RE: Control Dependencies vs C Compilers

[David Laight <David.Laight@ACULAB.COM>]

From: Peter Zijlstra
> Sent: 06 October 2020 12:47
> Hi,
> 
> Let's give this linux-toolchains thing a test-run...
> 
> As some of you might know, there's a bit of a discrepancy between what
> compiler and kernel people consider 'valid' use of the compiler :-)
> 
> One area where this shows up is in implicit (memory) ordering provided
> by the hardware, which we kernel people would like to use to avoid
> explicit fences (expensive) but which the compiler is unaware of and
> could ruin (bad).
...
> 
> In short, the control dependency relies on the hardware never
> speculating stores (instant OOTA) to provide a LOAD->STORE ordering.
> That is, a LOAD must be completed to resolve a conditional branch, the
> STORE is after the branch and cannot be made visible until the branch is
> determined (which implies the load is complete).
> 
> However, our 'dear' C language has no clue of any of this.
> 
> So given code like:
> 
> 	x = *foo;
> 	if (x > 42)
> 		*bar = 1;
> 
> Which, if literally translated into assembly, would provide a
> LOAD->STORE order between foo and bar, could, in the hands of an
> evil^Woptimizing compiler, become:
> 
> 	x = *foo;
> 	*bar = 1;
> 
> because it knows, through value tracking, that the condition must be
> true.
> 
> Our Documentation/memory-barriers.txt has a Control Dependencies section
> (which I shall not replicate here for brevity) which lists a number of
> caveats. But in general the work-around we use is:
> 
> 	x = READ_ONCE(*foo);
> 	if (x > 42)
> 		WRITE_ONCE(*bar, 1);

An alternative is to 'persuade' the compiler that
any 'tracked' value for a local variable is invalid.
Rather like the way that barrier() 'invalidates' memory.
So you generate:

	x = *foo
	asm ("" : "+r" (x));
	if (x > 42)
		*bar = 1;

Since the "+r" constraint indicates that the value of 'x'
might have changed it can't optimise based on any
presumed old value.
(Unless it looks inside the asm opcodes...)

	David

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