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updating domtrees dynamically, removing all unreachable blocks #33730

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updating domtrees dynamically, removing all unreachable blocks #33730

wants to merge 16 commits into from

Commits on Aug 3, 2020

  1. Decline to remove phi nodes when this might cause forward references 

    Phi nodes are optimized away when there is only one predecessor, but this can
cause problems in dead loops because forward references can be created, leading
to issues with optimization passes that look at all code, dead or not. This
fixes issue JuliaLang#29107 when DCE is turned on.
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  2. Remove edges from dead blocks from phi nodes

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  3. Fix possible merge cycle bug in cfg_simplify!

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  4. Fix bug in kill_edge! which caused edges to be removed from the wro… 

    …ng phi nodes
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  5. Fix bug in which uses of SSA values processed in just_fixup! were n… 

    …ot counted

This fixes JuliaLang#29253, which was caused by `simple_dce!` erroneously erasing SSA
values that did not appear to be used, because these uses were only discovered
in `just_fixup!` at the end of iterating over an `IncrementalCompact`.
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  6. Fix small bug in JuliaLang#36684 

    PR JuliaLang#36684 changes `iterate(IncrementalCompact)` to return an extra index, but
leaves its arguments unchanged. However, the PR decremented the index argument
in a particular recursive call to `iterate`. This caused `iterate` not to
recognise that it was done when `allow_cfg_transforms` was turned on.
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  7. Implement algorithm for updating dominator tree with CFG edge inserti… 

    …ons and deletions

The DFS tree associated with a CFG now keeps track of postorder as well as
preorder numbers. The DFS tree, as well as the state associated with the SNCA
algorithm for finding (immediate) dominators is now stored in DomTree and
reused for Dynamic SNCA.
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  8. Remove dead blocks as determined by reachability instead of number of… 

    … predecessors

For now, just construct the domtree when we make an `IncrementalCompact` rather
than try to update it (the domtree) incrementally.
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  9. Make domtree construction take Vector{BasicBlock} as input instead … 

    …of CFG

This is in anticipation of domtrees being added to CFGs.
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  10. Add a domtree to every CFG, avoiding constructing them explicitly whe… 

    …n needed

Every time a CFG is created, its corresponding dominator tree is as well.
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  11. Update domtrees dynamically when CFGs get modified, and fix bug in dy… 

    …namic domtree implementation
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  12. Update domtree dynamically in cfg_simplify

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  13. Kill statements in blocks that are unreachable, add tests for killing… 

    … edges

This change only affects statements that we have yet to encounter after killing
an edge, while iterating through `IncrementalCompact`. Statements in dead
blocks that come before the point at which the edge is killed are killed in
`kill_edge!`, when the edge is killed.
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  14. Fix infinite loop bug in kill_edge! 

    If a statement was `nothing`, `kill_edge!` would never move on from trying to
kill it because the index wasn't incremented.
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  15. Move BasicBlock to separate source file 

    This is so we can add type declarations to fields in ir.jl that are domtrees,
by breaking the dependency loop between domtree.jl (uses basic blocks but
defines domtrees) and ir.jl (uses domtrees but defined basic blocks).
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  16. Enable CFG transforms; turn DCE back on globally

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