "volatile if" 
> > 	appears in one translation unit and the dependent stores in
> > 	some other translation unit?
> 
> It can hardly lift anything outside a TU (barring the next point). So I
> don't see how it can go wrong here. This is in fact the case with the
> perf ringbuffer. The ctrl-dep lives in a different TU from the
> stores.

I don't see how it could either, but I have been surprised before.

> > o	LTO, as has already been mentioned in this thread.
> 
> So I would probably advocate the volatile-if to be a full sync point,
> and LTO would have to preserve that.

Completely agreed!  And probably not the only place that LTO needs
to be reined in a bit.

							Thanx, Paul