[PATCH] Conditional count update for fast coverage test in multi-threaded programs

[Rong Xu <xur at google dot com>]

Hi,

This patch injects a condition into the instrumented code for edge
counter update. The counter value will not be updated after reaching
value 1.

The feature is under a new parameter --param=coverage-exec_once.
Default is disabled and setting to 1 to enable.

This extra check usually slows the program down. For SPEC 2006
benchmarks (all single thread programs), we usually see around 20%-35%
slow down in -O2 coverage build. This feature, however, is expected to
improve the coverage run speed for multi-threaded programs, because
there virtually no data race and false sharing in updating counters.
The improvement can be significant for highly threaded programs -- we
are seeing 7x speedup in coverage test run for some non-trivial google
applications.

Tested with bootstrap.

Thanks,

-Rong

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