linux-kernel - Re: Control Dependencies vs C Compilers

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Message-ID: <47bfda8f-304e-5283-c6d6-0753037b0b2c@redhat.com>
Date:   Tue, 6 Oct 2020 16:16:26 +0100
From:   Nick Clifton <nickc@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "stern@...land.harvard.edu" <stern@...land.harvard.edu>
Cc:     David Laight <David.Laight@...lab.com>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Paul McKenney <paulmck@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "parri.andrea@...il.com" <parri.andrea@...il.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "j.alglave@....ac.uk" <j.alglave@....ac.uk>,
        "luc.maranget@...ia.fr" <luc.maranget@...ia.fr>,
        "akiyks@...il.com" <akiyks@...il.com>,
        "dlustig@...dia.com" <dlustig@...dia.com>,
        "joel@...lfernandes.org" <joel@...lfernandes.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>
Subject: Re: Control Dependencies vs C Compilers

Hi Guys,

  OK, so playing the devils advocate here...

> Mostly I just want the compiler people to say they'll guarantee the
> behaviour if we do 'X'. If 'X' happens to be 'any dynamic branch headed
> by a volatile load' that's fine by me.

  Is a compiler hack really the right way to go here ?

  After all, if you do get this feature added it will make it
  harder to compile the kernel with other compilers (*cough 
  LLVM cough*), or older versions of gcc.  Plus code like this
  is often subject to very aggressive optimization and all it
  takes is one bug in the compiler implementation and you lose
  the gains you were trying for.

  My suggestion as an alternative is to use assembler instead.
  That way you can guarantee that you get the instructions you
  want in the order that you want them.  It should be fairly
  straightforward to create a macro or inline function that
  contains the necessary code and this can be done once and 
  then used wherever the functionality is required.

Cheers
  Nick