Re: [RFC] LKMM: Add volatile_if()

[Will Deacon]

On Fri, Jun 04, 2021 at 11:43:59AM +0100, Will Deacon wrote:
> On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote:
> > With optimizing compilers becoming more and more agressive and C so far
> > refusing to acknowledge the concept of control-dependencies even while
> > we keep growing the amount of reliance on them, things will eventually
> > come apart.
> > 
> > There have been talks with toolchain people on how to resolve this; one
> > suggestion was allowing the volatile qualifier on branch statements like
> > 'if', but so far no actual compiler has made any progress on this.
> > 
> > Rather than waiting any longer, provide our own construct based on that
> > suggestion. The idea is by Alan Stern and refined by Paul and myself.
> > 
> > Code generation is sub-optimal (for the weak architectures) since we're
> > forced to convert the condition into another and use a fixed conditional
> > branch instruction, but shouldn't be too bad.
> > 
> > Usage of volatile_if requires the @cond to be headed by a volatile load
> > (READ_ONCE() / atomic_read() etc..) such that the compiler is forced to
> > emit the load and the branch emitted will have the required
> > data-dependency. Furthermore, volatile_if() is a compiler barrier, which
> > should prohibit the compiler from lifting anything out of the selection
> > statement.
> 
> When building with LTO on arm64, we already upgrade READ_ONCE() to an RCpc
> acquire. In this case, it would be really good to avoid having the dummy
> conditional branch somehow, but I can't see a good way to achieve that.

Thinking more on this, an alternative angle would be having READ_ONCE_CTRL()
instead of volatile_if. That would then expand (on arm64) to either
something like:

	LDR	X0, [X1]
	CBNZ	X0, 1f		// Dummy ctrl
1:

or, with LTO:

	LDAPR	X0, [X1]	// RCpc

and we'd avoid the redundancy.

Will