From: Markus Trippelsdorf via llvm-dev <llvm-dev@lists.llvm.org>
To: paulmck@linux.vnet.ibm.com
Cc: linux-arch@vger.kernel.org, gcc@gcc.gnu.org,
parallel@lists.isocpp.org, llvm-dev@lists.llvm.org,
will.deacon@arm.com, linux-kernel@vger.kernel.org,
dhowells@redhat.com, peterz@infradead.org,
Ramana.Radhakrishnan@arm.com,
Luc Maranget <luc.maranget@inria.fr>,
akpm@linux-foundation.org, Jade Alglave <j.alglave@ucl.ac.uk>,
Linus Torvalds <torvalds@linux-foundation.org>,
mingo@kernel.org
Subject: Re: [isocpp-parallel] Proposal for new memory_order_consume definition
Date: Sun, 28 Feb 2016 09:27:02 +0100 [thread overview]
Message-ID: <20160228082702.GA300@x4> (raw)
In-Reply-To: <20160227231033.GW3522@linux.vnet.ibm.com>
On 2016.02.27 at 15:10 -0800, Paul E. McKenney via llvm-dev wrote:
> On Sat, Feb 27, 2016 at 11:16:51AM -0800, Linus Torvalds wrote:
> > On Feb 27, 2016 09:06, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > wrote:
> > >
> > >
> > > But we do already have something very similar with signed integer
> > > overflow. If the compiler can see a way to generate faster code that
> > > does not handle the overflow case, then the semantics suddenly change
> > > from twos-complement arithmetic to something very strange. The standard
> > > does not specify all the ways that the implementation might deduce that
> > > faster code can be generated by ignoring the overflow case, it instead
> > > simply says that signed integer overflow invoked undefined behavior.
> > >
> > > And if that is a problem, you use unsigned integers instead of signed
> > > integers.
> >
> > Actually, in the case of there Linux kernel we just tell the compiler to
> > not be an ass. We use
> >
> > -fno-strict-overflow
>
> That is the one!
>
> > or something. I forget the exact compiler flag needed for "the standard is
> > as broken piece of shit and made things undefined for very bad reasons".
> >
> > See also there idiotic standard C alias rules. Same deal.
>
> For which we use -fno-strict-aliasing.
Do not forget -fno-delete-null-pointer-checks.
So the kernel obviously is already using its own C dialect, that is
pretty far from standard C.
All these options also have a negative impact on the performance of the
generated code.
--
Markus
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WARNING: multiple messages have this Message-ID (diff)
From: Markus Trippelsdorf <markus@trippelsdorf.de>
To: paulmck@linux.vnet.ibm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
linux-arch@vger.kernel.org, gcc@gcc.gnu.org,
parallel@lists.isocpp.org, llvm-dev@lists.llvm.org,
will.deacon@arm.com, linux-kernel@vger.kernel.org,
dhowells@redhat.com, peterz@infradead.org,
Ramana.Radhakrishnan@arm.com,
Luc Maranget <luc.maranget@inria.fr>,
akpm@linux-foundation.org, Jade Alglave <j.alglave@ucl.ac.uk>,
mingo@kernel.org
Subject: Re: [isocpp-parallel] Proposal for new memory_order_consume definition
Date: Sun, 28 Feb 2016 09:27:02 +0100 [thread overview]
Message-ID: <20160228082702.GA300@x4> (raw)
Message-ID: <20160228082702.-7-wHG-AW9RCnpfrSRYUAufpNJD7K8KAGtkwyZFYSds@z> (raw)
In-Reply-To: <20160227231033.GW3522@linux.vnet.ibm.com>
On 2016.02.27 at 15:10 -0800, Paul E. McKenney via llvm-dev wrote:
> On Sat, Feb 27, 2016 at 11:16:51AM -0800, Linus Torvalds wrote:
> > On Feb 27, 2016 09:06, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > wrote:
> > >
> > >
> > > But we do already have something very similar with signed integer
> > > overflow. If the compiler can see a way to generate faster code that
> > > does not handle the overflow case, then the semantics suddenly change
> > > from twos-complement arithmetic to something very strange. The standard
> > > does not specify all the ways that the implementation might deduce that
> > > faster code can be generated by ignoring the overflow case, it instead
> > > simply says that signed integer overflow invoked undefined behavior.
> > >
> > > And if that is a problem, you use unsigned integers instead of signed
> > > integers.
> >
> > Actually, in the case of there Linux kernel we just tell the compiler to
> > not be an ass. We use
> >
> > -fno-strict-overflow
>
> That is the one!
>
> > or something. I forget the exact compiler flag needed for "the standard is
> > as broken piece of shit and made things undefined for very bad reasons".
> >
> > See also there idiotic standard C alias rules. Same deal.
>
> For which we use -fno-strict-aliasing.
Do not forget -fno-delete-null-pointer-checks.
So the kernel obviously is already using its own C dialect, that is
pretty far from standard C.
All these options also have a negative impact on the performance of the
generated code.
--
Markus
next prev parent reply other threads:[~2016-02-28 8:27 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-18 1:10 Proposal for new memory_order_consume definition Paul E. McKenney
2016-02-20 2:15 ` [isocpp-parallel] " Tony V E
2016-02-20 19:53 ` Paul E. McKenney
2016-02-26 0:46 ` Hans Boehm via llvm-dev
2016-02-26 23:56 ` Lawrence Crowl
2016-02-27 17:06 ` Paul E. McKenney
2016-02-27 19:16 ` Linus Torvalds via llvm-dev
2016-02-27 23:10 ` Paul E. McKenney via llvm-dev
2016-02-27 23:10 ` Paul E. McKenney
2016-02-28 8:27 ` Markus Trippelsdorf via llvm-dev [this message]
2016-02-28 8:27 ` Markus Trippelsdorf
2016-02-28 16:13 ` Linus Torvalds
2016-02-28 16:50 ` via llvm-dev
2016-02-28 16:50 ` [llvm-dev] " cbergstrom
2016-02-29 17:37 ` Michael Matz
2016-02-29 17:57 ` Linus Torvalds via llvm-dev
2016-02-29 17:57 ` Linus Torvalds
2016-02-29 19:38 ` Lawrence Crowl
2016-02-29 21:10 ` James Y Knight via llvm-dev
2016-02-29 21:12 ` James Y Knight via llvm-dev
2016-02-29 21:12 ` [llvm-dev] " James Y Knight
2016-02-29 20:45 ` Toon Moene
2016-02-29 20:45 ` Toon Moene
2016-02-29 18:17 ` Michael Matz
2016-03-01 1:28 ` Paul E. McKenney via llvm-dev
2016-03-01 1:28 ` Paul E. McKenney
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