Andrew Trick 39db766257 Restrict CopyForwarding (and destroy hoisting) to OSSA SIL. ...

The premise of CopyForwarding was that memory locations have their own
ownership lifetime. We knew this was unmaintainable at the time, and
that was the original incentive for SIL opaque values, aided by
OSSA. In the meantime, we've been relying on SILGen producing
reasonable SIL patterns. Unfortunately, the CopyForwarding pass is
still with us while we make major changes to SIL ownership patterns
and agressively optimize ownership. That introduces risk.

Ultimately, the entire CopyForwarding pass should be redesigned for
OSSA-only and destroy hoisting should be a simple OSSA utility where
most of the work is done by AccessPath::collectUses.

But in the meantime, we should remove the source of risk by limiting
the CopyForwarding pass to OSSA. Any performance regressions will be
recovered as OSSA moves later in the pipeline. After that, opaque
values will improve even more over the current state by handling
generic SIL using the more powerful CopyPropagation pass.

Fixes rdar://71584600 (miscompile in CopyForwarding's release hoisting)

Here's an example of the kind of SIL the CopyForwarding does not
anticipate (although it only actually miscompiles in much more obscure
scenarios, which is why it's so dangerous):

bb0(%0 : $AnyObject):
  %alloc1 = alloc_stack $AnyObject
  store %0 to %objaddr : $*AnyObject
  %ref = load %objaddr : $*AnyObject
  %alloc2 = alloc_stack $ObjWrapper
  # The in-memory reference is destroyed before retaining the loaded ref.
  copy_addr [take] %alloc1 to [initialization] %alloc2 : $*ObjWrapper
  retain_value %ref : $AnyObject

2020-11-19 21:52:04 -08:00

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