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As part of sys_core_fold, variables involved in bit syntax
matching would be annotated when it would be safe for a later
pass to do the delayed sub-binary creation optimization.
An implicit assumption regarding the annotation was that the
code must not be further optimized. That assumption was broken
in 05130e48555891, which introduced a fixpoint iteration
(applying the optimizations until there were no more changes).
That means that a variable could be annotated as safe for
reusing the match context in one iteration, but a later iteration
could rewrite the code in a way that would make the optimization
unsafe.
One way to fix this would be to clear all reuse_for_context
annotations before each iteration. But that would be wasteful.
Instead I chose to fix the problem by moving out the annotation
code to a separate pass (sys_core_bsm) that is run later after
all major optimizations of Core Erlang has been done.
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