C++: Fix join order in 'isGuarded'.

Before:

Tuple counts for UnsignedDifferenceExpressionComparedZero::isGuarded#bff/3@ec24001m after 1.7s:
  97431    ~0%     {2} r1 = JOIN UnsignedDifferenceExpressionComparedZero::isGuarded#bff#join_rhs WITH project#BasicBlocks::Cached::basic_block_member ON FIRST 1 OUTPUT Rhs.1, Lhs.0 'sub'
  11809769 ~1%     {2} r2 = JOIN r1 WITH Guards::GuardCondition::controls_dispred#fff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1 'sub', Rhs.1
  11809769 ~0%     {4} r3 = JOIN r2 WITH project#BasicBlocks::Cached::basic_block_member ON FIRST 1 OUTPUT Lhs.1, Rhs.1, false, Lhs.0 'sub'
  629277   ~4%     {7} r4 = JOIN r3 WITH Guards::GuardCondition::ensuresLt_dispred#ffffff_045123#join_rhs ON FIRST 3 OUTPUT Lhs.3 'sub', Lhs.0, Lhs.1, false, Rhs.3 'left', Rhs.4 'right', Rhs.5
  628120   ~4%     {7} r5 = SELECT r4 ON In.6 >= 0
  628120   ~1%     {3} r6 = SCAN r5 OUTPUT In.0 'sub', In.4 'left', In.5 'right'
                    return r6

After:

Tuple counts for UnsignedDifferenceExpressionComparedZero::isGuarded#fff/3@i2#a5071x3a after 392ms:
  103763 ~0%     {2} r1 = SCAN UnsignedDifferenceExpressionComparedZero::exprIsSubLeftOrLess#ff#prev_delta OUTPUT In.0 'sub', 26
  103763 ~0%     {1} r2 = JOIN r1 WITH exprs ON FIRST 2 OUTPUT Lhs.0 'sub'
  97431  ~0%     {3} r3 = JOIN r2 WITH project#BasicBlocks::Cached::basic_block_member ON FIRST 1 OUTPUT Rhs.1, false, Lhs.0 'sub'
  629277 ~0%     {7} r4 = JOIN r3 WITH Guards::GuardCondition::ensuresLt_dispred#ffffff_450123#join_rhs ON FIRST 2 OUTPUT Lhs.2 'sub', Lhs.0, false, Rhs.2, Rhs.3 'left', Rhs.4 'right', Rhs.5
  628120 ~0%     {7} r5 = SELECT r4 ON In.6 >= 0
  628120 ~1%     {6} r6 = SCAN r5 OUTPUT In.0 'sub', In.1, In.3, In.4 'left', In.5 'right', In.6
  628120 ~1%     {6} r7 = r6 AND NOT UnsignedDifferenceExpressionComparedZero::isGuarded#fff#prev(Lhs.0 'sub', Lhs.3 'left', Lhs.4 'right')
  628120 ~0%     {5} r8 = SCAN r7 OUTPUT In.2, In.1, In.0 'sub', In.3 'left', In.4 'right'
  628120 ~1%     {3} r9 = JOIN r8 WITH Guards::GuardCondition::controls_dispred#fff ON FIRST 2 OUTPUT Lhs.2 'sub', Lhs.3 'left', Lhs.4 'right'
                  return r9

Author: MathiasVP

cpp/ql/src/Security/CWE/CWE-191/UnsignedDifferenceExpressionComparedZero.ql

@@ -22,11 +22,13 @@ import semmle.code.cpp.dataflow.DataFlow
  *  Holds if `sub` is guarded by a condition which ensures that
  * `left >= right`.
  */
-pragma[noinline]
+pragma[nomagic]
 predicate isGuarded(SubExpr sub, Expr left, Expr right) {
-  exists(GuardCondition guard, int k |
-    guard.controls(sub.getBasicBlock(), _) and
-    guard.ensuresLt(left, right, k, sub.getBasicBlock(), false) and
+  exprIsSubLeftOrLess(pragma[only_bind_into](sub), _) and // Manual magic
+  exists(GuardCondition guard, int k, BasicBlock bb |
+    pragma[only_bind_into](bb) = sub.getBasicBlock() and
+    guard.controls(pragma[only_bind_into](bb), _) and
+    guard.ensuresLt(left, right, k, bb, false) and
     k >= 0
   )
 }