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various DCE-related bugfixes (replaces #36888) #37147

Merged
merged 6 commits into from
Aug 22, 2020
Merged

various DCE-related bugfixes (replaces #36888) #37147

merged 6 commits into from
Aug 22, 2020

Commits on Aug 21, 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 #29107 when DCE is turned on.
    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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  2. Remove edges from dead blocks from phi nodes

    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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  3. Fix possible merge cycle bug in cfg_simplify!

    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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  4. Fix bug in kill_edge! which caused edges to be removed from the wro… 

    …ng phi nodes
    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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  5. Fix bug in which uses of SSA values processed in just_fixup! were n… 

    …ot counted

This fixes #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`.
    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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  6. Fix small bug in #36684 

    PR #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.
    @yhls @vchuravy
    yhls authored and vchuravy committed Aug 21, 2020
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