[Webkit-unassigned] [Bug 84553] DFG should have control flow graph simplification

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=84553





--- Comment #5 from Filip Pizlo <fpizlo at apple.com>  2012-04-24 01:33:11 PST ---
I've decided that:

- We should have Phantom nodes in place of GetLocals in the constant folder. This means that Phantom nodes participate in operand-related liveness. So you may have Phantom(SetLocal), Phantom(Flush), Phantom(SetArgument), or Phantom(Phi). The control flow simplification will forward Phantom(operand instruction) to Phantom(source of the operand instruction) if the operand's value is known locally due to a SetLocal.

- We need to clean up unreachable blocks correctly. This means removing dead Phi edges. More subtly, it means that if we jettison a block - which may not render it unreachable, yet - then we must remove those Phi edges outgoing from that block that targeted the predecessor that we are killing.

- We need IR verification. Many of the invariants that we're allegedly upholding in this phase are not documented anywhere and may already be invalidated by bugs in other parts of the compiler. We need a phase that enforces the invariants that we're interested in. We can disable this phase for release builds, but it should always be on in debug builds.

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