Re: [PATCH 00/22] add support for Clang LTO

Paul E. McKenney
2020-06-30T20:30:37Z

On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2020 at 09:19:31PM +0200, Marco Elver wrote:
> > I was asked for input on this, and after a few days digging through some
> > history, thought I'd comment. Hope you don't mind.
>
> Not at all, being the one that asked :-)
>
> > First of all, I agree with the concerns, but not because of LTO.
> >
> > To set the stage better, and summarize the fundamental problem again:
> > we're in the unfortunate situation that no compiler today has a way to
> > _efficiently_ deal with C11's memory_order_consume
> > [https://lwn.net/Articles/588300/]. If we did, we could just use that
> > and be done with it. But, sadly, that doesn't seem possible right now --
> > compilers just say consume==acquire.
>
> 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.

> > Will suggests doing the same in the
> > kernel: https://lkml.kernel.org/r/20200630173734.14057-19-will@kernel.org
>
> PowerPC would need a similar thing, it too will not preserve causality
> for control dependecies.
>
> > What we're most worried about right now is the existence of compiler
> > transformations that could break data dependencies by e.g. turning them
> > into control dependencies.
>
> Correct.
>
> > If this is a real worry, I don't think LTO is the magical feature that
> > will uncover those optimizations. If these compiler transformations are
> > real, they also exist in a normal build!
>
> Agreed, _however_ with the caveat that LTO could make them more common.
>
> After all, with whole program analysis, the compiler might be able to
> more easily determine that our pointer @ptr is only ever assigned the
> values of &A, &B or &C, while without that visibility it would not be
> able to determine this.
>
> Once it knows @ptr has a limited number of determined values, the
> conversion into control dependencies becomes much more likely.

Which would of course break dependent loads.

> > And if we are worried about them, we need to stop relying on dependent
> > load ordering across the board; or switch to -O0 for everything.
> > Clearly, we don't want either.
>
> Agreed.
>
> > Why do we think LTO is special?
>
> As argued above, whole-program analysis would make it more likely. But I
> agree the fundamental problem exists independent from LTO.
>
> > But as far as we can tell, there is no evidence of the dreaded "data
> > dependency to control dependency" conversion with LTO that isn't there
> > in non-LTO builds, if it's even there at all. Has the data to control
> > dependency conversion been encountered in the wild? If not, is the
> > resulting reaction an overreaction? If so, we need to be careful blaming
> > LTO for something that it isn't even guilty of.
>
> It is mostly paranoia; in a large part driven by the fact that even if
> such a conversion were to be done, it could go a very long time without
> actually causing problems, and longer still for such problems to be
> traced back to such an 'optimization'.
>
> That is, the collective hurt from debugging too many ordering issues.
>
> > So, we are probably better off untangling LTO from the story:
> >
> > 1. LTO or no LTO does not matter. The LTO series should not get tangled
> > up with memory model issues.
> >
> > 2. The memory model question and problems need to be answered and
> > addressed separately.
> >
> > Thoughts?
>
> How hard would it be to creates something that analyzes a build and
> looks for all 'dependent load -> control dependency' transformations
> headed by a volatile (and/or from asm) load and issues a warning for
> them?
>
> This would give us an indication of how valuable this transformation is
> for the kernel. I'm hoping/expecting it's vanishingly rare, but what do
> I know.
>

This could be quite useful!

Thanx, Paul