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SubjectRe: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()
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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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