C++: Add dataflow through '__builtin_bit_cast'.

Author: MathiasVP

cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -114,6 +114,13 @@ predicate conversionFlow(
       instrTo.(CheckedConvertOrNullInstruction).getUnaryOperand() = opFrom
       or
       instrTo.(InheritanceConversionInstruction).getUnaryOperand() = opFrom
+      or
+      exists(BuiltInInstruction builtIn |
+        builtIn = instrTo and
+        // __builtin_bit_cast
+        builtIn.getBuiltInOperation() instanceof BuiltInBitCast and
+        opFrom = builtIn.getAnOperand()
+      )
     )
     or
     additional = true and

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

@@ -52,3 +52,9 @@ void following_pointers( // $ ast-def=sourceStruct1_ptr ir-def=*cleanArray1 ir-d
   sink(stackArray); // $ ast,ir
   indirect_sink(stackArray); // $ ast ir=50:25 ir=50:35 ir=51:19
 }
+
+void test_bitcast() {
+  unsigned long x = source();
+  double d = __builtin_bit_cast(double, x);
+  sink(d); // $ ir MISSING: ast
+}

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

@@ -153,6 +153,7 @@ irFlow
 | clang.cpp:50:25:50:30 | call to source | clang.cpp:53:17:53:26 | *stackArray |
 | clang.cpp:50:35:50:40 | call to source | clang.cpp:53:17:53:26 | *stackArray |
 | clang.cpp:51:19:51:24 | call to source | clang.cpp:53:17:53:26 | *stackArray |
+| clang.cpp:57:21:57:28 | call to source | clang.cpp:59:8:59:8 | d |
 | dispatch.cpp:9:37:9:42 | call to source | dispatch.cpp:35:16:35:25 | call to notSource1 |
 | dispatch.cpp:9:37:9:42 | call to source | dispatch.cpp:43:15:43:24 | call to notSource1 |
 | dispatch.cpp:10:37:10:42 | call to source | dispatch.cpp:36:16:36:25 | call to notSource2 |