Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Wed, 1 Jul 2020 11:10:54 +0200
From: Peter Zijlstra <>
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 <>,,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <>
Subject: Re: [PATCH 00/22] add support for Clang LTO

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:
> > 
> >
> > 
> > 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.

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.