Messages in this thread | | | From | David Laight <> | Subject | RE: Control Dependencies vs C Compilers | Date | Tue, 6 Oct 2020 12:37:06 +0000 |
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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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