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Date: Fri, 9 Mar 2018 16:32:41 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>, Linux Kernel Mailing List
 <linux-kernel@...r.kernel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>, "Gustavo A. R. Silva"
 <gustavo@...eddedor.com>, "Tobin C. Harding" <me@...in.cc>, Steven Rostedt
 <rostedt@...dmis.org>, Jonathan Corbet <corbet@....net>, Chris Mason
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 "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov
 <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo
 Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas
 Gleixner <tglx@...utronix.de>, Masahiro Yamada
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 <rdunlap@...radead.org>, Ian Abbott <abbotti@....co.uk>, Sergey Senozhatsky
 <sergey.senozhatsky.work@...il.com>, Petr Mladek <pmladek@...e.com>, Andy
 Shevchenko <andriy.shevchenko@...ux.intel.com>, Pantelis Antoniou
 <pantelis.antoniou@...sulko.com>, Linux Btrfs
 <linux-btrfs@...r.kernel.org>, Network Development
 <netdev@...r.kernel.org>, Kernel Hardening
 <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in
 min()/max()

On Fri, 9 Mar 2018 16:28:51 -0800 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, Mar 9, 2018 at 4:07 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > A brief poke failed to reveal a workaround - gcc-4.4.4 doesn't appear
> > to know that __builtin_constant_p(x) is a constant.  Or something.
> 
> LOL.
> 
> I suspect it might be that it wants to evaluate
> __builtin_choose_expr() at an earlier stage than it evaluates
> __builtin_constant_p(), so it's not that it doesn't know that
> __builtin_constant_p() is a constant, it just might not know it *yet*.
> 
> Maybe.
> 
> Side note, if it's not that, but just the "complex" expression that
> has the logical 'and' etc, maybe the code could just use
> 
>   __builtin_constant_p((x)+(y))
> 
> or something.

I'll do a bit more poking at it.

> But yeah:
> 
> > Sigh.  Wasn't there some talk about modernizing our toolchain
> > requirements?
> 
> Maybe it's just time to give up on 4.4.  We wanted 4.5 for "asm goto",
> and once we upgrade to 4.5 I think Arnd said that no distro actually
> ships it, so we might as well go to 4.6.
> 
> So maybe this is just the excuse to finally make that official, if
> there is no clever workaround any more.

I wonder which gcc versions actually accept Kees's addition.

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