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Message-ID: <f6f7ae76-c6d9-9a5e-043d-07f6d0270ceb@csgroup.eu> 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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