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Re: [isocpp-parallel] Proposal for new memory_order_consume definition
- From: Linus Torvalds <torvalds at linux-foundation dot org>
- To: Markus Trippelsdorf <markus at trippelsdorf dot de>
- Cc: Paul McKenney <paulmck at linux dot vnet dot ibm dot com>, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, parallel at lists dot isocpp dot org, llvm-dev at lists dot llvm dot org, Will Deacon <will dot deacon at arm dot com>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, David Howells <dhowells at redhat dot com>, Peter Zijlstra <peterz at infradead dot org>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, Luc Maranget <luc dot maranget at inria dot fr>, Andrew Morton <akpm at linux-foundation dot org>, Jade Alglave <j dot alglave at ucl dot ac dot uk>, Ingo Molnar <mingo at kernel dot org>
- Date: Sun, 28 Feb 2016 08:13:51 -0800
- Subject: Re: [isocpp-parallel] Proposal for new memory_order_consume definition
- Authentication-results: sourceware.org; auth=none
- References: <20160218011033 dot GA1505 at linux dot vnet dot ibm dot com> <20160220021516 dot 4898897 dot 32908 dot 5212 at gmail dot com> <20160220195318 dot GF3522 at linux dot vnet dot ibm dot com> <CAPUmR1bw=N4NkjAK1zn_X0+84KEaEAM6HZCHZJy_txqC9hMgSg at mail dot gmail dot com> <20160227170615 dot GU3522 at linux dot vnet dot ibm dot com> <CA+55aFyHmykKc=YybJMo9ZUO352MY5noJVB4-K1Lkjmw4UHXfA at mail dot gmail dot com> <20160227231033 dot GW3522 at linux dot vnet dot ibm dot com> <20160228082702 dot GA300 at x4>
On Sun, Feb 28, 2016 at 12:27 AM, Markus Trippelsdorf
>> > -fno-strict-overflow
> 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.
They really don't.
Have you ever seen code that cared about signed integer overflow?
Yeah, getting it right can make the compiler generate an extra ALU
instruction once in a blue moon, but trust me - you'll never notice.
You *will* notice when you suddenly have a crash or a security issue
due to bad code generation, though.
The idiotic C alias rules aren't even worth discussing. They were a
mistake. The kernel doesn't use some "C dialect pretty far from
standard C". Yeah, let's just say that the original C designers were
better at their job than a gaggle of standards people who were making
bad crap up to make some Fortran-style programs go faster.
They don't speed up normal code either, they just introduce undefined
behavior in a lot of code.
And deleting NULL pointer checks because somebody made a mistake, and
then turning that small mistake into a real and exploitable security
hole? Not so smart either.
The fact is, undefined compiler behavior is never a good idea. Not for
Performance doesn't come from occasional small and odd
micro-optimizations. I care about performance a lot, and I actually
look at generated code and do profiling etc. None of those three
options have *ever* shown up as issues. But the incorrect code they
generate? It has.