The `multiple_invocation_paths` predicate had a bad join order where
we (essentially) joined `i1` with `i2` and only then joined `i1` and `i2`
separately to reduce the number of tuples. The join coming from `i1 != i2` had
little impact, but `i1.getFunction() = multi` made a big difference (and
similarly for `i2`). I factored out the code so that these joins would be done
more eagerly. Thus, we went from
```
[2019-11-06 16:53:05] (38s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff/4@2ce75a
[2019-11-06 16:53:35] (68s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff:
134547 ~9% {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<0>, I.<2>
235284431 ~3% {4} r2 = JOIN r1 WITH CallGraph::TInvocation#fff AS R ON FIRST 1 OUTPUT r1.<0>, r1.<1>, R.<1>, R.<2>
235149884 ~3% {4} r3 = SELECT r2 ON r2.<3> != r2.<1>
235149884 ~4% {3} r4 = SCAN r3 OUTPUT r3.<1>, r3.<0>, r3.<3>
166753634 ~5% {4} r5 = JOIN r4 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus#swapped AS R ON FIRST 1 OUTPUT R.<1>, r4.<2>, r4.<1>, r4.<0>
129778 ~0% {4} r6 = JOIN r5 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus AS R ON FIRST 2 OUTPUT r5.<0>, r5.<3>, r5.<1>, r5.<2>
return r6
[2019-11-06 16:53:35] (68s) Registering MethodCallOrder::multiple_invocation_paths#ffff + [] with content 1705dcbc08kd9aa40rp2g2e9civhv
[2019-11-06 16:53:35] (68s) >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff with 129778 rows and 4 columns.
```
to
```
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths_helper#ffff/4@586aec
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths_helper#ffff:
134547 ~0% {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<2>, I.<0>
88111 ~4% {3} r2 = JOIN r1 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus#swapped AS R ON FIRST 1 OUTPUT R.<1>, r1.<1>, r1.<0>
761305 ~0% {4} r3 = JOIN r2 WITH #CallGraph::FunctionInvocation::getACallee_dispred#ffPlus AS R ON FIRST 1 OUTPUT r2.<1>, r2.<2>, r2.<0>, R.<1>
673194 ~0% {4} r4 = SELECT r3 ON r3.<3> != r3.<1>
673194 ~0% {4} r5 = SCAN r4 OUTPUT r4.<2>, r4.<1>, r4.<3>, r4.<0>
return r5
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths_helper#ffff + [] with content 20edaaecf25nldgp24d9c4et8m3kv
[2019-11-06 17:22:22] (25s) >>> Wrote relation MethodCallOrder::multiple_invocation_paths_helper#ffff with 673194 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs/4@9e5441
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs:
673194 ~0% {4} r1 = SCAN MethodCallOrder::multiple_invocation_paths_helper#ffff AS I OUTPUT I.<2>, I.<3>, I.<0>, I.<1>
return r1
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs + [] with content 2069301e655fi9mcovngg9hetfqas
[2019-11-06 17:22:22] (25s) >>> Wrote relation MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs with 673194 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff/4@2f7c34
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff:
134547 ~0% {2} r1 = SCAN CallGraph::TInvocation#fff AS I OUTPUT I.<2>, I.<0>
129778 ~0% {4} r2 = JOIN r1 WITH MethodCallOrder::multiple_invocation_paths_helper#ffff_2301#join_rhs AS R ON FIRST 2 OUTPUT R.<2>, R.<3>, r1.<0>, r1.<1>
return r2
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths#ffff + [] with content 1705dcbc08kd9aa40rp2g2e9civhv
[2019-11-06 17:22:22] (25s) >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff with 129778 rows and 4 columns.
[2019-11-06 17:22:22] (25s) Starting to evaluate predicate MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs/4@9f9146
[2019-11-06 17:22:22] (25s) Tuple counts for MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs:
129778 ~0% {4} r1 = SCAN MethodCallOrder::multiple_invocation_paths#ffff AS I OUTPUT I.<0>, I.<3>, I.<1>, I.<2>
return r1
[2019-11-06 17:22:22] (25s) Registering MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs + [] with content 17c3fe1fcbf6ghhdr7hiukqp41rst
[2019-11-06 17:22:22] (25s) >>> Wrote relation MethodCallOrder::multiple_invocation_paths#ffff_0312#join_rhs with 129778 rows and 4 columns.
```
Execution time on `salt` went from 29.5s to somewhere below 299ms (the predicate
was not listed in the timing report).
As it turns out, there was a further bad join-order in the `global_name_used`
predicate. In this case, there was a common subexpression in the RA that was
being factored out and evaluated separately, producing a large number of tuples.
This is just good enough to cause no performance regressions and pass
the virtual-dispatch tests we have for `security.TaintTracking`. In
particular, it fixes the tests for `UncontrolledProcessOperation.ql`
when enabling `DefaultTaintTracking.qll`.
