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

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Date:   Sun, 18 Aug 2019 07:01:35 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
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()

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?

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


Segher