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In addition to being used to tell whether a particular unsafe template might get emitted, we can also use it to give better efficiency notes: – We can say what is wrong with the call types, rather than just saying we failed to open-code. – We can tell whether any of the “better” templates could possibly apply, i.e. is the inapplicability of a template because of inadequate type information or because the type is just plain wrong. We don’t want to flame people when a template that couldn’t possibly match doesn’t match, e.g. complaining that we can’t use fixnum+ when the arguments are known to be floats.
This is how we give better efficiency notes:
The Template-Note is a short noun-like string without capitalization or punctuation that describes what the template “does”, i.e. we say “Unable to do ~A, doing ~A instead.”
The Cost is moved from the Vop-Info to the Template structure, and is used to determine the “goodness” of possibly applicable templates. [Could flush Template/Vop-Info distinction] The cost is used to choose the best applicable template to emit, and also to determine what better templates we might have been able to use.
A template is possibly applicable if there is an intersection between all of the arg/result types and the corresponding arg/result restrictions, i.e. the template is not clearly impossible: more declarations might allow it to be emitted.