C++: Remove self edges #13059
C++: Remove self edges #13059
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| then preUpdate = [nFrom, getAPriorDefinition(defOrUse)] | ||
| else preUpdate = nFrom | ||
| ) | ||
| } |
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Can you explain what the change is here / why nodeFrom != nodeTo above is insufficient on its own?
Otherwise LGTM.
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Sure. This was also my initial idea, but then @hvitved pointed out that for flow out of post-update nodes specifically we need to allow SSA identity-steps. To see why, consider this example:
void setTaint(x) {
sink(x.field);
x.field = taint;
}
while (b) {
setTaint(y);
}the flow after leaving setTaint (i.e., from [post] y) to y is implemented as an SSA step from [post] y's pre-update node (which is exactly y) to the next use of y, but the next use of y is y itself! So in order to have flow from [post] y to y the ssaFlow predicate must include steps from y to y itself in order to support flow out of post-update nodes.
So the final creates a new postUpdateFlow predicate whose implementation is equivalent to ssaFlow, but allow identity steps.
Does that make sense?
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That makes sense, yep. Thanks for the explanation!
This removes all the cases of a dataflow node stepping to itself.
Commit-by-commit review recommended.