[SYCL] Avoid additional allocations in has_extension (#19765)

#19264 fixed an issue where
has_extension would allow partial matches to return true. However, the
proposed solution makes additional allocations to pad the strings used.
To avoid this, this commit changes the implementation to explicitly
check for the partial match conditions without padding either strings.

---------

Signed-off-by: Larsen, Steffen <[email protected]>

Author: steffenlarsen

sycl/source/detail/device_impl.cpp

@@ -106,13 +106,34 @@ device_impl::get_backend_info<info::device::backend_version>() const {
 #endif
 
 bool device_impl::has_extension(const std::string &ExtensionName) const {
+  if (ExtensionName.empty())
+    return false;
+
   const std::string AllExtensionNames{
       get_info_impl<UR_DEVICE_INFO_EXTENSIONS>()};
 
-  // We add a space to both sides of both the extension string and the query
-  // string. This prevents to lookup from finding partial extension matches.
-  return ((" " + AllExtensionNames + " ").find(" " + ExtensionName + " ") !=
-          std::string::npos);
+  size_t FoundExtPos = AllExtensionNames.find(ExtensionName);
+  while (FoundExtPos != std::string::npos) {
+    // If the extension name was found, we need to ensure it is not a partial
+    // match. That is, the following must hold:
+    //  * The match must be at the start of the list of names or have a
+    //    whitespace before it and
+    //  * the match must end at the end of the list of names or have a
+    //    whitespace after it.
+    bool IsStartOrTerminated =
+        FoundExtPos == 0 || AllExtensionNames[FoundExtPos - 1] == ' ';
+    bool IsEndOrTerminated =
+        FoundExtPos + ExtensionName.size() == AllExtensionNames.size() ||
+        AllExtensionNames[FoundExtPos + ExtensionName.size()] == ' ';
+    if (IsStartOrTerminated && IsEndOrTerminated)
+      return true;
+
+    // If the match was partial, the extension name could still be later in the
+    // list. As such, search for the next match and recheck.
+    FoundExtPos = AllExtensionNames.find(ExtensionName,
+                                         FoundExtPos + ExtensionName.size());
+  }
+  return false;
 }
 
 bool device_impl::is_partition_supported(info::partition_property Prop) const {

sycl/unittests/context_device/HasExtensionWordBoundary.cpp

@@ -91,7 +91,6 @@ TEST_F(HasExtensionWordBoundaryTest, SingleExtension) {
 
   sycl::platform Plt{sycl::platform()};
   sycl::device Dev = Plt.get_devices()[0];
-  auto DevImpl = detail::getSyclObjImpl(Dev);
 
   EXPECT_TRUE(Dev.has_extension("cl_khr_fp64"));
   EXPECT_FALSE(Dev.has_extension("cl_khr_fp6"));
@@ -102,13 +101,28 @@ TEST_F(HasExtensionWordBoundaryTest, FirstMiddleLastExtensions) {
 
   sycl::platform Plt{sycl::platform()};
   sycl::device Dev = Plt.get_devices()[0];
-  auto DevImpl = detail::getSyclObjImpl(Dev);
 
   EXPECT_TRUE(Dev.has_extension("cl_first_ext"));
   EXPECT_TRUE(Dev.has_extension("cl_middle_ext"));
   EXPECT_TRUE(Dev.has_extension("cl_last_ext"));
 }
 
+TEST_F(HasExtensionWordBoundaryTest, MatchAfterPartialMatch) {
+  MockExtensions = "cl_khr_fp64_with_more cl_khr_fp64";
+
+  sycl::platform Plt{sycl::platform()};
+  sycl::device Dev = Plt.get_devices()[0];
+
+  EXPECT_TRUE(Dev.has_extension("cl_khr_fp64"));
+}
+
+TEST_F(HasExtensionWordBoundaryTest, MatchEmptyString) {
+  sycl::platform Plt{sycl::platform()};
+  sycl::device Dev = Plt.get_devices()[0];
+
+  EXPECT_FALSE(Dev.has_extension(""));
+}
+
 TEST_F(HasExtensionWordBoundaryTest, NonUniformGroupExtensions) {
   MockExtensions = "cl_khr_subgroup_non_uniform_vote "
                    "cl_khr_subgroup_ballot "
@@ -118,7 +132,6 @@ TEST_F(HasExtensionWordBoundaryTest, NonUniformGroupExtensions) {
 
   sycl::platform Plt{sycl::platform()};
   sycl::device Dev = Plt.get_devices()[0];
-  auto DevImpl = detail::getSyclObjImpl(Dev);
 
   EXPECT_TRUE(Dev.has_extension("cl_khr_subgroup_non_uniform_vote"));
   EXPECT_TRUE(Dev.has_extension("cl_khr_subgroup_ballot"));