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6 Local call analysis

All calls to local functions (known named functions and LETs) are resolved to the exact LAMBDA node which is to be called. If the call is syntactically illegal, then we emit a warning and mark the reference as :notinline, forcing the call to be a full call. We don’t even think about converting APPLY calls; APPLY is not special-cased at all in ICR. We also take care not to convert calls in the top-level component, which would join it to normal code. Calls to functions with rest args and calls with non-constant keywords are also not converted.

We also convert MV-Calls that look like MULTIPLE-VALUE-BIND to local calls, since we know that they can be open-coded. We replace the optional dispatch with a call to the last optional entry point, letting MV-Call magically default the unsupplied values to NIL.

When ICR optimizations discover a possible new local call, they explicitly invoke local call analysis on the code that needs to be reanalyzed.

[### Let conversion. What it means to be a let. Argument type checking done by caller. Significance of local call is that all callers are known, so special call conventions may be used.] A lambda called in only one place is called a “let” call, since a Let would turn into one.

In addition to enabling various ICR optimizations, the let/non-let distinction has important environment significance. We treat the code in function and all of the lets called by that function as being in the same environment. This allows exits from lets to be treated as local exits, and makes life easy for environment analysis.

Since we will let-convert any function with only one call, we must be careful about cleanups. It is possible that a lexical exit from the let function may have to clean up dynamic bindings not lexically apparent at the exit point. We handle this by annotating lets with any cleanup in effect at the call site. The cleanup for continuations with no immediately enclosing cleanup is the lambda that the continuation is in. In this case, we look at the lambda to see if any cleanups need to be done.

Let conversion is disabled for entry-point functions, since otherwise we might convert the call from the XEP to the entry point into a let. Then later on, we might want to convert a non-local reference into a local call, and not be able to, since once a function has been converted to a let, we can’t convert it back.

A function’s return node may also be deleted if it is unreachable, which can happen if the function never returns normally. Such functions are not lets.