Merge pull request #2860 from jbj/isInCycle-neighbors

C++: Manual magic for `isInCycle`

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/IRConstruction.qll

@@ -102,6 +102,19 @@ private module Cached {
     result = getMemoryOperandDefinition(instr, _, _)
   }
 
+  /**
+   * Gets a non-phi instruction that defines an operand of `instr` but only if
+   * both `instr` and the result have neighbor on the other side of the edge
+   * between them. This is a necessary condition for being in a cycle, and it
+   * removes about two thirds of the tuples that would otherwise be in this
+   * predicate.
+   */
+  private Instruction getNonPhiOperandDefOfIntermediate(Instruction instr) {
+    result = getNonPhiOperandDef(instr) and
+    exists(getNonPhiOperandDef(result)) and
+    instr = getNonPhiOperandDef(_)
+  }
+
   /**
    * Holds if `instr` is part of a cycle in the operand graph that doesn't go
    * through a phi instruction and therefore should be impossible.
@@ -115,7 +128,7 @@ private module Cached {
   cached
   predicate isInCycle(Instruction instr) {
     instr instanceof Instruction and
-    getNonPhiOperandDef+(instr) = instr
+    getNonPhiOperandDefOfIntermediate+(instr) = instr
   }
 
   cached