Combinatorial code explosion with nested functions of unknown arity
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ClojureScript 0.0-2322
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Micha thanks for clarifying the issue further. I see the problem and I think we can do the SSA bit when we don't have arity information.
Comment made by: michaniskin
David, I have seen this with Hoplon applications. Hoplon code tends to have the sort of deeply nested expressions you see there, because the DOM tree can be both quite broad and quite deep.
In a real application we found that when attempting to compile with advanced optimizations GClosure would generate gigabytes and gigabytes of JavaScript and never finish. Without optimizations the application would take five minutes or more to load in FireFox and Safari (Chrome, for some reason, was okay, which threw us off initially.)
In fact, I ended up writing a macro to perform a sort of SSA unfolding and flattening in order to work around the issue. The macro basically transforms this:
into this:
The macro is here: https://github.com/tailrecursion/hoplon/blob/5.10.23/src/tailrecursion/hoplon.clj#L101-L105 and it's applied automatically to the contents of the <body> of the document here: https://github.com/tailrecursion/hoplon/blob/5.10.23/src/tailrecursion/hoplon.clj#L119-L124
When I wrap large expressions with this macro the problem is mitigated. It's obviously a filthy hack and not an optimal solution by any means, but we can't make real applications without it.
Brendan do you have an actual example where this issue matters in practice? Thanks.
Comment made by: brendanyounger
This is an unknown arity issue, not one of native versus non-native. The indirection introduced by the explicit apply function leads to the duplication of code. Compiling with :static-fns doesn't materially change anything with :whitespace or :advanced compilation.
Whether or not gzip is able to undo the code explosion, the browser still has to parse O(2^{nesting depth}) characters which means the parsing time is increased in proportion and, likely, the internal representation of the javascript is as well. For a mobile phone with constrained memory and processing power, this is a big deal. I would argue that any code duplication leading to an exponential increase in code size is a serious defect.
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The following code, when compiled with :advanced optimizations leads to 32 repetitions of the inner number 2. On less trivial examples, it leads to multi-megabyte outputs very quickly. I have attached the compiler output for :whitespace and :advanced for the latest version of the CLJS compiler.