cXprop
Postpass optimization for TinyOS applications
Nathan Cooprider
John Regehr
School of Computing, University of Utah
{coop,regehr}@cs.utah.edu
Features
Summary
Creating TinyOS applications from
generic components often leads to
applications containing dead code and
constant variables. These unnecessarily
consume SRAM and Flash memory. We
address the problem with cXprop, a
static-analysis-based
post-processing
tool for the C code produced by the
nesC
compiler.
cXprop
performs
"conditional
X
propagation,"
a
generalization
of
the
well-known
conditional
constant
propagation
algorithm where X is an abstract value
propagation domain supplied by the
user. cXprop is interprocedural and
achieves reasonable precision on
pointer-rich codes. We use novel
concurrency abstractions to analyze
global state inside of TinyOS' restrictive
concurrency model. The dataflow
information
produced
by
cXprop
supports reduction in code size through
interprocedural dead code elimination,
and reduction in data size by finding
global variables that do not use their
full bitwidth. For example, state
variables and boolean flags often use
an entire byte to store a single bit. We
have validated the code produced by
cXprop through random testing and by
dynamically checking its results against
actual executions. cXprop reduces
application code size on average by
10% for TinyOS applications and
predicts savings of 50-100 bytes of
SRAM. It is available as open source
software.
Tool components
●
●
●
●
Three modes of execution
● CP and DCE
● Program state assertion
● Dynamic state printing
Pluggable abstract domains
Handles concurrent code
● Three concurrency abstractions
Models pointers and structs
● limited array support at this time
Background Work
Patrick Cousot and Radhia Cousot. Abstract interpretation: A
unified lattice model for static analysis of programs by
construction or approximation of fixpoints. In Proc. of the 4th
Symp. on Principles of Programming Languages (POPL), pages
238–252, Los Angeles, CA, January 1977.
George C. Necula, Scott McPeak, S. P. Rahul, and Westley
Weimer. CIL: Intermediate language and tools for analysis and
transformation of C programs. In Proc. of the Intl. Conf. on
Compiler Construction (CC), pages 213–228, Grenoble, France,
April 2002.
Mark N.Wegman and F. Kenneth Zadeck. Constant propagation
with conditional branches. ACM Transactions on Programming
Languages and Systems (TOPLAS), 13(2):181–210, April 1991.
Pluggable abstract domains
Value-set
Interval
Bitwise
Parity
Constant
Current results
Graph 1: The percentage of information collected by cXprop during its analysis with the different domains.
average 9.2%
code size
reduction
average 11.9% reduction in
global data size
Graph 2: Comparison of cXprop processed code size
code size from normal nesC compilation.
Graph 3: Estimated savings in RAM through global bit
compression.
Future work
The abstract interpreter is made up
of several parts. However, there is a
clean interface for the abstract
domains. This makes implementing
new domains straightforward.
cXprop
provides
a
novel
concurrency model for analyzing
global variables in the presence of
interrupts and atomic sections.
Atomic sections are connected in
the control flow graph.
We are continuing to improve and
enhance cXprop. We plan to continue to
improve the speed, precision, and
expressiveness of the analysis while
looking for new ways to apply the
information gained by cXprop.
Acknowledgments
“spaghetti” concurrency mode
This work is supported by National
Science Foundation CAREER Award
CNS-0448047
http://www.cs.utah.edu/~coop/research/cxprop