Poor scaling with optimized builds involving OSSA & copy propagation in the 6.2 compiler

[jamieQ]

Description

copy propagation in optimized builds scales poorly on code with large basic blocks, and can end up taking a very long time to compile due to redundant work when canonicalizing OSSA lifetimes.

Reproduction

using code of the following form:

final class A {}

final class B {
  let prop1: A
  let prop2: A
  ...
  let propN: A

  init() {
    self.prop1 = switch () { default: A() }
    self.prop2 = switch () { default: A() }
    ...
    self.propN = switch () { default: A() }
  }
}

build with:

swiftc -swift-version 5 -O -emit-sil <file>

build takes ~3.5 minutes with 1024 properties and scales up to ~43 minutes with 2048 properties

Expected behavior

compile time should increase more slowly on these examples.

Environment

Apple Swift version 6.2 (swiftlang-6.2.0.9.909 clang-1700.3.9.907)
Target: arm64-apple-macosx15.0

Additional information

more details can be found in this forum thread: https://forums.swift.org/t/poor-scaling-with-optimized-builds-involving-ossa-copy-propagation-in-the-6-2-compiler/80436