gh-106581: Fix two bugs in the code generator's copy optimization #108380
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I was comparing the last preceding poke with the *last* peek, rather than the *first* peek. Unfortunately this bug obscured another bug, which I have yet to fix: When the last preceding poke is UNUSED, the first peek disappears, leaving the variable unassigned.
Rename CopyEffect to CopyItem. Change CopyItem to contain StackItems instead of StackEffects. Update those StackItems when adjusting the manager higher or lower. Assert that those StackItems' offsets are equivalent.
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I apologize, when using this to split |
(I had different code in one of the branches during debugging. :-) Co-authored-by: Irit Katriel <[email protected]>
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When replacing matched pairs of push/pop (poke/peek), I was matching up the last poke in the previous "manager" with the wrong peek in the new one, causing some copying opportunities to be missed.
Fixing this revealed a more serious bug where we could end up skipping the copy of an 'unused' poke to a used peek, leaving the variable uninitialized. The fix for this required me to keep track of the original StackItems in the CopyEffect object, and use that to reconstruct the source when copying from 'unused'.
The net result so far is that this found two redundant initializations.