It's common for a build command to contain a reference like:
${INCPREFIX}${CPPPATH}
Where ${INCPREFIX} is "-I" and ${CPPPATH} is an array of include
paths. Without this change, Rscons expands ${INCPREFIX} (and then
tries to expand "-I") repeatedly, once for each path.
This change simply makes Rscons intelligent enough to expand
${INCPREFIX} only once, then join the expanded value to each path.
In a mid-sized rake project, ruby-prof shows that the number of
calls to expand_varref is reduced from ~80,000 to ~30,000., and the
total CPU time spent in expand_varref is reduced by about 30%.
In a larger project, the improvement in real build time is small
but consistent enough to be measurable (~0.5 seconds off a total
of ~20 seconds).
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