linux-kernel - Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN

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Date:   Thu, 26 Aug 2021 08:47:58 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nathan Chancellor <nathan@...nel.org>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto

On Thu, Aug 26, 2021 at 08:37:09AM +0200, Christophe Leroy wrote:
> Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
> >This patch seems to fix it. Not sure if that's just papering over it 
> >though.
> >
> >diff --git a/arch/powerpc/include/asm/bug.h 
> >b/arch/powerpc/include/asm/bug.h
> >index 1ee0f22313ee..75fcb4370d96 100644
> >--- a/arch/powerpc/include/asm/bug.h
> >+++ b/arch/powerpc/include/asm/bug.h
> >@@ -119,7 +119,7 @@ __label_warn_on:					 \
> >  								\
> >  			WARN_ENTRY(PPC_TLNEI " %4, 0",		\
> >  				   BUGFLAG_WARNING | 
> >  				   BUGFLAG_TAINT(TAINT_WARN),	\
> >-				   __label_warn_on, "r" (x));	\
> >+				   __label_warn_on, "r" (!!(x))); \
> >  			break;					\
> >  __label_warn_on:						\
> >  			__ret_warn_on = true;			\
> 
> But for a simple WARN_ON() call:
> 
> void test(unsigned long b)
> {
> 	WARN_ON(b);
> }
> 
> Without your change with GCC you get:
> 
> 00000000000012d0 <.test>:
>     12d0:	0b 03 00 00 	tdnei   r3,0
>     12d4:	4e 80 00 20 	blr
> 
> 
> With the !! change you get:
> 
> 00000000000012d0 <.test>:
>     12d0:	31 23 ff ff 	addic   r9,r3,-1
>     12d4:	7d 29 19 10 	subfe   r9,r9,r3
>     12d8:	0b 09 00 00 	tdnei   r9,0
>     12dc:	4e 80 00 20 	blr

That is because the asm (unlike the builtin) cannot be optimised by the
compiler.


Segher