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Address review comments

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This commit is contained in:
Oliver Scherer 2019-12-20 19:46:38 +01:00
parent 9158e17997
commit 5f08df1047
1 changed files with 9 additions and 3 deletions
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12
src/librustc_mir/dataflow/mod.rs
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@ -651,16 +651,22 @@ pub trait BottomValue {
/// Merges `in_set` into `inout_set`, returning `true` if `inout_set` changed.
///
/// You usually don't need to override this, since it automatically applies
/// It is almost certainly wrong to override this, since it automatically applies
/// * `inout_set & in_set` if `BOTTOM_VALUE == true`
/// * `inout_set | in_set` if `BOTTOM_VALUE == false`
///
/// This means that if a bit is not `BOTTOM_VALUE`, it is propagated into all target blocks.
/// For clarity, the above statement again from a different perspective:
/// A block's initial bit value is `!BOTTOM_VALUE` if *any* predecessor block's bit value is
/// A bit in the block's entry set is `!BOTTOM_VALUE` if *any* predecessor block's bit value is
/// `!BOTTOM_VALUE`.
///
/// There are situations where you want the opposite behaviour: propagate only if *all*
/// predecessor blocks's value is `!BOTTOM_VALUE`. In that case you need to
/// predecessor blocks's value is `!BOTTOM_VALUE`.
/// E.g. if you want to know whether a bit is *definitely* set at a specific location. This
/// means that all code paths leading to the location must have set the bit, instead of any
/// code path leading there.
///
/// If you want this kind of "definitely set" analysis, you need to
/// 1. Invert `BOTTOM_VALUE`
/// 2. Reset the `entry_set` in `start_block_effect` to `!BOTTOM_VALUE`
/// 3. Override `join` to do the opposite from what it's doing now.