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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Heiko Carstens' <hca@linux.ibm.com>,
        Nick Desaulniers <ndesaulniers@google.com>
CC:     Jens Axboe <axboe@kernel.dk>, Stefan Haberland <sth@linux.ibm.com>,
        =?iso-8859-1?Q?Jan_H=F6ppner?= <hoeppner@linux.ibm.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "llvm@lists.linux.dev" <llvm@lists.linux.dev>
Subject: RE: [PATCH 1/1] s390/dasd: fix string length handling
Thread-Topic: [PATCH 1/1] s390/dasd: fix string length handling
Thread-Index: AQHZ2cWnhKdW55JwuUOen/9fPqAWJK//8duQgAD1MkKAAAt+0A==
Date:   Tue, 29 Aug 2023 08:32:09 +0000
Message-ID: <51f70b0ad37f469a93242db9cfc6d9e8@AcuMS.aculab.com>
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From: Heiko Carstens
> Sent: 29 August 2023 08:49
> 
> 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...

And, if -ffreestanding is set (as in some kernel builds), the compiler
can't assume what strlen() does.

	David

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