The hand-rolled notion of flow was causing some severe performance
issues (on a few databases):
```
Tuple counts for Pythagorean::square#168e234a#f#loop_invariant_prefix/2@c86989kr after 6m35s:
175000 ~5% {2} r1 = JOIN SSA::SsaVariable::getDefinition#dispred#f0820431#ff_10#join_rhs WITH Flow::ControlFlowNode::getNode#dispred#f0820431#bf ON FIRST 1 OUTPUT Lhs.1, Rhs.1 'arg0'
174500 ~6% {2} r2 = JOIN r1 WITH SSA::SsaVariable::getVariable#dispred#f0820431#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'arg0'
1467782500 ~5% {3} r3 = JOIN r2 WITH AstGenerated::Name_::getVariable#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT 3, Rhs.1 'arg1', Lhs.1 'arg0'
1467553000 ~0% {2} r4 = JOIN r3 WITH py_expr_contexts_12#join_rhs ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.1 'arg1'
return r4
```
Rewriting it to use the data flow library made all of this go away. 🎉
This was what it looked like (at the point when I killed the evaluation):
```
Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#c5fa2be7#ff/2@i1#be98bwif after 1m50s:
274000 ~7% {4} r1 = SCAN SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#c5fa2be7#ffff OUTPUT In.1, In.0 'def', In.2, In.3
2731768000 ~1% {7} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::variableSourceUse#c5fa2be7#ffff ON FIRST 1 OUTPUT Rhs.0, Lhs.2, Lhs.3, Rhs.2, Rhs.3, Rhs.1 'use', Lhs.1 'def'
178000 ~4% {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#c5fa2be7#fffff ON FIRST 5 OUTPUT Lhs.6 'def', Lhs.5 'use'
return r3
```
And this is what it looks like now:
```
Tuple counts for SsaCompute::SsaComputeImpl::AdjacentUsesImpl::firstUse#c5fa2be7#ff/2@i1#f9d6ewsi after 207ms:
931353 ~2% {4} r1 = SCAN SsaCompute::SsaComputeImpl::AdjacentUsesImpl::variableSourceUse#c5fa2be7#ffff OUTPUT In.0, In.2, In.3, In.1 'use'
1050477 ~0% {4} r2 = JOIN r1 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::adjacentVarRefs#c5fa2be7#fffff_03412#join_rhs ON FIRST 3 OUTPUT Lhs.0, Rhs.3, Rhs.4, Lhs.3 'use'
506626 ~0% {2} r3 = JOIN r2 WITH SsaCompute::SsaComputeImpl::AdjacentUsesImpl::definesAt#c5fa2be7#ffff_1230#join_rhs ON FIRST 3 OUTPUT Rhs.3 'def', Lhs.3 'use'
return r3
```
We were building essentially a CP of all control flow nodes:
```
Tuple counts for Essa::AssignmentDefinition::getValue#dispred#f0820431#ff/2@dd1f67vl after 2m45s:
733365 ~6% {3} r1 = JOIN Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Rhs.0
376588 ~0% {2} r2 = JOIN r1 WITH SsaDefinitions::SsaSource::assignment_definition#9197156e#fff ON FIRST 2 OUTPUT Lhs.2 'this', Rhs.2 'result'
376588 ~0% {3} r3 = JOIN r2 WITH Essa::TEssaNodeDefinition#24e22a14#ffff_30#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'this', Lhs.1 'result'
6965593033 ~2% {3} r4 = JOIN r3 WITH project#SsaDefinitions::SsaSource::assignment_definition#9197156e ON FIRST 1 OUTPUT Lhs.1 'this', Rhs.1, Lhs.2 'result'
376588 ~0% {2} r5 = JOIN r4 WITH Essa::EssaNodeDefinition::getDefiningNode#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.0 'this', Lhs.2 'result'
return r5
```
We first tried preventing the join on `result`, but this caused the
characteristic predicate to blow up instead. Finally, we figured just
putting the `value` part in a field would be sufficient, and this did
the trick.
