linux-kernel - Re: [PATCH] powerpc: optimise WARN

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Date:   Mon, 19 Aug 2019 07:40:42 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: optimise WARN_ON()



Le 18/08/2019 à 14:01, Segher Boessenkool a écrit :
> On Sat, Aug 17, 2019 at 09:04:42AM +0000, Christophe Leroy wrote:
>> Unlike BUG_ON(x), WARN_ON(x) uses !!(x) as the trigger
>> of the t(d/w)nei instruction instead of using directly the
>> value of x.
>>
>> This leads to GCC adding unnecessary pair of addic/subfe.
> 
> And it has to, it is passed as an "r" to an asm, GCC has to put the "!!"
> value into a register.
> 
>> By using (x) instead of !!(x) like BUG_ON() does, the additional
>> instructions go away:
> 
> But is it correct?  What happens if you pass an int to WARN_ON, on a
> 64-bit kernel?

On a 64-bit kernel, an int is still in a 64-bit register, so there would 
be no problem with tdnei, would it ? an int 0 is the same as an long 0, 
right ?

It is on 32-bit kernel that I see a problem, if one passes a long long 
to WARN_ON(), the forced cast to long will just drop the upper size of 
it. So as of today, BUG_ON() is buggy for that.

> 
> (You might want to have 64-bit generate either tw or td.  But, with
> your __builtin_trap patch, all that will be automatic).
> 

Yes I'll discard this patch and focus on the __builtin_trap() one which 
should solve most issues.

Christophe