Avoid quadratic switch case intermediate

Author: smowton

java/ql/lib/semmle/code/java/ControlFlowGraph.qll

@@ -82,6 +82,7 @@
 import java
 private import Completion
 private import controlflow.internal.Preconditions
+private import controlflow.internal.SwitchCases
 
 /** A node in the expression-level control-flow graph. */
 class ControlFlowNode extends Top, @exprparent {
@@ -436,34 +437,6 @@ private module ControlFlowGraphImpl {
     )
   }
 
-  /**
-   * Gets the `i`th `SwitchCase` defined on `switch`, if one exists.
-   */
-  private SwitchCase getCase(StmtParent switch, int i) {
-    result = switch.(SwitchExpr).getCase(i) or result = switch.(SwitchStmt).getCase(i)
-  }
-
-  /**
-   * Gets the `i`th `PatternCase` defined on `switch`, if one exists.
-   */
-  private PatternCase getPatternCase(StmtParent switch, int i) {
-    result =
-      rank[i + 1](PatternCase pc, int caseIdx | pc = getCase(switch, caseIdx) | pc order by caseIdx)
-  }
-
-  /**
-   * Gets the PatternCase after pc, if one exists.
-   */
-  private PatternCase getNextPatternCase(PatternCase pc) {
-    exists(int idx, StmtParent switch |
-      pc = getPatternCase(switch, idx) and result = getPatternCase(switch, idx + 1)
-    )
-  }
-
-  private int lastCaseIndex(StmtParent switch) {
-    result = max(int i | any(SwitchCase c).isNthCaseOf(switch, i))
-  }
-
   // Join order engineering -- first determine the switch block and the case indices required, then retrieve them.
   bindingset[switch, i]
   pragma[inline_late]

java/ql/lib/semmle/code/java/controlflow/Guards.qll

@@ -7,6 +7,7 @@ import java
 private import semmle.code.java.controlflow.Dominance
 private import semmle.code.java.controlflow.internal.GuardsLogic
 private import semmle.code.java.controlflow.internal.Preconditions
+private import semmle.code.java.controlflow.internal.SwitchCases
 
 /**
  * A basic block that terminates in a condition, splitting the subsequent control flow.
@@ -72,6 +73,35 @@ class ConditionBlock extends BasicBlock {
   }
 }
 
+// Join order engineering -- first determine the switch block and the case indices required, then retrieve them.
+bindingset[switch, i]
+pragma[inline_late]
+private predicate isNthCaseOf(StmtParent switch, SwitchCase c, int i) { c.isNthCaseOf(switch, i) }
+
+/**
+ * Gets a switch case >= pred, up to but not including `pred`'s successor pattern case,
+ * where `pred` is declared on `switch`.
+ */
+private SwitchCase getACaseUpToNextPattern(PatternCase pred, StmtParent switch) {
+  // Note we do include `case null, default` (as well as plain old `default`) here.
+  not result.(ConstCase).getValue(_) instanceof NullLiteral and
+  exists(int maxCaseIndex |
+    switch = pred.getParent() and
+    if exists(getNextPatternCase(pred))
+    then maxCaseIndex = getNextPatternCase(pred).getCaseIndex() - 1
+    else maxCaseIndex = lastCaseIndex(switch)
+  |
+    isNthCaseOf(switch, result, [pred.getCaseIndex() .. maxCaseIndex])
+  )
+}
+
+/**
+ * Gets the closest pattern case preceding `case`, including `case` itself, if any.
+ */
+private PatternCase getClosestPrecedingPatternCase(SwitchCase case) {
+  case = getACaseUpToNextPattern(result, _)
+}
+
 /**
  * A condition that can be evaluated to either true or false. This can either
  * be an `Expr` of boolean type that isn't a boolean literal, or a case of a
@@ -113,17 +143,10 @@ class Guard extends ExprParent {
     result = this.(Expr).getBasicBlock()
     or
     // Return the closest pattern case statement before this one, including this one.
-    result =
-      max(int i, PatternCase c |
-        c = this.(SwitchCase).getSiblingCase(i) and i <= this.(SwitchCase).getCaseIndex()
-      |
-        c order by i
-      ).getBasicBlock()
+    result = getClosestPrecedingPatternCase(this).getBasicBlock()
     or
     // Not a pattern case and no preceding pattern case -- return the top of the switch block.
-    not exists(PatternCase c, int i |
-      c = this.(SwitchCase).getSiblingCase(i) and i <= this.(SwitchCase).getCaseIndex()
-    ) and
+    not exists(getClosestPrecedingPatternCase(this)) and
     result = this.(SwitchCase).getSelectorExpr().getBasicBlock()
   }
 

java/ql/lib/semmle/code/java/controlflow/internal/SwitchCases.qll

@@ -0,0 +1,31 @@
+/** Provides utility predicates relating to switch cases. */
+
+import java
+
+/**
+ * Gets the `i`th `SwitchCase` defined on `switch`, if one exists.
+ */
+SwitchCase getCase(StmtParent switch, int i) {
+  result = switch.(SwitchExpr).getCase(i) or result = switch.(SwitchStmt).getCase(i)
+}
+
+/**
+ * Gets the `i`th `PatternCase` defined on `switch`, if one exists.
+ */
+PatternCase getPatternCase(StmtParent switch, int i) {
+  result =
+    rank[i + 1](PatternCase pc, int caseIdx | pc = getCase(switch, caseIdx) | pc order by caseIdx)
+}
+
+/**
+ * Gets the PatternCase after pc, if one exists.
+ */
+PatternCase getNextPatternCase(PatternCase pc) {
+  exists(int idx, StmtParent switch |
+    pc = getPatternCase(switch, idx) and result = getPatternCase(switch, idx + 1)
+  )
+}
+
+int lastCaseIndex(StmtParent switch) {
+  result = max(int i | any(SwitchCase c).isNthCaseOf(switch, i))
+}