Changeset a2545593 for doc/theses/aaron_moss_PhD/phd/resolution-heuristics.tex

Timestamp:

Mar 13, 2019, 10:20:24 AM (4 years ago)

Author:

Aaron Moss <a3moss@…>

Branches:

ADT, aaron-thesis, arm-eh, cleanup-dtors, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, pthread-emulation, qualifiedEnum

Children:

320e72a2, ab3a69c

Parents:

53bb8f1

Message:

Add ascription casts to future work

File:

• doc/theses/aaron_moss_PhD/phd/resolution-heuristics.tex (modified) (1 diff)

doc/theses/aaron_moss_PhD/phd/resolution-heuristics.tex

@@ -381 +381 @@
 The main challenge to implement this approach in \CFACC{} is applying the implicit conversions generated by the resolution process in the code-generation for the thunk functions that \CFACC{} uses to pass type assertions to their requesting functions with the proper signatures.
 
-Though performance of the existing algorithms is promising, some further optimizations do present themselves. 
+One \CFA{} feature that could be added to improve the ergonomics of overload selection is an \emph{ascription cast}; as discussed in Section~\ref{expr-cost-sec}, the semantics of the C cast operator are to choose the cheapest argument interpretation which is convertable to the target type, using the conversion cost as a tie-breaker. 
+An ascription cast would reverse these priorities, choosing the argument interpretation with the cheapest conversion to the target type, only using interpretation cost to break ties\footnote{A possible stricter semantics would be to select the cheapest interpretation with a zero-cost conversion to the target type, reporting a compiler error otherwise.}. 
+This would allow ascription casts to the desired return type to be used for overload selection:
+
+\begin{cfa}
+int f$\(_1\)$(int);
+int f$\(_2\)$(double);
+int g$\(_1\)$(int);
+double g$\(_2\)$(long);
+
+f((double)42);  $\C[4.5in]{// select f\(_2\) by parameter cast}$
+(as double)g(42); $\C[4.5in]{// select g\(_2\) by return ascription cast}$
+(double)g(42); $\C[4.5in]{// select g\(_1\) NOT g\(_2\) because of parameter conversion cost}$
+\end{cfa}
+
+Though performance of the existing resolution algorithms is promising, some further optimizations do present themselves. 
 The refined cost model discussed in Section~\ref{conv-cost-sec} is more expressive, but requires more than twice as many fields; it may be fruitful to investigate more tightly-packed in-memory representations of the cost-tuple, as well as comparison operations that require fewer instructions than a full lexicographic comparison. 
 Integer or vector operations on a more-packed representation may prove effective, though dealing with the negative-valued $specialization$ field may require some effort.