linux-kernel - Re: [PATCH] compiler.h: don't use temporary variable in __compiletime

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Date:	Mon, 12 May 2014 16:38:01 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	James Hogan <james.hogan@...tec.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel Santos <daniel.santos@...ox.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-next <linux-next@...r.kernel.org>,
	linux-metag <linux-metag@...r.kernel.org>
Subject: Re: [PATCH] compiler.h: don't use temporary variable in
 __compiletime_assert()

Hi,

> Unfortunately this breaks the build of today's linux-next for the Meta
> architecture (arch/metag), which happens to use a fairly old compiler
> (based on gcc 4.2.4) which I presume is the reason why.

That's very odd.

Unfortunately, I don't have most of arch/metag, it seems, where could I
get it? In particular no gup.c exists for metag in Linus's current tree.

> A bunch of compile time asserts fail, even in code which should be
> optimised out. E.g. here's one which I analysed:
> 
> mm/gup.c: In function ‘follow_page_mask’:
> mm/gup.c:208: error: size of array ‘type name’ is negative
> 
> Line 208 uses HPAGE_PMD_NR which expands to a HPAGE_PMD_SHIFT, which
> expands to a BUILD_BUG(). However that line is inside an if block
> conditioned on pmd_trans_huge(*pmd) which include/asm-generic/pgtable.h
> defines inline to return 0, so the whole block should already be being
> optimised out.
> 
> I don't understand why your patch should break things, I suspect it's
> related to the sparse behaviour you're trying to work around, but can we
> please drop this patch until a more portable workaround can be found?
> I'm happy to test further patches with metag if it helps.

I don't really understand that either - if the compiler could prove that
the assignment to __cond was a constant, and remember that __cond is now
constant, I don't really see why it can't follow that through and
consider "!(condition)" a const??

I suppose the other option for the original problem is to ignore
_compiletime_assert() for sparse, like we do for BUG_ON(), but it'd
probably be good to analyse more why this particular code is broken now.

johannes

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