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

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Message-ID: <20210211114910.GA28121@gate.crashing.org>
Date:   Thu, 11 Feb 2021 05:49:10 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        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 07:41:52AM +0000, Christophe Leroy wrote:
> powerpc BUG_ON() is based on using twnei or tdnei instruction,
> which obliges gcc to format the condition into a 0 or 1 value
> in a register.

Huh?  Why is that?

Will it work better if this used __builtin_trap?  Or does the kernel only
detect very specific forms of trap instructions?

> By using a generic implementation, gcc will generate a branch
> to the unconditional trap generated by BUG().

That is many more instructions than ideal.

> As modern powerpc implement branch folding, that's even more efficient.

What PowerPC cpus implement branch folding?  I know none.

Some example code generated via __builtin_trap:

void trap(void) { __builtin_trap(); }
void trap_if_0(int x) { if (x == 0) __builtin_trap(); }
void trap_if_not_0(int x) { if (x != 0) __builtin_trap(); }

-m64:

trap:
	trap
trap_if_0:
	tdeqi 3,0
	blr
trap_if_not_0:
	tdnei 3,0
	blr

-m32:

trap:
	trap
trap_if_0:
	tweqi 3,0
	blr
trap_if_not_0:
	twnei 3,0
	blr


Segher