a611f8dbfc
Fix issue #78496 EarlyOtherwiseBranch finds MIR structures like: ``` bb0: { ... _2 = discriminant(X) ... switchInt(_2) -> [1_isize: bb1, otherwise: bb3] } bb1: { ... _3 = discriminant(Y) ... switchInt(_3) -> [1_isize: bb2, otherwise: bb3] } bb2: {...} bb3: {...} ``` And transforms them into something like: ``` bb0: { ... _2 = discriminant(X) _3 = discriminant(Y) _4 = Eq(_2, _3) switchInt(_4) -> [true: bb4, otherwise: bb3] } bb2: {...} // unchanged bb3: {...} // unchanged bb4: { switchInt(_2) -> [1_isize: bb2, otherwise: bb3] } ``` But that is not always a safe thing to do -- sometimes the early `otherwise` branch is necessary so the later block could assume the value of `discriminant(X)`. I am not totally sure what's the best way to detect that, but fixing #78496 should be easy -- we just check if `X` is a sub-expression of `Y`. A more precise test might be to check if `Y` contains a `Downcast(1)` of `X`, but I think this might be good enough. Fix #78496 |
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