linux-kernel - Re: Fedora's latest gcc produces unbootable kernels

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Message-ID: <20071203085817.GQ16835@devserv.devel.redhat.com>
Date:	Mon, 3 Dec 2007 03:58:17 -0500
From:	Jakub Jelinek <jakub@...hat.com>
To:	Thomas Gleixner <tglx@....tglx.de>
Cc:	Pierre Ossman <drzeus-list@...eus.cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Fedora's latest gcc produces unbootable kernels

On Mon, Dec 03, 2007 at 09:17:22AM +0100, Thomas Gleixner wrote:
> I looked at the disassembly but I can not spot the problem.
> 
> I think the real problem is somewhere else. Likely candidates are
> hrtimer_forward() or hrtimer_start() - in that order.

Should be hopefully fixed in latest Fedora gcc.  The problem was in code like
typedef union { long long int s; } U;
typedef struct { U u; } S;

void foo (S *s, long long int x, unsigned long int y)
{
  s->u = ({ (U) { .s = s->u.s + x * y }; });
}

where a backport of a recent optimization of mine, without which gcc handles
terribly initializers from compound literals (which is something hrtimer
uses just everywhere - why can't ktime.h for #if BITS_PER_LONG == 64 || defined(CONFIG_KTIME_SCALAR)
just use a scalar rather than union with a scalar in it??), sets the LHS
object to the compound literal's initializer rather than forcing creation of
a temporary object (the compound literal).  Unfortunately the gimplifier
had some bugs in case the initializer references (or at least might
reference) parts of LHS object.  Fixed by backporting 2 Ada bugfixes for the
gimplifier from GCC trunk (Ada was hitting those bugs even without this
compound literal optimization).

	Jakub
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