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Date:   Sat, 27 Feb 2021 11:31:08 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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()



Le 11/02/2021 à 15:30, Segher Boessenkool a écrit :
> On Thu, Feb 11, 2021 at 03:09:43PM +0100, Christophe Leroy wrote:
>> Le 11/02/2021 à 12:49, Segher Boessenkool a écrit :
>>> 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?
>>
>> We already made a try with __builtin_trap() 1,5 year ago, see
>> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr/
>>
>> The main problems encountered are:
>> - It is only possible to use it for BUG_ON, not for WARN_ON because GCC
>> considers it as noreturn. Is there any workaround ?
> 
> A trap is noreturn by definition:
> 
>   -- Built-in Function: void __builtin_trap (void)
>       This function causes the program to exit abnormally.
> 
>> - The kernel (With CONFIG_DEBUG_BUGVERBOSE) needs to be able to identify
>> the source file and line corresponding to the trap. How can that be done
>> with __builtin_trap() ?
> 
> The DWARF debug info should be sufficient.  Perhaps you can post-process
> some way?
> 
> You can create a trap that falls through yourself (by having a trap-on
> condition with a condition that is always true, but make the compiler
> not see that).  This isn't efficient though.
> 
> Could you file a feature request (in bugzilla)?  It is probably useful
> for generic code as well, but we could implement this for powerpc only
> if needed.
> 

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99299

Christophe

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