Andrew Trick f764b8b615 Fix a bug in CopyForwarding. Bailout during destoy hoisting. ...

Once the algorithm has begun hoisting destroys globally, there's no
way to cleanly bailout. The previous attempt to bailout could result
in an assert or lost destroy in release mode.

This is continued fall-out from changes in the previous release to
upstream SILGen or mandatory passes, such as PredictableMemOps, that
no longer preserve natural variable lifetimes.

In this case, we end up with SIL like this before CopyForwarding:

bb(%arg)
%local_addr = alloc_stack
store %arg to %local_addr
%payload = switch_enum(%arg)
retain %arg
store %arg to %some_addr
destroy_addr %local_addr
release_value %arg

We're attempting to hoist the destroy_addr to its last use, but can't
because the lifetimes of the alloc_stack (%local_addr) and the value
being stored on the stack (%arg) have become mixed up by an upstream
pass. We actually detect this situation now in order to bail-out of
destroy hoisting. Sadly, the bailout might only partially recover in
the case of interesting control flow, as happens in the test case's
Graph.init function. This triggers an assert, but in release mode it
simply drops the destroy.

Fixed <rdar://problem/43888666> [SR-8526]: Memory leak after switch in
release configuration.

2018-08-31 18:29:15 -07:00

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