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Informally, a call graph represents calls between entities in a
given program. The call graphs that compilers compute to determine
the applicability of an optimization must typically be conservative:
a call may be omitted only if it can never occur in any execution of
the program. Numerous software engineering tools also extract call
graphs with the expectation that they will help software engineers
increase their understanding of a program. The requirements placed
on software engineering tools when computing call graphs are
typically more relaxed than for compilers. For example, some false
negatives---calls that can in fact take place in some execution of
the program, but which are omitted from the call graph---may be
acceptable, depending on the understanding task at hand. In this
paper we empirically show a consequence of this spectrum of
requirements by comparing the C call graphs extracted from three
software systems (mapmaker, mosaic, and gcc) by five extraction
tools (cflow, CIA, Field, mkfunctmap, and rigiparse). A
quantitative analysis of the call graphs extracted for each system
shows considerable variation, a result that is counterintuitive to
many experienced software engineers. A qualitative analysis of these
results reveals a number of reasons for this variation: differing
treatments of macros, function pointers, input formats, etc. In
this paper, we describe and discuss the study, sketch the design
space, and discuss the impact of our study on practitioners, tool
evelopers, and researchers.
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Last modified: June 28, 1996
Gail Murphy