#12150: Compile time performance degradation on code that uses undefined/error with
CallStacks
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Reporter: thomie | Owner: (none)
Type: bug | Status: new
Priority: high | Milestone: 8.2.2
Component: Compiler | Version: 8.0.1
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Compile-time | Unknown/Multiple
performance bug | Test Case:
Blocked By: | Blocking:
Related Tickets: #10844 | Differential Rev(s):
Wiki Page: |
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Comment (by dfeuer):
OK, I have a guess about what's going on now. I didn't notice this before,
because the shapes were similar, but there's a big difference between how
7.10 and 8.0 handle this code in the very earliest stages. In particular,
`-ddump-ds` for 7.10 shows that `bool` has already inlined, exposing the
`undefined`s (which it already knows to be bottom), whereas that has not
happened in 8.0. Presumably, the slight optimization that takes place
along with desugaring sees `bool = undefined`, concludes that there's no
reason to have the extra identifier, and inlines it away altogether.
In 8.0 and 8.2, `bool` is not just `undefined`, but `undefined` ''applied
to a dictionary''. So suddenly it needs to actually produce a `bool`
binding, and doesn't figure out until demand analysis that `bool` is
always bottom.
I suspect that what we want to do is to shift just a bit of demand
analysis way up. `undefined` has demand signature
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