C++: Don't use GVN in AST DataFlow BarrierNode

It turns out that the evaluator will evaluate the GVN stage even when no
predicate from it is needed after optimization of the subsequent stages.
The GVN library is expensive to evaluate, and it'll become even more
expensive when we switch its implementation to IR.

This PR disables the use of GVN in `DataFlow::BarrierNode` for the AST
data-flow library, which should improve performance when evaluating a
single data-flow query on a snapshot with no cache. Precision decreases
slightly, leading to a new FP in the qltests.

There is no corresponding change for the IR data-flow library since IR
GVN is not very expensive.

Author: jbj

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowUtil.qll

@@ -6,7 +6,6 @@ private import cpp
 private import semmle.code.cpp.dataflow.internal.FlowVar
 private import semmle.code.cpp.models.interfaces.DataFlow
 private import semmle.code.cpp.controlflow.Guards
-private import semmle.code.cpp.valuenumbering.GlobalValueNumbering
 
 cached
 private newtype TNode =
@@ -689,9 +688,9 @@ class BarrierGuard extends GuardCondition {
 
   /** Gets a node guarded by this guard. */
   final ExprNode getAGuardedNode() {
-    exists(GVN value, boolean branch |
-      result.getExpr() = value.getAnExpr() and
-      this.checks(value.getAnExpr(), branch) and
+    exists(SsaDefinition def, Variable v, boolean branch |
+      result.getExpr() = def.getAUse(v) and
+      this.checks(def.getAUse(v), branch) and
       this.controls(result.getExpr().getBasicBlock(), branch)
     )
   }

cpp/ql/test/library-tests/dataflow/dataflow-tests/BarrierGuard.cpp

@@ -48,7 +48,7 @@ struct XY {
 void bg_stackstruct(XY s1, XY s2) {
   s1.x = source();
   if (guarded(s1.x)) {
-    sink(s1.x); // no flow
+    sink(s1.x); // no flow [FALSE POSITIVE in AST]
   } else if (guarded(s1.y)) {
     sink(s1.x); // flow
   } else if (guarded(s2.y)) {

cpp/ql/test/library-tests/dataflow/dataflow-tests/test.expected

@@ -3,6 +3,7 @@
 | BarrierGuard.cpp:25:10:25:15 | source | BarrierGuard.cpp:21:17:21:22 | source |
 | BarrierGuard.cpp:31:10:31:15 | source | BarrierGuard.cpp:29:16:29:21 | source |
 | BarrierGuard.cpp:33:10:33:15 | source | BarrierGuard.cpp:29:16:29:21 | source |
+| BarrierGuard.cpp:51:13:51:13 | x | BarrierGuard.cpp:49:10:49:15 | call to source |
 | BarrierGuard.cpp:53:13:53:13 | x | BarrierGuard.cpp:49:10:49:15 | call to source |
 | BarrierGuard.cpp:55:13:55:13 | x | BarrierGuard.cpp:49:10:49:15 | call to source |
 | BarrierGuard.cpp:62:14:62:14 | x | BarrierGuard.cpp:60:11:60:16 | call to source |

cpp/ql/test/library-tests/dataflow/dataflow-tests/test_diff.expected

@@ -1,3 +1,4 @@
+| BarrierGuard.cpp:49:10:49:15 | BarrierGuard.cpp:51:13:51:13 | AST only |
 | BarrierGuard.cpp:60:11:60:16 | BarrierGuard.cpp:62:14:62:14 | AST only |
 | clang.cpp:12:9:12:20 | clang.cpp:22:8:22:20 | AST only |
 | clang.cpp:28:27:28:32 | clang.cpp:30:27:30:34 | AST only |