linux-kernel - Re: [PATCH 1/1] s390/dasd: fix string length handling

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Message-ID: <20230829074854.7031-A-hca@linux.ibm.com>
Date:   Tue, 29 Aug 2023 09:48:54 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Jens Axboe <axboe@...nel.dk>,
        Stefan Haberland <sth@...ux.ibm.com>,
        Jan Höppner <hoeppner@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 1/1] s390/dasd: fix string length handling

On Mon, Aug 28, 2023 at 03:51:00PM -0700, Nick Desaulniers wrote:
> On Mon, Aug 28, 2023 at 05:18:37PM +0000, David Laight wrote:
> > From: Heiko Carstens
> > > Sent: 28 August 2023 16:32
> > >  	if (strlen(uid.vduit) > 0)
> > 
> > Does the compiler know enough to optimise that brain-dead test?
> > 
> 
> For the purposes of skipping diagnostics, no; clang performs semantic
> analysis BEFORE optimization (which is handled by LLVM). As such, clang
> will produce diagnostics on dead code.
> 
> Partly because LLVM isn't very ergonomic at emitting diagnostics from
> the backend, partly because Clang code owner and developers don't want
> clang to emit diagnostics dependent on optimization level.
> 
> I disagree with my compatriots, and you can read more thoughts here:
> https://discourse.llvm.org/t/rfc-improving-clangs-middle-and-back-end-diagnostics/69261?u=nickdesaulniers

Maybe I misunderstand what you write above, however clang (latest+greatest)
does indeed optimize the strlen() away and generates code which only tests
if uid.vduit[0] is zero or not.

Unlike gcc, which does not optimize this away and which uses the strlen()
inline assembly provided via string.h...