linux-kernel - Re: [PATCH] powerpc/bug: Remove specific powerpc BUG

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Date:   Thu, 11 Feb 2021 06:22:11 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()

On Thu, Feb 11, 2021 at 08:04:55PM +1000, Nicholas Piggin wrote:
> Excerpts from Christophe Leroy's message of February 11, 2021 5:41 pm:
> > As modern powerpc implement branch folding, that's even more efficient.

Ah, it seems you mean what Arm calls branch folding.  Sure, power4
already did that, and this has not changed.

> I think POWER will speculate conditional traps as non faulting always
> so it should be just as good if not better than the branch.

Right, these are not branch instructions, so are not branch predicted;
all trap instructions are assumed to fall through, like all other
non-branch instructions.


Segher