linux-kernel - Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for Clang

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Message-ID: <alpine.LNX.2.23.453.2011261031290.6@nippy.intranet>
Date:   Thu, 26 Nov 2020 11:30:36 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
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Subject: Re: [Intel-wired-lan] [PATCH 000/141] Fix fall-through warnings for
 Clang



On Wed, 25 Nov 2020, Nick Desaulniers wrote:

> On Wed, Nov 25, 2020 at 1:33 PM Finn Thain <fthain@...egraphics.com.au> wrote:
> >
> > Or do you think that a codebase can somehow satisfy multiple checkers 
> > and their divergent interpretations of the language spec?
> 
> Have we found any cases yet that are divergent? I don't think so. 

You mean, aside from -Wimplicit-fallthrough? I'm glad you asked. How about 
-Wincompatible-pointer-types and -Wframe-larger-than?

All of the following files have been affected by divergent diagnostics 
produced by clang and gcc.

arch/arm64/include/asm/neon-intrinsics.h
arch/powerpc/xmon/Makefile
drivers/gpu/drm/i915/Makefile
drivers/gpu/drm/i915/i915_utils.h
drivers/staging/media/atomisp/pci/atomisp_subdev.c
fs/ext4/super.c
include/trace/events/qla.h
net/mac80211/rate.c
tools/lib/string.c
tools/perf/util/setup.py
tools/scripts/Makefile.include

And if I searched for 'smatch' or 'coverity' instead of 'clang' I'd 
probably find more divergence.

Here are some of the relevant commits.

0738c8b5915c7eaf1e6007b441008e8f3b460443
9c87156cce5a63735d1218f0096a65c50a7a32aa
babaab2f473817f173a2d08e410c25abf5ed0f6b
065e5e559555e2f100bc95792a8ef1b609bbe130
93f56de259376d7e4fff2b2d104082e1fa66e237
6c4798d3f08b81c2c52936b10e0fa872590c96ae
b7a313d84e853049062011d78cb04b6decd12f5c
093b75ef5995ea35d7f6bdb6c7b32a42a1999813

And before you object, "but -Wconstant-logical-operand is a clang-only 
warning! it can't be divergent with gcc!", consider that the special cases 
added to deal with clang-only warnings have to be removed when gcc catches 
up, which is more churn. Now multiply that by the number of checkers you 
care about.