Go: Avoid combinatorial explosion in mostRecentSideEffect when there are multiple entry points

Author: hvitved

go/ql/lib/semmle/go/dataflow/GlobalValueNumbering.qll

@@ -127,10 +127,11 @@ private predicate sideEffectCfg(ControlFlow::Node src, ControlFlow::Node dst) {
 
 /**
  * Holds if `dominator` is the immediate dominator of `node` in
- * the side-effect CFG.
+ * the side-effect CFG belonging to `entry`.
  */
-private predicate iDomEffect(ControlFlow::Node dominator, ControlFlow::Node node) =
-  idominance(entryNode/1, sideEffectCfg/2)(_, dominator, node)
+private predicate iDomEffect(
+  ControlFlow::Node entry, ControlFlow::Node dominator, ControlFlow::Node node
+) = idominance(entryNode/1, sideEffectCfg/2)(entry, dominator, node)
 
 /**
  * Gets the most recent side effect. To be more precise, `result` is a
@@ -182,12 +183,15 @@ private predicate iDomEffect(ControlFlow::Node dominator, ControlFlow::Node node
  * Therefore, the most recent side effect for line 015 is line 009.
  */
 cached
+private ControlFlow::Node mostRecentSideEffect(ControlFlow::Node entry, ControlFlow::Node node) {
+  iDomEffect(entry, entry, result) and
+  result = node
+  or
+  iDomEffect(entry, result, mostRecentSideEffect(entry, node))
+}
+
 private ControlFlow::Node mostRecentSideEffect(ControlFlow::Node node) {
-  exists(ControlFlow::Node entry |
-    entryNode(entry) and
-    iDomEffect(entry, result) and
-    iDomEffect*(result, node)
-  )
+  result = mostRecentSideEffect(_, node)
 }
 
 /** Used to represent the "global value number" of an expression. */