[SYCL] Refactor unit-tests to ignore fallback libs (#7534)

Updated default constructor of mock property set to contain a property
about devicelib mask so unit-tests depend on one less env variable.

This resolves #6945

Author: AlexeySachkov

sycl/test/Unit/lit.cfg.py

@@ -41,9 +41,9 @@
 
 if 'SYCL_DEVICELIB_NO_FALLBACK' in os.environ:
     config.environment['SYCL_DEVICELIB_NO_FALLBACK'] = os.environ['SYCL_DEVICELIB_NO_FALLBACK']
-else:
-    # Disable device library fallback for unit tests by default.
-    config.environment['SYCL_DEVICELIB_NO_FALLBACK'] = "1"
+# We do not have any default for SYCL_DEVICELIB_NO_FALLBACK, which means that
+# env variable won't be defined. That is ok, because we expect tests to pass
+# even without it.
 
 # Propagate path to symbolizer for ASan/MSan.
 for symbolizer in ['ASAN_SYMBOLIZER_PATH', 'MSAN_SYMBOLIZER_PATH']:

sycl/unittests/helpers/PiImage.hpp

@@ -160,7 +160,27 @@ template <typename T> class PiArray {
 /// Convenience wrapper for pi_device_binary_property_set.
 class PiPropertySet {
 public:
-  PiPropertySet() = default;
+  PiPropertySet() {
+    // Most of unit-tests are statically linked with SYCL RT. On Linux and Mac
+    // systems that causes incorrect RT installation directory detection, which
+    // prevents proper loading of fallback libraries. See intel/llvm#6945
+    //
+    // Fallback libraries are automatically loaded and linked into device image
+    // unless there is a special property attached to it or special env variable
+    // is set which forces RT to skip fallback libraries.
+    //
+    // Setting this property here so unit-tests can be launched under any
+    // environment.
+
+    std::vector<char> Data(/* eight elements */ 8,
+                           /* each element is zero */ 0);
+    // Name doesn't matter here, it is not used by RT
+    // Value must be an all-zero 32-bit mask, which would mean that no fallback
+    // libraries are needed to be loaded.
+    PiProperty DeviceLibReqMask("", Data, PI_PROPERTY_TYPE_UINT32);
+    insert(__SYCL_PI_PROPERTY_SET_DEVICELIB_REQ_MASK,
+           PiArray{DeviceLibReqMask});
+  }
 
   /// Adds a new array of properties to the set.
   ///