From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 00/22] add support for Clang LTO Date: Wed, 1 Jul 2020 11:10:54 +0200 Message-ID: <20200701091054.GW4781@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200630203016.GI9247@paulmck-ThinkPad-P72> Sender: linux-kbuild-owner@vger.kernel.org To: "Paul E. McKenney" Cc: Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" List-Id: linux-arch.vger.kernel.org On Tue, Jun 30, 2020 at 01:30:16PM -0700, Paul E. McKenney wrote: > On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote: > > I'm not convinced C11 memory_order_consume would actually work for us, > > even if it would work. That is, given: > > > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > > > only pointers can have consume, but like I pointed out, we have code > > that relies on dependent loads from integers. > > I agree that C11 memory_order_consume is not normally what we want, > given that it is universally promoted to memory_order_acquire. > > However, dependent loads from integers are, if anything, more difficult > to defend from the compiler than are control dependencies. This applies > doubly to integers that are used to index two-element arrays, in which > case you are just asking the compiler to destroy your dependent loads > by converting them into control dependencies. Yes, I'm aware. However, as you might know, I'm firmly in the 'C is a glorified assembler' camp (as I expect most actual OS people are, out of necessity if nothing else) and if I wanted a control dependency I would've bloody well written one. I think an optimizing compiler is awesome, but only in so far as that optimization is actually helpful -- and yes, I just stepped into a giant twilight zone there. That is, any optimization that has _any_ controversy should be controllable (like -fno-strict-overflow -fno-strict-aliasing) and I'd very much like the same here. In a larger context, I still think that eliminating speculative stores is both necessary and sufficient to avoid out-of-thin-air. So I'd also love to get some control on that. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57626 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729571AbgGAJLG (ORCPT ); Wed, 1 Jul 2020 05:11:06 -0400 Date: Wed, 1 Jul 2020 11:10:54 +0200 From: Peter Zijlstra Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200701091054.GW4781@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200630203016.GI9247@paulmck-ThinkPad-P72> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Paul E. McKenney" Cc: Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Message-ID: <20200701091054.QX3_iq3u-CNsdboCE1WkLzsGTG3hImWELOYPCGz-jnY@z> On Tue, Jun 30, 2020 at 01:30:16PM -0700, Paul E. McKenney wrote: > On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote: > > I'm not convinced C11 memory_order_consume would actually work for us, > > even if it would work. That is, given: > > > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > > > only pointers can have consume, but like I pointed out, we have code > > that relies on dependent loads from integers. > > I agree that C11 memory_order_consume is not normally what we want, > given that it is universally promoted to memory_order_acquire. > > However, dependent loads from integers are, if anything, more difficult > to defend from the compiler than are control dependencies. This applies > doubly to integers that are used to index two-element arrays, in which > case you are just asking the compiler to destroy your dependent loads > by converting them into control dependencies. Yes, I'm aware. However, as you might know, I'm firmly in the 'C is a glorified assembler' camp (as I expect most actual OS people are, out of necessity if nothing else) and if I wanted a control dependency I would've bloody well written one. I think an optimizing compiler is awesome, but only in so far as that optimization is actually helpful -- and yes, I just stepped into a giant twilight zone there. That is, any optimization that has _any_ controversy should be controllable (like -fno-strict-overflow -fno-strict-aliasing) and I'd very much like the same here. In a larger context, I still think that eliminating speculative stores is both necessary and sufficient to avoid out-of-thin-air. So I'd also love to get some control on that.