linux-kernel - Re: [PATCH] btrfs: fix __ucmpdi2 compile bug on 32 bit builds

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Date:	Sat, 28 Mar 2009 10:38:50 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Chris Mason <chris.mason@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] btrfs: fix __ucmpdi2 compile bug on 32 bit builds

Heiko Carstens <heiko.carstens@...ibm.com> writes:

> From: Heiko Carstens <heiko.carstens@...ibm.com>
>
> We get this on 32 builds:
>
> fs/built-in.o: In function `extent_fiemap':
> (.text+0x1019f2): undefined reference to `__ucmpdi2'

>
> Happens because of a switch statement with a 64 bit argument.
> Convert this to an if statement to fix this.

To be honest that sounds more like a bug in your architecture.

I don't think it's the right solution to make a new rule
"you shall not do 64bit switch()", because that's a reasonable
thing to do and will be hard to enforce over millions of lines
of random Linux code.

There was a explicit decision to not support implicit 64bit 
divides on 32bit because they're very costly, but that doesn't
really apply to 64bit switch(). At least they shouldn't be very costly
in theory. It seems indeed weird to call a function to compare
a 64bit value. I bet the call sequence is larger than just 
doing two cmps. Perhaps your gcc should be fixed? Or alternatively
at least that function be added to the kernel runtime library.

-Andi

ak@...ux.intel.com -- Speaking for myself only.
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