This document discusses some of the design decisions made during the development of Darjeeling.
Unlike the majority of search-based program repair tools that implement
GenProg-style statement transformations, Darjeeling relies on
a prepending (rather than appending) statements to insert new statements
into the program.
By restricting the only prepending before statements, liveness and scope
information only needs to be computed immediately before the statement.
In contrast, GenProg computes the set of variables that are live before
and after a given statement. Unfortunately, Clang's liveness analysis
API only exposes the set of variables that are live before a statement.
Since append-statement is the only transformation that requires knowledge
of the set of variables that are live after the target statement, we can
avoid the need to compute this information by using prepend-statement
instead.
Regardless of whether one chooses to use prepend-statement or
append-statement, the search will not be able to insert statements at
certain points in the program. append-statement prevents insertion
at the start of a block, and prepend-statement prevents insertion at
the end of a block. Darjeeling solves this problem by using
prepend-statement exclusively and injecting an empty (and invisible!)
statement at the end of each block.