Fix speculative devirtualization to correctly use casted value [5.2] #29004
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…d value We used to emit this: checked_cast_br [exact] %0 : $Foo to $Bar, bb1, bb2 bb1(%1 : $Bar): %2 = unchecked_ref_cast %0 : $Foo to %1 : $Bar apply ...(..., %2) br ... bb2: ... This is not ownership SIL-safe, because we're re-using the original operand, which will have already been consumed at that point. The more immediate problem here is that this is actually not safe when combined with other optimizations. Suppose that after the speculative devirtualization, our function is inlined into another function where the operand was an upcast. Now we have this code: %0 = upcast ... checked_cast_br [exact] %0 : $Foo to $Bar, bb1, bb2 bb1(%1 : $Bar): %2 = unchecked_ref_cast %0 : $Foo to %1 : $Bar apply ...(..., %2) br ... bb2: ... The SILCombiner will simplify the unchecked_ref_cast of the upcast into an unchecked_ref_cast of the upcast's original operand. At this point, we have an unchecked_ref_cast between two unrelated class types. While this means the block bb1 is unreachable, we might perform further optimizations on it before we run the cast optimizer and delete it altogether. In particular, the devirtualizer follows chains of reference cast instructions, and it will get very confused if it finds this invalid cast between unrelated types. Fixes <rdar://problem/57712094>, <https://bugs.swift.org/browse/SR-11916>.
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@swift-ci Please test |
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@DougGregor do you mind approving this once @gottesmm reviews the master branch PR? #29003 |
DougGregor
approved these changes
Jan 6, 2020
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We used to emit this:
This is not ownership SIL-safe, because we're re-using the original
operand, which will have already been consumed at that point.
The more immediate problem here is that this is actually not safe
when combined with other optimizations. Suppose that after the
speculative devirtualization, our function is inlined into another
function where the operand was an upcast. Now we have this code:
The SILCombiner will simplify the unchecked_ref_cast of the upcast
into an unchecked_ref_cast of the upcast's original operand. At
this point, we have an unchecked_ref_cast between two unrelated
class types. While this means the block bb1 is unreachable, we
might perform further optimizations on it before we run the cast
optimizer and delete it altogether.
In particular, the devirtualizer follows chains of reference cast
instructions, and it will get very confused if it finds this invalid
cast between unrelated types.
Fixes rdar://problem/57712094, https://bugs.swift.org/browse/SR-11916.