Merge pull request #15637 from MathiasVP/fix-joins-in-irguards

C++: Fix joins in `controlsBlock`

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll

@@ -203,30 +203,42 @@ private class GuardConditionFromIR extends GuardCondition {
    * `&&` and `||`. See the detailed explanation on predicate `controls`.
    */
   private predicate controlsBlock(BasicBlock controlled, boolean testIsTrue) {
-    exists(IRBlock irb, Instruction instr |
+    exists(IRBlock irb |
       ir.controls(irb, testIsTrue) and
-      instr = irb.getAnInstruction() and
-      instr.getAst().(ControlFlowNode).getBasicBlock() = controlled and
-      not isUnreachedBlock(irb) and
-      not this.excludeAsControlledInstruction(instr)
+      nonExcludedIRAndBasicBlock(irb, controlled) and
+      not isUnreachedBlock(irb)
     )
   }
+}
 
-  private predicate excludeAsControlledInstruction(Instruction instr) {
-    // Exclude the temporaries generated by a ternary expression.
-    exists(TranslatedConditionalExpr tce |
-      instr = tce.getInstruction(ConditionValueFalseStoreTag())
-      or
-      instr = tce.getInstruction(ConditionValueTrueStoreTag())
-      or
-      instr = tce.getInstruction(ConditionValueTrueTempAddressTag())
-      or
-      instr = tce.getInstruction(ConditionValueFalseTempAddressTag())
-    )
+private predicate excludeAsControlledInstruction(Instruction instr) {
+  // Exclude the temporaries generated by a ternary expression.
+  exists(TranslatedConditionalExpr tce |
+    instr = tce.getInstruction(ConditionValueFalseStoreTag())
     or
-    // Exclude unreached instructions, as their AST is the whole function and not a block.
-    instr instanceof UnreachedInstruction
-  }
+    instr = tce.getInstruction(ConditionValueTrueStoreTag())
+    or
+    instr = tce.getInstruction(ConditionValueTrueTempAddressTag())
+    or
+    instr = tce.getInstruction(ConditionValueFalseTempAddressTag())
+  )
+  or
+  // Exclude unreached instructions, as their AST is the whole function and not a block.
+  instr instanceof UnreachedInstruction
+}
+
+/**
+ * Holds if `irb` is the `IRBlock` corresponding to the AST basic block
+ * `controlled`, and `irb` does not contain any instruction(s) that should make
+ * the `irb` be ignored.
+ */
+pragma[nomagic]
+private predicate nonExcludedIRAndBasicBlock(IRBlock irb, BasicBlock controlled) {
+  exists(Instruction instr |
+    instr = irb.getAnInstruction() and
+    instr.getAst().(ControlFlowNode).getBasicBlock() = controlled and
+    not excludeAsControlledInstruction(instr)
+  )
 }
 
 /**