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

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Date:	Sun, 29 Mar 2009 15:22:03 -0400
From:	Kyle McMartin <kyle@...artin.ca>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	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

On Sat, Mar 28, 2009 at 10:38:50AM +0100, Andi Kleen wrote:
> 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.
> 

i386 will do this kind of stupidity too if you let it. I had to fix
this in nouveau a few weeks back because they were doing a u64 modulus
(a power of 2 too, no idea why gcc was so clueless as to not properly
 reduce it...)

GCC is getting much worse in this regard...

regards, Kyle
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