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Diffstat (limited to 'Documentation/memory-barriers.txt')
| -rw-r--r-- | Documentation/memory-barriers.txt | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 22a969cdd476..1073e019ef06 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -694,6 +694,24 @@ Please note once again that the stores to 'b' differ. If they were identical, as noted earlier, the compiler could pull this store outside of the 'if' statement. +You must also be careful not to rely too much on boolean short-circuit +evaluation. Consider this example: + + q = ACCESS_ONCE(a); + if (a || 1 > 0) + ACCESS_ONCE(b) = 1; + +Because the second condition is always true, the compiler can transform +this example as following, defeating control dependency: + + q = ACCESS_ONCE(a); + ACCESS_ONCE(b) = 1; + +This example underscores the need to ensure that the compiler cannot +out-guess your code. More generally, although ACCESS_ONCE() does force +the compiler to actually emit code for a given load, it does not force +the compiler to use the results. + Finally, control dependencies do -not- provide transitivity. This is demonstrated by two related examples, with the initial values of x and y both being zero: |