From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: linuxppc-dev@lists.ozlabs.org, Paul Mackerras <paulus@samba.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc: optimise WARN_ON()
Date: Sun, 18 Aug 2019 07:01:35 -0500 [thread overview]
Message-ID: <20190818120135.GV31406@gate.crashing.org> (raw)
In-Reply-To: <20190817090442.C5FEF106613@localhost.localdomain>
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
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc: optimise WARN_ON()
Date: Sun, 18 Aug 2019 07:01:35 -0500 [thread overview]
Message-ID: <20190818120135.GV31406@gate.crashing.org> (raw)
In-Reply-To: <20190817090442.C5FEF106613@localhost.localdomain>
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
next prev parent reply other threads:[~2019-08-18 12:03 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-17 9:04 [PATCH] powerpc: optimise WARN_ON() Christophe Leroy
2019-08-17 9:04 ` Christophe Leroy
2019-08-18 12:01 ` Segher Boessenkool [this message]
2019-08-18 12:01 ` Segher Boessenkool
2019-08-19 5:40 ` Christophe Leroy
2019-08-19 5:40 ` Christophe Leroy
2019-08-19 7:57 ` Segher Boessenkool
2019-08-19 7:57 ` Segher Boessenkool
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