Andrew Trick 7205e46f41 Disable -O copy propagation (lifetime shortening) by default ...

Problem: We continue to uncover code that assumes either precise local
variable lifetimes (to the end of the lexical scope) or extended
temporary lifetimes (to the end of the statement). These bugs require
heroic debugging to find the root cause. Because they only show up in
Release builds, they often manifest just before the affected project
“ships” under an impending deadline.

We now have enough information from projects that have been tested
with copy propagation that we can both understand common patterns and
identify some specific APIs that may cause trouble. We know what API
annotations the compiler will need for helpful warnings and can begin
adding those annotations.

Disabling copy propagation now is only a temporary deferral, we will
still need to bring it back by default. However, by then we should
have:

- LLDB and runtime support for debugging deinitialized objects

- A variant of lifetime sortening that can run in Debug builds to
  catch problems before code ships

- Static compiler warnings for likely invalid lifetime assumptions

- Source annotations that allow those warnings to protect programmers
  against existing dangerous APIs

In the meantime...

Projects can experiment with the behavior and gradually migrate.

Copy propagation will automatically be enabled in -enable-ossa-modules
mode. It is important to work toward a single performance
target. Supporting full OSSA and improving ARC performance without
copy propagation would be prohibitively complicated.

rdar://76438920 (Temporarily disable -O copy propagation by default)

2021-04-09 00:12:09 -07:00

21 KiB

Raw Blame History

View Raw