linux-kernel - Re: [PATCHv3, resend] powerpc: mm: radix

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Message-ID: <20220810135621.GX25951@gate.crashing.org>
Date:   Wed, 10 Aug 2022 08:56:21 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     nathan@...nel.org, ndesaulniers@...gle.com, mpe@...erman.id.au,
        Arnd Bergmann <arnd@...db.de>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCHv3, resend] powerpc: mm: radix_tlb: rearrange the if-else block

Hi!

On Wed, Aug 10, 2022 at 01:43:18PM +0200, Anders Roxell wrote:
> Clang warns:
> 
> arch/powerpc/mm/book3s64/radix_tlb.c:1191:23: error: variable 'hstart' is uninitialized when used here [-Werror,-Wuninitialized]
>                                 __tlbiel_va_range(hstart, hend, pid,
>                                                   ^~~~~~
> arch/powerpc/mm/book3s64/radix_tlb.c:1175:23: note: initialize the variable 'hstart' to silence this warning

This note often is bad advice: hiding problems instead of investigating
and solving them.  Bah.

If silencing warnings is your goal, look no further than "-w" :-)

> Rework the 'if (IS_ENABLE(CONFIG_TRANSPARENT_HUGEPAGE))' so hstart/hend
> always gets initialized, this will silence the warnings. That will also
> simplify the 'else' path. Clang is getting confused with these warnings,
> but the warnings is a false-positive.

If it is, please report that bug to clang?  It says "*is* uninitialized
when used here", there can not be false positives to statements like
that.  If the analysis was heutistical it should say "may be" or such.


Segher