This gains some performance by not computing locations for all
expressions since we are only interested in calls and variable accesses.
The `Top::hasLocationInfo` predicate goes from 2m28s to 1m32s on
Chromium.
This predicate was slow, mostly because it's just very large. A manual
join order cuts the run time on Chromium from
definitions::Top::hasLocationInfo_dispred#ffffff ..................... 3m23s
definitions::MacroAccessWithHasLocationInfo::hasLocationInfo#ffffff .. 1m56s
to
definitions::Top::hasLocationInfo#ffffff .... 2m28s
The main slowdown was the two uses of `SCAN` to reorder columns in the
RA.
The `reachable` predicate is large and slow to compute. It's part of a
mutual recursion that's non-linear, meaning it has a recursive call on
both sides of an `and`.
This change removes a part of the base case that has no effect on
recursive cases. The removed part is added back after the recursion has
finished.
Before, on Wireshark:
ControlFlowGraph::Cached::reachable#f .......... 20.8s (executed 9800 times)
ConstantExprs::successors_adapted#ff ........... 4.2s (executed 615 times)
ConstantExprs::potentiallyReturningFunction#f .. 3.9s (executed 9799 times)
ConstantExprs::possiblePredecessor#f ........... 2.9s (executed 788 times)
After, on Wireshark:
ConstantExprs::reachableRecursive#f ............ 13.2s (executed 9800 times)
ConstantExprs::successors_adapted#ff ........... 4.2s (executed 615 times)
ConstantExprs::potentiallyReturningFunction#f .. 4.3s (executed 9799 times)
ConstantExprs::possiblePredecessor#f ........... 2.6s (executed 788 times)
I've verified that this change doesn't change what's computed by
checking that the output of the following query is unchanged:
import cpp
import semmle.code.cpp.controlflow.internal.ConstantExprs
select
strictcount(ControlFlowNode n | reachable(n)) as reachable,
strictcount(ControlFlowNode n1, ControlFlowNode n2 | n2 = n1.getASuccessor()) as edges,
strictcount(FunctionCall c | aborting(c)) as abortingCall,
strictcount(Function f | abortingFunction(f)) as abortingFunction
This change only moves code around -- there are no changes to predicate
bodies or signatures.
The predicates that go in `ConstantExprs.Cached` after this change were
already cached in the same stage or, in the case of the `aborting*`
predicates, did not need to be cached. This is a fortunate consequence
of how the mutual recursion between the predicates happens to work, and
it's not going to be the case after the next commit.
